Café Racer Conversion (BMW K100)
About the project
Choosing a base model
There is a wide range of motorcycles on the market that would can be converted into a café racer. Let me lay out the main points that have let me to chose a BMW K100 from 1984 with its one of a kind engine layout.
Converting a vehicle takes a lot of time and effort and for me it is of great importance that once a project is finished that it can be enjoyed for a long time to come, which brings us to reliability and maintenance. The BMW K100 was used as a police motorcycle in the past showing its trustworthiness (the police needs to focus on their tasks and frequent vehicle repairs are excluded from the get go by choosing reliable models).
There are quite a few reports on BMW K100 bikes that have reached with their original engine over 500.000km. I have heard of a report stating someone made over 1 Million kilometers with one engine on a K100 bike (however I could not find the article).
Apart from the widely known longevity of the engine, there are also very few things that need replacement on the bike over time. It runs on a cardan shaft which automatically excludes the frequent maintenances that chained driven bikes need.
On top of all, this bike is very very affordable… Often you find it cheaper than buying a used scooter.
Considering all factors, this was the perfect basis as a café racer project bike.
My recommendation:
Using a BMW K100 as a café racer base, its reliability, robustness and affordable purchase price makes it a no-brainer!
2. Building a workshop
Even a well planned project could fail by having a hard accessible or only time limited space to build. For me it was that I had no place to build this project, so I had to convert our old garden house into a workshop.
A quick garden house upgrade turned into a 3 months project (click here to read more about the workshop project).
This garden house was build by my grandpa a long time ago, therefore I tried to safe the most of it and rebuild the rest.
Also I added a tool room and several quick access tool walls to make the upcoming café racer conversion a joy rather than a nerv wrecking tool search. I guess anyone with a workshop knows what I am talking about, often we do spent a ridiculous amount of the building time searching for a tool.
Many litres of coffee later I had my own heated workshop.
Ready to bring in the bike.
3. Disassemble of the motorcycle
A café racer is known for its minimalistic design, meaning the bodywork will be reduced to a minimum. And this is another strong point for getting a BMW K100 or its smaller sibling the BMW K75 for a café racer project as you can sell most of its original bodywork. I was able to earn around 50% of the purchase price of this used bike back (through selling the now unneeded bodywork parts).
This BMW K100 had 62.000 km on its back, therefore I decided to not only convert it into a café racer but make a complete restoration of all parts.
So I started the disassembly, taking it into pieces.
My recommendation:
Make use of the original repair book of the BMW K100/K75, it is of great help at parts. Especially Nm of every screw are beautifully laid out throughout the chapters.
4. Into the Workshop
It is time to bring the bike into the workshop and get serious.
This is my first motorcycle project. But luckily we live in the modern world where information is readily available, one could literally become one of the most knowledgable motorcycle builder within 5 years by just being home dedicating several hours a day studying the art and mechanics of motorcycles. Ive once heard the following phrase which stock with me ever since:
One hour per day of study in your chosen field is all it takes. One hour per day of study will put you at the top of your field within three years. Within five years you'll be a national authority. In seven years, you can be one of the best people in the world at what you do. -Earl Nightingale
We live in wonderful and simultaneously very tense times. Wonderful because we have so many options/informations available like never before and very tense because we tend to fall into the social media trap of pretentious lifestyles and distorted realities.
I guess we should all use our phones and social media a bit less and if we use social media than rather to study and educate. I myself fall into the Reel or YouTube trap frequently… it is so hard to avoid being caught in its addictiveness.
5. Complete Disassemble
Here you can see another BIG reason why I chose this bike for the café racer conversion, its incredible BIG engine… Did you know that the idea behind this engine came from Josef Fritzenwenger (a german engineer) it was a unprecedented idea at BMW:
Taking a car engine and placing it on its side and making a motorcycle out of it!
The engine comes originally from a Peugeot 104. So easy it was not, I believe the engineering took more than 2 years as the engine and drive train had to be adapted to work in such a small form factor of a motorcycle.
Now what I stated in Point 1 makes very much sense:
The BMW K100 was used as a police motorcycle in the past showing its trustworthiness .
There are quite a few reports on BMW K100 bikes that have reached with their original engine over 500.000km.
A car engine is generally made for much more kilometres than a motorcycle engine.
My recommendation:
Want a more than reliable motorcycle? Place a car engine in a motorcycle frame!
—> And that is exactly what you get with the BMW K100!
6. Cleaning the Engine
62.000 km have left its traces, additionally I have found some sealing rings that started leaking oil making especially the gear box and clutch housing into an oil pan.
I could use breaking cleaner and spray the entire engine several times and see the dirt in front of my eyes melting away. However I wouldn’t recommend doing so many breaking cleaner advise against spraying on rubber or plastic parts. You can test your breaking cleaners strength by spraying it on a plastic glove.
Breaking cleaner is not safe for rubber and plastic parts and I advice against cleaning an entire engine with it.
