A quick look at Siebenrock pistons for a R75/5

These piston and cylinder kits seem to be the rage right now. I used one on the last build and had very little problems aside from the pushrod seals being extremely difficult to install.
The pistons are made to mate with the R75/5 heads and you can use the same carbs (might need to change jetting).

Here is a quick look at the piston and how it compares to a stock R75/5 piston that i have lying around.


It is a lighter, yet wider piston. The piston on the right has the rings on it still. The siebenrock doesn’t have the rings on it.


I didn’t measure heights when i snapped this photo but they seem close.

The piston in the cylinder. The cylinder wall is very thin compared to a R75/5 cylinder.

I haven’t heard of any issues with these yet. Supposedly they give you a 20% gain in power… so that makes it around 900cc.
I’m pretty sure they are cheaper then replacement BMW parts too.

Race Tech Gold Valve Emulators on a BMW R75/5

Phew. After racking my brain and tearing the front forks apart a few dozen times, I got these all figured out. Hopefully this blog post will help others to clarify and not make the same trial and errors I have made. I can now say I am an expert at the innards of the R75/5 fork system. I’m not an expert with the Gold Valve emulators but I sure learned A LOT!

First off, Read this from RaceTech.com – BMW Gold Valve instructions
And use this as a reference as well:
Emulator Tuning Guide
For information on how the emulators work, please read through this link.
Dampening Rod Forks and Emulators
For instructions and diagrams of a BMW /5 fork leg, check out an old Beemers and Bits link:

When you buy the emulators for a early 70’s airhead, ask for a set of sliver emulator springs (more on that later).

The Race Tech BMW page talks about how to modify the fork’s internal dampener rod and piston.
Make sure you remove the spring and check ball at the bottom of the rod.

Here is a shot of the eight dampener rod holes being increased to 1/4″

I was initially too conservative with the modifications. Based on my initial readings, I thought by simply drilling a hole into the top of the piston and shaving down the hex head a bit would suffice. I wanted to leave part of the hex head so I could have something to wrench on if the dampener rod ever needs disassembly.

It turned out that I needed to chop the entire top of the piston off to increase oil flow. You want to encourage ample oil flow through the piston and dampener rod.

After you modified your dampener rod/piston, be sure to clean out the metal shavings. Use a pipe cleaner, air compressor, etc… get it clean!

Next came emulator modifications. You have to customize the emulators for BMW use.
First take the emulators apart and you will notice that under the spring, there is a small top hat like plate with 1 or two holes and markings for a total of 4 holes. Get a small drill bit and drill out the remaining holes so you have a total of 4. This will help low speed fork response over road bumps and such.

Hopefully you were sent a set of silver springs to use on your emulator. I recommend the silver springs because they aren’t as stiff. The springs will control high speed fork movement and how fast the valve will open to allow oil flow.
When I reassembled the emulator, I got the spring to a point where it was snug between the washer/bolt head and the emulator body. I could still spin the spring but I couldn’t wiggle it up and down. This was my ‘ZERO’ point.
After that is when I made my preload turns. I did 3 full preload turns on the silver spring.

I got a foot of 3/4″ PVC pipe from the local hardware store. goldValve_1030198
You will want to cut two identical lengths of PVC about 40mm in length to sit on top of the piston and under the emulator.


Now is a good time to check and make sure your forks are aligned properly. I recommend Duane Ausherman’s web site regarding fork stiction and alignment http://w6rec.com/duane/bmw/forks.htm
I also recommend getting a fork rebuild kit with all the necessary seals, washers and such.
Here is a fork seal replacement tip: http://beemersandbits.com/2012/07/fork-seal-install/

Assemble your fork lowers and dampener rod on the ALIGNED fork legs with all the new seals and rubber rings, washers etc…

After your dampener rod has been installed in the fork tube and your fork lowers have been attached, then insert the 40mm PVC spacer to sit above the dampener rod piston.
Then install your Gold Valve Emulator on top of the PVC spacer with the spring side pointing upwards.

