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  • Writer's pictureKai

BMW K100 RS Caferacer project

Updated: Apr 21, 2020

Hi everyone,

In this blog I'll show you how to give a whole new life to an old motorcycle that deserve to be taken care of, and probably save it from ending in a junkyard. How? By turning it into a Cafe Racer!

Let´s start from the beginning.

The donour bike is a 1985 K100 RS in perfect state. When I picked it up it only had 29.000km on the clock.

Step one. Disassemble all body parts. I bet the bike already lost 25 kilograms by just removing all that stuff.

The result after day one.

After the speedo is gone and the first cut was made on the tail 😱 it’s now time digging into the lines. It’s very important understanding how the current lines find or don’t find themselves so I get a better understanding where I need to adopt/ changing them to achieve my vision of how the bike shall look in future. The yellow lines (tank) will need finding the red ones (frame) going forward one or the other way.

The BEP 3.0 had initially been developed for those who were looking to replace the original cockpit (also labeled „Brotbox“ in German-> “Lunch Box”) of their K75, K100, K1000 or K1 bike with a speedo of a third-party manufacturer. And everything done completely stress-free without soldering and tangled wires. No alterations to the wiring harness or modifications of the vehicle are required. Simply plug in and off you go!, fully universal   interface fits to all connectors ! (round/square)

First test after replacing the speedo with BEP 3.0

Work in progess...

Day one on my fork mockup for the K100. The Original front wheel carries a telescopic fork with 41.3 mm standpipe diameter, which is a big disadvantage compared to a fork with 41.0 diameter. You simply don´t get cool clamps in 41.3.

I decided swapping against a R100 R fork. Et voila, the 43.0mm stub handlebars fit. 🤓

And so we are starting the fork adoption from BMW R100 R.

Weekend so let´s swap the fork.Two different fork manufactures are used on BMW K models: Showa with an outer upper tube diameter of 1.612 in (40.9 mm) and Fichtel and Sachs measuring 1.627 in (41.3 mm). The new Showa is already waiting in the background.

Mock-up done!

That is how a K100 tank looks from the inside.

Schrott or not? I am going to sell the used break discs and go for new ones. There´s however maybe still some pocket money in it for me if the discs diameter is not below 3.6mm. The tool addict I am my collection also contains a micrometer. The discs still have some good 4.1mm left.

Over one kilo of solid BSK Speedworks exhaust, beautiful sound and quality.

The mounted exhaust is coming along great!

I´m waiting for the weekend ahead to continue working on the bike.

Reorganised my workshop the other day. The K now is on the ramp. I’m now waiting for my tail since 2 months...

From a 3D PC design to reality. The rear mudguard was printed with ABS plus. ABS plus is much more robust against heat and impacts as you would expect it for a fender and the extra strut in the middle even improves torsional strength. There´s however a small challenge when it comes to proper mounting at the bike.

The original plan was using two mount points right over the swing arm at the gear box. Just it turned out that a) after removing the spring nuts there´s no threads in the holes and b) the fender would sit too close towards the wheel and swing arm. See picture below.

Time for plan B. A complete new mount using exiting threads from the battery holder. Made from stainless steel, the new mount accomplishes a high quality look, increases overall stabilty and moves the guard away from the wheel and swing arm.

Job done!

Some rim porn for you.

“The first cut is the deepest, baby, I know “. Cutting the back of the frame, but only roughly. Better a step by step process rather than cutting away too much.

Garage life scene.

Adopting the new tail. unit As it stands the original tank mounts need being reconstructed.

A new rear brake disc was a must have. Prefering the look of the perforated disc with the black center in combination with my overhauled brakes.

Time for a tiny wedding. Mrs. Tire and Mr. Rim do themselves the honour.

Talking to other builders across the world, one of the works a lot of them are concerned of is modifying an original wiring harness. From my point of view it´s not my most prefered works but unfortunetly necessary. Apart from removing overhead connectors it also has a pure athetic reason; if you look at the harness, it´s 30 years old, dirty and the tape is so dry it almost goes off by itself.

In my case as I will go for BEP 3 and keep the original harness the task was only removing 2 overhead connectors from the fuse box. I white round plug for bulb testing and a black big plug for additional electric gear like navigation systems. Just cutting off the cables is howver NOT an option. You´re in danger creating a shortcut if you leave open cables in the harness.

To be on the very safe side measure the circuit/cables end to end to make sure you´ve really catched the right cable. A multimeter does the job here. Once the overhead cables are completely removed start wrapping the harness back, starting at the very end points. (The smaller ends of the harness)

I´m using Tesa 51608 tape often used by the German car industrie for harnesses. It´s heat resitant, doesn´t glue rather than stick and is easily removable. The end result can look like this:

A sunday early morning greeting from my garage.

When it´s spring.

Tip: I see a lot of people close engines only using fabric plugged into the holes. Just if you intend using high pressure abrasives that doesn’t really work well. In this case here I just leave the intake manifolds attached and use the clamps to attach wood discs.

Note: that’s exactly why manhole covers are round. If the have the exact diameter of the holes they can’t fall in.

While I talked and showed off my works on the switch gear, regulator and starter engine in seperate blog posts here, there is also progress on the tank and seat alignment. So this is not the original tank. Unfortunetly it´s one of the tank models with a single bolt connector sitting on a bridge, which is prone to brake. The one below has two connection points underneath the tank held by clips.

Changing the tank is the only way getting the tail perfectly alligned, but it also means modifying the connection points on the frame. Remove the old ones and get new ones welded to it.

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6 commenti

Where can I buy one of those rear fenders? It's exactly what I'm looking for, for my build.

Mi piace

Hello Kai,

I love what you wrote and I would like to ask you a quest.

Im planning to buy an k 75, I actually must re wire all the connection or I can use the original one with new meter and change them in a second time?

Thanks in advance


Mi piace
22 mar 2021
Risposta a

Yes, it’s plug n play. Also most Acewell Speedometer support the BEP. Just Google.

Mi piace

Hallo Kai,

mit Interesse verfolge ich den K100 Umbau. Meinen K100 Umbau habe ich letztes Jahr verkauft. Die Sache mit den K100 wird mir zu sehr Mainstream, sorry. Was ich sehr interessant finde ist das Bild mit dem aufgeschnittenen Tank. Wenn Du schon so weit gegangen bist, könntest Du ja mal das tun was sich bisher noch niemand getraut hat, und zwar den Tank im hinteren Bereich so weit nach unten zu senken, dass die untere Tanklinie nicht mehr so steil nach oben geht. Das würde die Linie des ganzen Motorrades verbessern, vor allem den blöden Knick im Heckrahmen. Das abfallende Rahmenrohr unterm Sitz ließe sich somit auch besser kaschieren. Oder hast Du vielleicht genau das vor?

Viele Grüße,


Mi piace
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