There are other alternatives like:
Sandblasting (abrasive and sand could entere the engine)
Dry ice blasting (nonabrasive and environmentally friendly)
Engine cleaner (time consuming job)
I didn’t choose sandblasting as I wanted to maintain and restore the original look of the engine and I didn’t want to risk sand potentially reach into sensible engine compartments. So I started with engine cleaner and a set of tooth brushes. And after some hours of cleaning I loaded the engine in the trunk of my dads car and went for dry ice blasting. I was very curious how it works which was a big motive for me to try it out in this project.
7. Dry Ice Blasting
In dry ice blasting the solid form of carbon dioxide (dry ice) is expelled with high speeds at an object and on impact it converts back into a gaseous state causing shear stress (thermal shock) and micro shockwaves therefore effectively removing the layer of dirt.
No chemical residuos
No secondary waste
Non toxic
Non abrasive
Seemingly the perfect treatment for my plan to restore the original beauty of the BMW K100 engine.
The results turned out well, but the oxidation the aluminium endured throughout the last 40 years still need treatment. Later on I will use a self made sanding fleece attachment for my cordless screwdriver to reduce the extent of oxidation that have already occurred.
8. Results of Dry Ice Blasting
So how do 5 hours of tooth brush cleaning and dry ice blasting look like?
Here a picture for the answer.
The lightning and angle made it somewhat better looking than in reality. However it was already a satisfying result for such an old engine.
I am ready for the next steps.
9. The Cylinder Bore Surface
Ever wondered how the cylinder bore surfaces of a BMW K100 after 62.000km do look like?
I was very curious and I made a picture for you while looking at it. The original cross-grinding of the bore surfaces made in 1984 are still visible like when the engine came from the production line. This is again an unbelievable example on the workings of an oil film!
Just imagine this for a second the piston glides on the cylinder bore surface which is metal on metal for 40 years and the surfaces look unused due to the tiny oil film between both.
The BMW K100 has a rpm (revolution per minute) of around 1000rpm (standing) to 9000rpm (top speed). This means:
the piston moves once up and once down per each revolution (2000 glides per minute standing and 18000 glides when going top speed).
going 1 hour on top speed would equal to 18000glides*60min=1.080.000! Around 1 Million glides per hour.
Can you imagine how many glides this piston did on the cylinder bore surface? Me neither, but lets make an approximate calculation to find out:
The bike is 40 years old and drove 62.000km. Let us assume that all 62.000km were done at a speed of 100km/h with 5000rpm. So the bike needed 62.000km:100km/h=620h to drive it (Woah that sounds so little, suddenly the bike is not very used in my mind).
How many minutes did the bike need to drive 62.000km in 40 years?
620h*60= 37200min
Now we assume the BMW K100 makes 5000rpm at 100km/h and we multiply it with the total min the bike was running:
37200min*5000rpm= 186.000.000 revolutions of the crankshaft
Each revolution of the crankshaft the piston glides twice
so 186.000.000*2=372.000.000 glides!!
372 Million glides and no signs of wear whatsoever, thanks to all the engineers out there!
10. High pressure cleaner
The engine is already in the workshop, but a motorcycle consists of many more parts and they are all in equally used and dirty condition. So I thought to start with a high pressure steam jet to make my life somewhat easier.
In the background you see my old VW T4 with its 350.000km. Sometime ago it was ready for the scrap yard but the beautiful memories I made with it gave me the motivation for a full restoration and to build a camper out of it (if you want to read more about the VW T4 project you can find it by clicking here).
Once the parts are relatively clean it is time for a management system. Cartons are a great way to order each system of the motorcycle separately. A café racer conversion will take time and over the course of building one might find her/himself wondering where some parts have gone or it became hard to remember where it belonged. Also make use of plastic bags and label them with waterproof pencils to find the right screws/parts in no time.
After I went through a unmanaged building phase of the old mofa (moped) of my grandpa when I was 14 years of age, I truly know how much time and nerves one can safe with a management system.
11. Restoring the exhaust
The exhaust manifolds are build from of stainless steel. That means that the superficial layer which has changed substantially over the course of 40 years can be polished back to an even shinier tone than it has left its production line in 1984.
Let me show you the before and after pictures.
Later I will replace the original exhaust (which acts as a 4in1 collector) with a custom made stainless steel 4in1 collector from a shop in England. The current owners grandpa was one of the first motorcycle sellers in England (click here to get to the BSK SpeedWorks page).
Extra Information:
If you plan to have a shorter exhaust system like I wanted for my final café racer then the exhaust manifolds must be shortened before welding the spring connection (that pulls the 4in1 collector towards the manifolds) onto one of the manifolds.
12. Finishing the exhaust system
The exhaust manifolds are now cut to the desired length. Time to weld hooks/tabs onto the exhaust parts to secure the system with extension springs. This makes mounting and disassembly so comfortable!
In the background you see my old VW T4 with its 350.000km. Sometime ago it was ready for the scrap yard but the beautiful memories I made with it gave me the motivation for a full restoration and to build a camper out of it (if you want to read more about the VW T4 project you can find it by clicking here).
Once the parts are relatively clean it is time for a management system. Cartons are a great way to order each system of the motorcycle separately. A café racer conversion will take time and over the course of building one might find her/himself wondering where some parts have gone or it became hard to remember where it belonged. Also make use of plastic bags and label them with waterproof pencils to find the right screws/parts in no time.