Oil level comes into play at this point.
BMW recommends between 5 – 10 wt on a stock set up. Do not use the BMW recommended weight.
Because of the dampener rod modifications, you have to start with a minimum of 15wt fork oil. I had to buy my 15wt fork oil at the local Harley Davidson dealer because the other motorcycle shops only had up to 10wt.
Race Tech recommends 150mm oil level. I beat my head against the wall trying to figure out what that meant or how it equated to CCs.
After a conversation with Race Tech’s Matt Wiley, he clarified it for me.
You need to compress the forks fully. See Photo:IMG_1930
With the forks raised as high as they can go, you add your fork oil until the oil level is 150mm from the top of the fork tube.
This equated to 275cc of fork oil in each leg. (A stock BMW setup requires 250cc).
Other BMW models might differ so it is recommended that you measure by compressing your forks and taking note of how many CCs it takes to get 150mm from the top.
NOTE! Fork Oil Viscosity varies between brands! So use same brand of oil when changing oil…

After adding the oil, you can lower your legs to the full extension.

Now comes the theory about springs. The BMW or Progressive springs will NOT work with the Gold Valve emulators. They are much too soft and spongy… and importantly, too long! The Gold Valve takes up 55mm space in the fork tube so you will need shorter springs as a result.
Race Tech recommends using their springs… which is what I currently have installed. Before your order the springs, make sure you buy the proper springs that are recommended for the rider’s weight.
The Race Tech springs appear too short. Not to worry because they recommend a large PVC spacer at the top of the forks between the spring and the upper fork caps. IMG_1932
PVC Spacer should come just to top of chrome fork tube with forks fully extended.
15-20mm preload is recommended. When PVC spring spacer is just to top of chrome tube you will be in that range.

Unfortunately, the bike now belongs to the owner who commissioned me to build it. If I still owned it, I would love to experiment with a set of custom Sonic Springs. This was written by a fellow BMW rider by the name of Craig : ” I had Sonic Springs wind springs for me of the proper length and rate for the emulators. Instead of the race tech 425mm spring at a .8kg/mm rate, Sonic springs wound me an 0.65kg/mm spring at a length of 485mm. This way, the ride is stiffer (but not ungodly stiff), the bike rides a bit higher in the stroke, and it eats bumps like crazy. Stock springs are about 530 mm long, but if you use an emulator, it consumes about 55 mm of space inside the fork. The shorter spring accounts for the reduction in space available when the emulator is installed. “
If I was keeping the bike, i would gamble the $100-$150 on trying the Sonic Springs. The logic behind the idea makes sense to me.

So to sum up:
I used the silver spring with a preload of 3 turns
I modified the valve plate bleed holes to 4 holes.
275cc of 15wt fork oil
and Race Tech springs with a large PVC spacer.

Lastly, to check everything out, i measured my Sag amount. The sag is measured by taking note of where the forks settle when the bike is on the ground without a rider. If getting help is an issue, use a zip tie on the fork tube and a stick. With the bike off the center stand and you off the bike, push on bars and let the bike rise. Then use the stick to push the zip tie flush with the fork seal cover. Then put it on the center stand without disturbing the position of the zip tie. Then measure the gap between the zip tie and the seal with the fork fully extended. Then do the same thing with you on the bike. Might take a couple of tries before you get it on the stand without moving the zip tie.
Without a rider, the sag is 20mm.
Once I sat on the bike, they compressed 35mm.
Those are the specs I was to achieve for and they are dead on.

My review of the set up?

The bike is a canyon killer. I hit high and low speed canyon turns and the bike handled amazing. No dipping or diving. Zero dive when braking. No springy pop when accelerating. It would glide in and out of turns and keep you glued to the road. I love it.
On the horrendous Los Angeles roads… the bike handles 70% of the potholes and bumps OK. But I can’t help but notice that at any speed, fast or slow, sometimes the front forks hit the bumps a bit too hard for my taste. Also when on the rain-groove freeways the bike can get a bit bouncy.
There is a driveway lip that I ride over going to my home and for some reason that one lip was a harsh hit every time I hit it. It was if the front forks weren’t working at all. However, according to Matt at Race Tech, that is always an issue and if the suspension is great everywhere else, then just deal with it. Which is what I did.