After I went through a unmanaged building phase of the old mofa (moped) of my grandpa when I was 14 years of age, I truly know how much time and nerves one can safe with a management system.
13. Restoring the Tank
The tank system will have to undergo restoration and modifications in order to assure correct working and an desired café racer design.
The BMW K100/K75 tank has 3 functional components in the tank itself:
The fuel pump
The fuel filter
The fuel sensor
The interior of my K100 tank looked pretty shocking to me, especially because I always thought that an aluminium tank would always stay in good condition throughout the years. Regarding this discovery I got curious if the saying that fuel became more aggressive/corrosive through increased environmental legislations had something to do with the unpleasant tank discovery.
So I found this study in the Energy Fuels Journal:
Gasolines containing higher amounts of bioethanol significantly influence corrosion of ferrous and nonferrous metals.
-Study of Corrosion of Metallic Materials in EthanolGasoline Blends
Energy Fuels Journal August 28, 2017
However high or low the bioethanol content, fuel is corrosive over time and some corrosion is to be expected. Much of the contaminant layer seen in the tank is from the degeneration of the fuel rubber tubes. It would be a wonder if 40 years submerged in fuel wouldn’t have caused some sediments of the rubber parts. Additionally does the fuel sensor not accurately show the fuel levels anymore and needs replacement. The fuel pump is also a common point of failure over the years so I decided to replace all parts.
If you look at the pictures you can see the before and after condition. All it takes is a long brush and dishwasher gel and about 2 hours of intense scrubbing. Afterwards I left the tank filled with the washing liquid and the next day I sprayed it through with a high pressure water steam jet. Of course there are traces of aluminium oxidation visible afterwards but that is no reason to concern in the case of my tank.
14. Tank Modifications and Paintwork
Remember what the characteristics that make a motorcycle into a café racer?
A very minimalistic design, originally coming from London (England) started by the ton-up boys designed to reduce weight and comfort to increase speed.
Therefore I will remove everything from the tank that make a more minimalistic design possible, with that I mean the original attachment points for the chassis. No chassis; no chassis attachment points! Any metal cutting tool would do the job here. I have found the Dremel tool especially valuable here. There are in total 4 places that have to be cut (see the picture).
Attention: Do not cut too deep into the welding line of the tank. I have heard of stories where the tank got a great paintwork (after chass. att. removal) and some days being filled with fuel the paint started to bubble up at the cut line through micro openings.
My recommendation:
Sand and spray the cuts then fill the tank with fuel and leave it be for 48h. If fuel would find a way through one of your cuts it would be fairly visible on the paint (see picture, no reaction on sprayed area meaning the tank can go to the painter)
15. Restoring the throttle body
The throttle body has surely seen some better days.
The vacuum 4 vacuum caps for each throttle body became brittle and cracky a clear indicator for false air. Additionally the inlet manifolds which give the air passage from the throttle body into the cylinder heads are hardened to a great extent (hardened old rubber parts are a potential source of false air).
What is false air?
Any air that leaks unmeasured into the engine system between the airflow meter and intake valves.
The throttle body is generally speaking a robust part and there is not much to mention here. However it is often said that one shouldn’t change the position of the synchronisation screws (noticeable through the blue paint on them), those screws were adjusted in the factory of BMW and without a comparative vacuum measuring device/ carburetor synchronizer. Do not play with the synchronisation unless needed.
The other parts can be disassembled for a polish/paintwork. You can see the results after my treatment (attached pictures).
16. The BMW K100 frame
The original BMW K100/K75 frame has two rather unfortunate characteristics that most certainly will be a thorn in the eye for the vary vast majority of café racer enthusiasts:
1: The overly long rear end
If the BMW K100 frame is seen apart from the bike it is rather difficult to imagine why such a long rear end would be necessary.
Why such an overly long frame rear end?
I added a picture with the chassis mounted to give a demonstration why the frame has such a long end: To create a strong base for the passenger seat; Topcase and side boxes.
The solution is to cut the frame rear almost 40cm shorter. Afterwards one has two options:
Option A: Buying a ready to weld in loop
Option B: Forming your own rear end
This opens a design and ergonomics question, every modification done on the bike will have a huge impact on the final result. I like to see the entire motorcycle like an organism and every part of it is like an organ and it is crucial to ensure the optima compatibility between each organ. Looking at each modification separately will no to yield in a wholesome design concept.
So I began to study the rear…
17. K100 Rear End Studies
Cutting the rear end is fairly easy. Make sure the cut line is clean, also you should check the regulations of your country on how far you are allowed to cut or if you are allowed to cut at all. In Germany (2022) a good orientation was until the last welding line (as seen in the picture).
I chose to study the bike and create accordingly angles for the rear end. I tried to consider all factors to come up with the angles shown in the 6 second video attached.
I recommend a 30mm tube, just as thick as the original frame and to create 3 individual pieces. Each of the 3 pieces will have to have a variety of specific angles to really mach the rest of the bike. The 2 horns seen in the video are not equally bend throughout their length, they start with a 28° angle and reduce by reaching the tail, also they are slightly twisted meaning they bend inwards and upwards trying to mimic the tanks prominent shapes. Also its length is exactly thought through to reach the needed height for the rear to elevate the entire design more towards racing visuals, however it had to be just short enough to emphasise the imposing volume of the engine. Creating a look that seems like one is riding on an engine rather than on a motorcycle.
18. K100 Rear End Welding
Once all 3 rear end pieces are carefully created it is time to weld it to the original BMW K100 frame. Do not forget to create a substructure to be placed at place of junction for increased strength rear end strength (see picture).
TIG welding is used to weld the rear end (in German we call it WIG-schweißen).
TIG = Tungsten Inert Gas. The tungsten electrode is non-consumable and produces the weld. An inert gas like argon or helium protects the welding area. Additionally we use a filler metal (hold in the left hand).
How does it work?
The welding machine uses a constant high electric current that conducts through the electrode to the metal while the used gas keeps contaminants out of the welding area creating consistent high quality welds.
19. K100 Rear End Results
The results are exactly like planned. Now we have a perfect foundation to build up upon. I transported the frame in my self-build VW T4 camper (click here for the VW T4 camper project).
Next step is hand shaping a custom carbon seat from a piece that I found in the trash container in front of a construction site.
But you remember that I mentioned before:
The original BMW K100/K75 frame has two rather unfortunate characteristics that most certainly will be a thorn in the eye for the vary vast majority of café racer enthusiasts.
Number 1 was the overly long rear end for which most find a solution without giving the road legislating engineer a headache and potentially risking the failure of road exam: Cut and attach a shorter rear.
But what about the second issue with the frame?
The broken back look of the original BMW K100 frame
This issue is a severe problem, a problem that would need an entire reconstruction of the frame makeup (which is exactly what some builders did outside of Germany). BUT in Germany the exam criteria to pass customisation is very strict and almost every modification must have official papers stating for its examined safety. Shortening the rear end will leave you with a one seater and it doesn’t change the stability and safety of the bike on a considerable level, therefore not a big issue. Lets dive deeper into it…
20. The K100 Broken Back Problem
2: The broken back visuals
The BMW K100 appeared to me being the full café racer package: reliable like a car, huge engine, brute appearance, ease of maintenance and cheap as a used scooter…
…IF THERE WOULDNT BE THE BROKEN BACK FRAME…
So I had to find a solution avoiding a complete reconstruction of the frame which would include having to hire an engineer for several thousands of Euros to calculate the stability of the new frame for the street legalisation exam.
So I had to play magician:
The solution: Making the broken back visually disappear without it physically disappearing.
—> The first of its kind: A floating frame.
The floating frame is as the name suggests a purely design functional extension of the original frame that has to be implemented without modifying the original frame in any kind (no holes, no cuts, nothing can be changed). I will later hand shape a carbon seat to adapt to its design, therefore giving it also a stability relevant function.
21. Rear & Floating Frame Metal Fairing
Time to create fairings out of metal for the rear end and the floating frame.
I bought a big metal sheet and cut fairings with an angle grinder out of it. Afterwards I use my body weight to bend the fairings accordingly. Fairings are necessary to protect the wiring harness and electric components that I will place later on the floating frame and below the tank. To safe space the Fuel injection unit will be placed on the floating frame and incorporated into the hand shaped carbon seat.
22. Handmade Carbon Seat/Rear
(Want to see my Handmade Carbon Seat project in detail? Click here for more information.)
I invested a lot of energy into finding the optimal café racer rear angles and in finding a never done before solution to change the emphasis of the frame constellation (the floating frame). Therefore I needed a seat-rear chassis-combination that would be worthy of being placed on it. So I drove to the biggest local motorcycle dealer and had a look at a variety of seat forms and rear chassis to understand the thoughts behind the leading motorcycle companies in building seats/chassis. I went through the entire spectrum from a Ducati Panigale to an Indian US military Scout to electro motorcycles. In making an in depth visit of motorcycle seats I came to the conclusion that factory produced parts lack in general the little extra mile to give it the “Woah-effect”. Producing a motorcycle or a vehicle in general is a wonder by itself, from digging out resources for production to the uncountable variety of manufacturing steps including all transportation steps and employees involved in all steps makes one question how something so elaborate can even be in reach of purchase. Therefore not every original supplied seat and rear end can be produced and engineered to its maximum potential. It would clearly affect sales and therefore the sustainability of the producing company. But apart from the logic reason why I do not find a “WOAH” seat and rear chassis on every bike in the store lets talk purely from the view of a someone who wants to create a perfect match for his K100 café racer project:
The most visual appealing seat regarding finding thoughts for my project had a Ducati Panigale V4.
The most unique seat had the Ducati Diavel, basically everything behind the tank became part of the seat cushion. It made me think of making a seat entirely of foam and giving the shaped form to a saddler, therefore avoiding having to hand make a rear chassis… But it would not have looked well, so I kept on looking for ideas.
The only café racer looking bike I found was the BMW NineT racer. It was interesting to see how BMW build a modern bike in a café racer style. Great! What better inspiration than from BMW itself! But the rear design looked forced compared to the rest of the bike. Its half oval shaped rear chassis didn’t compliment the tank nor the seat shape effectively. The narrow dark plastic corners on the plastic rear alienated it even more from the rest of the bike. The seat also appears to be relatively high in orientation to the tank.