I have a very similar R75/5 with stock BMW fork set up and Progressive springs. The bike handles the bumps in town very well however after experiencing the Gold Valve set up, I can’t help but notice that the bike feels much too squishy when braking and hitting the twisty roads. I think I will be adding a gold valve set up to that bike sometime in the future.

Feel free to contact me with any questions.

Let the wiring begin

I can’t be happier to have the bodywork back (and my garage back too!). I can finally start putting some overdue time in on the bike. The zen of wiring begins. A part of the build i actually enjoy problem solving.


Instead of the old, original key switch, we are upgrading to a Rocky Point key ignition. The first step is installing the ‘doughnut’ retainer. You use the same bend-tabs for the ignition board but you bend the tabs on the giant washer instead. This is much easier to install when the headlight is off the bike.


I then installed the chrome key housing cover. Installing the small spring on the black sliding cover was a bit tricky. That black slider needs to weave into the chrome cover and then you have to wrap the spring around the round tab that sticks out of the center of the headlight bucket. Again, another item easier to install when the headlight is off the bike.
Bending the tabs for the chrome cover is really tricky. An assortment of needle-nose pliers helps.


The speedometer is simple to install. Don’t forget the rubber gasket that goes between the headlight and the speedometer.
The orange turn signal indicator is simple to install as well.


Don’t forget to thread the wires through the handlebar switch support piece before you install it into the headlight bucket.


And so it begins….


Transmission Install

A Transmission install is actually not difficult.

Step #1 is making sure the swingarm is unmounted from the frame. It can remain connected to the shocks.

I then took caution to protect the frame:

Protection. Notice the rags too:

Time to prep the transmission. It helps if the shifter and the clutch arm are removed. In my case, i removed the shifter but left the clutch arm mounted:

Time to lube the splines. Make sure the splines are clean of dirt and grime. I use a BMW moly grease. It is sticky stuff. :

Keeping the swingarm and rear wheel pulled back, gently lift the transmission into place. It may take a small wiggle here and there to get the splines to mate with the engine.

Make sure the top right bolt of the engine housing aligns properly with the transmission housing as you mate the splines into the clutch:

Time to tighten everything down.
You need a M8 nut and washer for the top right engine stud bolt.
a M8 x 40mm bolt and washer for the top left mount. (Note, i am not installing the BMW airbox. If i was, there is a retaining clip that mounts to the top left bolt along with a washer intended for the airbox).
a M8 X 85mm bolt, washer and nut for the bottom right mount
and a M8 x 40mm bolt and washer for the bottom left mount.
A 6mm hex socket and wrench does the trick along with a 13mm wrench/socket.

Lastly before you mount the swingarm, Make sure you attach the driveshaft boot. Notice the different ends of the boot. The round end is for the transmission, the oval/rectangular end is for the drive shaft.

Next step… aligning and mounting the swing-arm… stay tuned!

Swing arm race and bearing install

Installation of the swing arm bearings.

Here is the order of the parts. The first part is a cup that only goes on the side of the swingarm opposite of the drive shaft. All it does is prevents grease from traveling into the crossmember of the swingarm. The other side does not require one.

First install that cup/plate into the swingarm and make sure it sits flush on the small lip.

I stuck the races in the freezer with hopes they would shrink enough to make the instal a bit easier.
The races can be driven in by hammering them in with a 30mm socket or if you have the Cycleworks race puller, the aluminum plate is also meant to be used as a driver.

Here is a 30mm socket (i know, the socket is upside down. I threw it in there for the photo):

Here is the Cycleworks plate being used as a drift:

Race installed:

Greased up bearing (be sure to pack it well with grease):

Then goes the collar with the flat side down, against the bearing:

Then goes the dust seal:

I prefer to install the collar through the rear of the dust seal, and then insert the dust seal into the swinarm:

One side done:

The process is exactly the same for the other side except for the grease protection cup.