After my visit I was still unsure how to proceed in building my own seat and chassis. And I slept a night over it…
23a. Creating a prototype
The day after my visit to the bike shop, where I caught a glimpse of modern motorcycle seats, I decided to create my own prototype. For the prototype, I required a soft and easily moldable material, which led me to use two old construction foam bottles. To avoid any contact between the foam and bike surfaces, I wrapped the bike in plastic.
I sprayed the contents of both bottles and created a big pile of foam. After waiting for 12 hours, I began shaping the foam. However, I soon realized that the foam was very unstable due to large air bubbles, which made it challenging to shape precisely. Despite this, the prototype served its purpose, giving me a better idea of how to shape the seat.
Now it was time to create the actual custom cafe racer seat.
23b. Turning Trash Into A Carbon Seat
I needed a denser foam and thought about insulation material, also known as “Styrodur”. So I drove to a nearby construction site and asked the construction owner if I could have a look into the waste container where I found a relative big block of scrap insulation. The construction site owner with a smile in his face saw me leaving with a piece. I arrived in my workshop and began shaping with a garden hacksaw a kitchen knife and an old metal file. This is when I heard the lyrics of Eminem in my mind:
If you had
One shot
Or one opportunity….
Would you capture it
Or just let it slip?-Eminem
I had literally one shot as I did have only this waste container treasure and no time for a second try. Shaping slowly, very carefully became my mantra, which I kept throughout almost 2 weeks. The entire shaping process you can find here (click).
After around 2 weeks I couldn’t believe my eyes, there was a real carbon fibre seat laying in front of me which complimented the bike design like in my dearest dreams.
24. Restoring Injectors
The injectors are also almost 40 years old. I always imagined injectors to create the injecting force themselves, but the truth is that they basically only have an open/close switch. Meaning the pressure is build up in the fuel line before reaching the injectors. When now the signal comes to open there is enough pressure present through the fuel lines that it creates the injecting force through the injectors and its spray nozzle to create a fine spray (for better combustion).
An Injector does not control the amount of injected fuel nor fuel flow nor timing of injection. An injector is nothing more than a on/off spray switch.
To restore the surface of the injector a thrill machine is of great help! Carefully insert the spray nozzle into the drilling machine and lock it into position. Now by the drill rotating at several thousands reps per minute one has to solely hold the sanding fleece against it. I did hold it for a bit longer to get a polished look. Careful if you polish the injectors: They will rust within a short period of time unless clear coated.
There are 4 small filters on the top of the injectors that need replacement after such a long time. Also the o rings and seals should be replaced.
My recommendation:
As the fuel was flowing for 40 years in one direction and the injector is just a on/off switch which doesn’t care about the direction of flow we can attach a fuel hose the the spray side. Now attach a 12V power source to the +/- of the injector and create a pressure at the end of the hose. With repeatedly creating an electric circuit the injectors open and close leaving fuel to flow out at the top. This way we make sure that the injectors get the chance to let go of any debris that could be stuck in the fine spray nozzles.
25. Restoring Starter
The starter needs an exterior and interior makeup.
Exterior: Sanding and paint or polish
Interior: Replacement of ball bearings, sealing and copper contacts.
Exterior: I sanded the exterior with fine grit and proceeded to 1000grit sanding fleece. The middle part of the starter is steel coated in some alloy. By polishing its the alloy gets removed and the steel is prone to rust. Either a good clear coat or yearly re-polishing is required. I loved the polished look and so I chose to re-polish from now and then.
Interior: The ball bearings were still in fairly good condition, however I want a project to be enjoyable for a long period of time after completion therefore I didn’t hesitate a second in fitting new bearings and seals. The copper contacts wear over time in any case I recommend replacing them after 40 years / 60.000km.
26. Swing Arm Axle Pin Bolt
No screw should be left untouched in a proper motorcycle build, or thats what I thought. So I beautified every screw with passion, the swing arm axle pin bolts were one of them.
Here I thought:
A picture is worth a thousend words.
So I was curious were does this saying come from?
Henrik Ibsen first said more than a hundred years ago: "A thousand words leave not the same deep impression as does a single deed” which was para-phrased into what we know now. Additionally I found this newspaper article from 1913 (see small picture).
27. The BMW K100 Clutch
The Clutch was fairly worn but still in good condition. But I found a leaky o-ring seal that is a common cause of an oily clutch in the BMW K100/75. The o-ring hardens over time reducing its sealing strength and letting engine oil into the clutch housing.
I have the engine already on the floor so I replace the entire clutch with a new one, place new seals and Staburags.
My recommendation:
Use a clutch disc centering tool to align the clutch disc while tightening the pressure plate (see picture).
28. The BMW K100 Oil/Water-Pump
A common point of needed maintenance is the two-part Oil/Water-pump of the BMW K100/K75. The anatomy of the pump is to distribute oil through a gear-type propellor and water through a propellor. Between both lays a seal that prevents oil and water from mixing when being driving by the same shaft. Over time the seal can start leaking, the fluids wont start mixing however because BMW engineered a space between both cavities that in case of leakage the leaking fluid can drain through a hole in the oil pan below the bike. Additionally over time the area around the seal starts wearing on the shaft surface worsening the leaks.
My BMW K100 had the same issue when I bought it, a liquid was coming out of the drainage hole between the oil and water pump. So I removed the Pump and upgraded the seal to the an upgraded version. I also replaced the shaft and placed an upgraded propellor that was no longer made from cast but from stamped steel that was introduced by BMW in 1990.
Furthermore the Pump housing was powder coated.
My recommendation:
Using only the BMW Drei Bond (Three bond) silicon sealer, this seals your engine parts very effectively when mounting and it is very easy to remove once maintenance is needed.
29. The BMW K100 Rims
The original rims can definitely be used for the conversion, the spokes pattern is quite interesting. There are some beauty mistakes (blemishes) from the casting process form the day they were produced. The following steps are needed to make them suitable for this project:
Removing visible casting lines
Placing new wheel bearings
Powder coating the rims
My recommendation:
Make sure to NOT powder coat the mounting surfaces! The powder coat layer is softer than metal and you do not want any powder coating on the mounting surfaces for your rims. I saw many coating also the mounting surfaces but I highly recommend not doing so for everyones safety.
30. The BMW K100 Fork Modifications
The BMW K100 was delivered with 3 different diameters of the fork pipes: 41.0mm, 41.3mm and 41.7mm. Make sure to measure your fork before ordering handle bars (in Germany if installing handle bars for the wrong fork diameter could result in loosing the insurance cover).
The first picture attached was after a thorough clean-up. What modifications need to be done to adapt the original BMW K100 fork to a café racer conversion?
The total fork length needs to be reduced. If you have a look at the attached a picture of the original BMW K100 you can see that the fork is so long that the oil pan is not running parallel to the floor surface. This is undesirable for a good looking café racer design. The engine needs to be lowered in the front by shortening the fork.
New groove must be milled into the fork pipes
The original fork springs are not progressive enough for the desired café racer driving style. Harder and more progressive fork pipes from Wirth will be fitted.
The plastic spacer inside the fork needs to be shortened in equal fashion as the fork pipes.
Powder coating
In total the fork was shortened around 70mm. The new groove was milled and I gave it to powder coat. After I installed the new fork springs with new seals the fork looked better than ever, ready for reassembly.
31. The Tuning Parts arrived
If you need parts for your K100/K75 or similar models I can recommend the following contacts:
www.kpartsholland.com; here you find every part that you need to have replaced. Thom has an entire warehouse full with K100/K75 parts. Additionally his repertoire for new upgrade parts increases with every year:
Tills.de; the probably biggest and most knowledgeable BMW K100/K75 supplier in Germany I know of.
https://www.instagram.com/fasermanufaktur/?hl=en; if you are looking for some unique handmade carbon parts for your BMW K100/75.
Please leave them a greeting from me (El Veterinario/Lukas Kiemer) in the purchase order if you were to order something there. They are all 3 super helpful and they stand for their products with all their heart.
32. The BMW K100 Electrics
The original BMW K100 cable harness is very elaborated. When the BMW K100 was produced it had most advanced electronics fitted and when I disassembled the bike I was surprised by the extent of the wiring.
The fuse box has the size of a car fuse box! Also the fuel injection module unit is of the same size one finds in a car. We are truly dealing with a car on 2 wheels here.
The big problem is that a car sized fuse box and a visible fuel injection module unit would not fit with the desired café racer look that I have in mind.
In short: all electronics need to disappear!
A café racer uses a minimalistic design, removing as much as possible to increase the chance of crossing the 100km/h mark, which was a big number back in the days. This BMW K100 with its 90 horse powers easily hits the 200km/h mark, for this project it is more about the café racer aesthetics than anything else. So we need to smartly hide all electronics below the already little chassis we have to work with due to the café racer design nature. Additionally I want to upgrade the electronics with some real impressive modern technology.
Stay tuned for whats about to come!
33. Modernising The Cable Harness
I always dreamed of having an old motorcycle that does have the electric functions of a newly produced high tech bike. So lets combine this café racer conversion with some high tech to reduce the voluminous technique from 1980.
In order to do that I found the m-unit blue from Motogadget. It is a digital control unit with 10 independent circuits. If any fault is detected the circuits are automatically switched off. Also I can connect via bluetooth to the unit and see live all currents of all circuits, allowing the unit to detect instantly if a lamp would have a fault. The break light can be modulated, so as the flashing lights. It allows GPS tracking as well. A great function is the sensor in the unit which acts as an alarm system by signalling the horn and flash lights while sending the place, time and if the vehicle was on the floor via Bluetooth to the Motogadget (MoRide) app. My favourite part is the Keyless-Go function, which switches the electrical-circuit of the motorcycle on when reaching the bike, allowing the ignition lock to be removed —> Creating a perfect café racer look.
The Mo.Unit Blue has several more functions and I would need an entire page to go through all of them. If you want to read more about the Motogadget Mo.Unit Blue click here.
34. Hiding the BMW K100 electronics
To create a strong base for all electrics I cut a metal plate and fixed it with 4 stainless steel clamps. Through the clamps the strict laws in Germany are obeyed by not modifying anything on the original frame. This to be electro baseplate is also used to secure the floating frame even stronger to the frame of the bike.
Time to place the first components of the modified wiring harness on it.
This approach enables all the electrical components to fit just beneath the tank, with some parts barely fitting within millimeters. It was quite a headache to install, but the effort was worth it for the exceptionally clean result. Check out the next step to view the electrical setup.
35. The Modernised K100 Harness
Several steps later (after powder coating and assembling the engine), I managed to complete work on the K100 harness. As you can see from the photos, a lot has changed. A whole box of wires is no longer on the bike, and a new computer is connected to all my push buttons on the motorcycle handlebars (clip-on handlebars).
Modernizing the electronics is one thing, but doing it both beautifully and professionally is another. Therefore, I recommend taking some extra time to lay each cable carefully and with a clear overview. This approach will enable you to quickly identify and resolve any issues in the future, rather than having to trace each cable to its destination. Also, consider the next owner of your bike. Someday, even if it takes 70 years, someone else will ride your bike. So, to make things easier for mechanics, yourself, and future owners, organize all cables well. Do things right, or better yet, don't start them at all.
36. Powder-Coating
After the frame and many other parts were ready for assembly, I needed to find a solution to give all surfaces a fresh shine. However, a fresh shine alone isn't sufficient to protect the mostly steel surfaces from rusting. Therefore, I visited the local powder coater and brought all the parts in.
But be cautious, it's not that simple! Your bike has very tight tolerances in some areas, meaning that parts are designed to fit perfectly into one another. The coated surface will increase in thickness since the powder isn't as thin as regular car paint, for example. Hence, it's crucial to prepare all surfaces accordingly. It's best to discuss with your powder coater before handing over your items. Make sure to specify which surfaces should “not” be coated. Some powder coaters will provide instructions and heat-resistant tape for you to apply yourself, helping to save some money on the order. You'll be surprised at how long it can take to perfectly prepare all objects. For a simple garden rail, you can coat the entire thing, but that's not the case with a complex machine like a motorbike.
For some parts, I fabricated pieces from thin metal sheets and mounted them to protect large openings. One of the attached pictures shows how I mark such a metal sheet using parts from the bike. Then, I cut along the markings and mount them to provide perfect protection. Remember, before powder coating, the operator will sandblast the objects, which can cause severe damage to delicate surfaces, such as areas where bearings are pressed in. Therefore, using mounted metal sheets offers the best protection. The only drawback is that preparing such custom-made metal sheet pieces can be quite time-consuming.
Another great option I plan to try for the next build is “Cerakote.” It is said to be as durable as powder coating but only adds a minimal fraction to the surface thickness compared to powder coating, making it much easier to assemble parts later on, especially if parts were coated that should have been avoided.
37. After the Powder Coating
After a few days, the powder coater called to inform me that the parts were ready for pickup. In the attached pictures, you can see how I began the assembly process. As mentioned earlier, the surface for the steering head bearing needs to be protected from the coating. In one of the pictures, I have already placed it in the newly coated frame.
This is an exciting step, as your old parts suddenly look like they're from a newly produced motorcycle.
I highly recommend powder coating for your project parts, but be aware that some post-treatment might be necessary.
38. Engine Assembly
Finally, it's time to reassemble the engine. Here too, parts like the water/oil pump and the oil pump were powder coated.
This, combined with the dry-ice blasted and subsequently polished engine, makes this old engine appear modern and up-to-date.
With that set and done, the engine is now updated as well.
Don't skip replacing old gaskets and sealants: But don't use anything other than the BMW engine sealant. It's called Drei Bond in German, so I guess it would be Three Bond in English. It comes in a small tube and is very easy to remove if you need to disassemble some parts later on. Some off-the-shelf sealants can cause major headaches when you try to remove them later. So, stick with the BMW-approved Drei Bond.
Also, it's advisable to upgrade the output shaft to the 12-rivet nut version, as it has proven to be more stable and reliable than the 8-rivet nut drive shaft used in older models, if I remember correctly. My model had the less stable drive shaft, so I ordered a replacement from a salvaged BMW K100 from Lithuania.
39. Merging Frame & Engine
The moment I've been waiting for so long has finally arrived:
Placing the frame on top of the engine.
This moment can only be described as a “Magical Moment”. Seeing all the parts come together and the structure of a motorcycle reemerging is simply beautiful.
Now, all electronics and parts can be connected. Ensure that you closely follow the BMW maintenance manual so you can tighten all screws to their exact Nm torque.
Work patiently, proceed slowly, and enjoy every second of this process. The goal is now within sight.
40. Test Run
My heart skipped a beat as I tried to turn the keys to start the engine... only to realize there was no keyhole. I had forgotten to install the ignition lock.
Just kidding!
This bike is keyless, and now my phone serves as the key. I simply have to be within a predetermined distance (which I set to 5 meters), and the bike is ready to start. Additionally, I installed the Mo.lock, which uses RFID technology (Radio Frequency Identification) for keyless unlocking in case my mobile phone dies. This means simply swiping a penny-sized chip close to the RFID device that I mounted through the air filter.
So, I activated Bluetooth on my phone and approached the bike. I pressed the button I installed on the clip-on handlebars, and the bike started within 3 seconds... I was ecstatic at that moment. All the effort was worth it.
But wait! The exhausts and my handmade carbon seat still needed to be placed to see if my design concept would create a visually pleasing and cohesive experience. Time for the final steps.
41. DIY Carbon Seat Meets BMW K100
Previously, as I outlined in an earlier step, I first had to create a prototype seat out of construction foam. The goal was to design a seat that was not only aesthetically pleasing but also ergonomically comfortable and would pass inspection. The process of creating the seat and covering it with real carbon fiber took me about a week of intense focus. And now, the moment of truth arrived: would it match the planned design?
You can see the detailed steps of crafting the carbon seat by clicking here (click).
The design of the seat is unprecedented in the world of the BMW K100. The combination of the subframe and the seat represents a novel approach for a BMW K100, which admittedly made me quite nervous.
So, without further ado, let's see if it fits (refer to pictures).
P.S.: The DIY carbon seat also required a cushion, so I crafted a carbon seat plate and handed it over to an upholsterer. She covered it with 2cm thick breast implant material and sewed it in BMW colors.
And the relief... it was exactly as I had envisioned it... Beautiful, to say the least.
42. Lithium Battery
As mentioned in the beginning of this page, a cafe racer is all about the minimalistic appearance of the bike, stripping off as much as possible. Why stop with the battery?
Not long ago, more and more bikers started adopting lithium batteries for their bikes for several reasons:
Lighter and smaller.
Much longer lifetime.
Really low discharge rate.
Maintenance-Free.
Better for the environment.
The biggest reason for the cafe racer bike's adoption of lithium batteries is their smaller size, which gives the bike an even better cafe racer look.
Why a low discharge rate is especially important for motorbikers:
We tend to store our bikes for the entire winter. If your battery loses too much stored energy, it can drain to such low levels that it becomes harmful to its chemistry, possibly even irreversibly damaging it. This makes the need for a replacement a common occurrence each spring. So, a lithium battery won't be nearly as drained after the winter as a standard lead-acid battery would be.
Have a look at the pictures and see the difference in size. It's unbelievable.
43. Final Results: The BMW K100 CR
Ready for the final reveal?
What a bike! The BMW K100, now in its café racer version, is absolutely beautiful. Its remarkably large engine, which earned it the nickname “Flying Brick,” is a key element that imparts a brutal and raw appearance to the café racer version.
This aligns with a truth I’ve discovered in life: Your biggest weakness can become your biggest asset. Often, the things we viewed as failures or hardships in the past end up providing us with experiences or understanding that propel us to unforeseen heights in the future. This means that if there is a problem in your life you wish would disappear, it might very well be the best thing that could have happened to you, looking back from a future perspective.
In Summary:
I find this bike to be the ideal Café Racer base. It combines so many attributes that make it such a special machine. Café racer or not, I tip my hat to all the engineers and talented people involved in creating such a complex machine. You all did a phenomenal job. Thank you.
Bonus #1: My Uncle Thomas
My uncle was the first to rush over and take a look at the BMW K100 as I rolled it in front of the house. He owns several motorcycles himself, but his passion lies with much older models. He's a big fan of BSA motorcycles, and his oldest bike is about 100 years old.
I captured the very first moment he saw the bike, and his enthusiasm was clearly evident, just as mine was on that day. So, I felt it was important to include him as a bonus in this story. I also want to thank him for lending me many tools during my build.
I believe that only someone who has also restored a motorcycle can truly understand the struggle and the amount of energy required to reach the final stage.
Bonus #2: K100 next to the Ducato
If you're interested in a different type of project, you might want to check out the conversion I undertook after completing the BMW K100. This time, it's a four-wheeled vehicle.
The DIY Fiat Ducato Camper
This project is particularly appealing to motorcycle enthusiasts and the like, as it offers the convenience of bringing a bed to events like motorcycle shows. But it's also great for leisure travel, allowing you to spend time with loved ones without the constraints of a schedule and hotel check-in and check-out times. So, I got my hands on a Fiat Ducato and transformed it into a self-sufficient camper.
I documented the entire Ducato camper conversion process in a video and also wrote an article about it. You can find it by clicking here (click).
Bonus #3: K100 next to the Mini-Camper
I would like to present to you yet another conversion: The VW Caddy Mini Camper.
This project, which I completed after the Ducato Camper Conversion, aimed to include everything a larger camper has, but within the form factor of a regular car. It's not the long version of a VW Caddy, but the standard Length 1, making it truly a small car. Its length is even shorter than a VW Passat, for example.
It was really an exciting project because the results show how much can fit into such a small space if organized efficiently. One of the features is a pull-out coffee machine that runs on 230V, produced inside the car.
You can have a look at the project by clicking here (click).
Additionally, I wrote a book for anyone who wants to build their own mini-camper. It is a comprehensive guide to help turn the dream of ultimate freedom into reality for all of us. If you own a car, why not make it your little private escape as well? You can find the book by clicking here (click).
This concludes this K100 cafe-racer article. I wish you many safe rides and, above all, a happy life.
Lukas Kiemer