I have at long last discovered that my front forks are Fichtel and Sachs (F&S).     There is a red plastic cover with an o-ring that fits into a small spring 20mm down the stanchion.  It seems to be adjustable with "rebound - fast slow" options.  I have looked through the "interweb" for break down diagrams of K75 forks and can't find anything like it. At least one of these is broken, the one in the photo is the best one.  I would like to get a new set.  Any ideas anyone?

Red plastic thing

Never seen one. Could be an after market bit.

I think that Fichtel and Sachs is the key here. I had never heard of them before but it seems they supplied front forks to BM for the K series for a while.

Hi,  The F&S forks were certainly supplied on K-bike models from the beginning of the line in 1983 until they were replaced by Showa forks around 1988-1989.  See this table.

What Rick is saying is that particular red plastic part is not a standard OEM part supplied on all F&S equipped K-bike models.  It must therefore be a part from a 3rd party supplier of performance kits for this fork.  You'll have to research what upgrades were applied to your bike by previous owners.

May be back to square one. But perhaps someone knows what goes in there if the red plastic thingee doesn't?

Damage wrote:There is a red plastic cover with an o-ring that fits into a small spring 20mm down the stanchion. . . . what goes in there if the red plastic thingee doesn't?

What is below the spring that holds the red plastic cover thingee?

Can you identify which part of the fork assembly this red plastic cylinder engages with? That way we might be able to guess as to what purpose it serves?

Damper Assembly:


Stanchion assembly:

Hi Damage, just to add some more "help" to the mix.

My K75 is a Feb 90 build and has the "S" type forks fitted that I believe were an option on the non-faired '90 K75 and would probably have been on your '89 K75S from the factory. They are the ones at the top of the table linked to in Robmac's post 4 of this thread as they have "Brembo" cast into the sliders.

The distinguishing marks are the "S" stamped into the fork cap.


and the rib down the front of the slider. I don't think this rib was on the later Showa forks and not sure whether it was on the F&S forks found on the early K100.


When I was doing my renovation on my K75 I disassembled the forks and the springs and spacer arrangement was different in each leg however with help from the forum (Inge, I believe from memory) I was shown the correct layout from the factory as in the photo below.


Another note if you have these forks is that the damper rod is different in the left leg to the right. They also take a different amount of oil to the K100 forks - these are 280ml each side rather than 330ml I think is spec'ed for the K100.

Hoping this info is help rather than confusion.

Cheers

Gaz wrote:Hi Damage, just to add some more "help" to the mix.

My K75 is a Feb 90 build and has the "S" type forks fitted that I believe were an option on the non-faired '90 K75 and would probably have been on your '89 K75S from the factory. They are the ones at the top of the table linked to in Robmac's post 4 of this thread as they have "Brembo" cast into the sliders.

The distinguishing marks are the "S" stamped into the fork cap.


and the rib down the front of the slider. I don't think this rib was on the later Showa forks and not sure whether it was on the F&S forks found on the early K100.


When I was doing my renovation on my K75 I disassembled the forks and the springs and spacer arrangement was different in each leg however with help from the forum (Inge, I believe from memory) I was shown the correct layout from the factory as in the photo below.


Another note if you have these forks is that the damper rod is different in the left leg to the right. They also take a different amount of oil to the K100 forks - these are 280ml each side rather than 330ml I think is spec'ed for the K100.

Hoping this info is help rather than confusion.

Cheers

Gaz

You have almost nailed it I think. On the photo that shows the Allen head screws in the fork stanchions, mine are 20 mm below this.  There is no S mark. 

I should have mentioned that there is not an allen key 20mm below the surface but some type of spring that changes the preload.  Also that the only way of getting oil into them seems to be by taking off the bottom fork and putting it in there.

Laitch wrote:

Damage wrote:There is a red plastic cover with an o-ring that fits into a small spring 20mm down the stanchion. . . . what goes in there if the red plastic thingee doesn't?

What is below the spring that holds the red plastic cover thingee?

Laitch

I haven't been able to extract that.

hmm.... looks to me to be another type of valve , perhaps intended to modify the rebound of the fork leg

from what I can see so far , the red plastic bit locates into a clip type arrangement  and below a spring loaded valve of sorts into the leg proper ?


the top nut removed still shows the ring retainer around the fork cap , so im guessing pretty much after market gear , rather than anything "factory "

perhaps removal of the cap ...(good luck prying that ring clip out , but should be perdy easy if you can hold the tube properly and push down on the cap against the preloaded spring internal )

you might even find some kind of brand manufacture marks on the cap  once removed  giving more clues to look it up ?
interesting none the less , would love to see who made it and why ..?

good luck !

Hi Damage, me again! Been thinking about your forks and you need to get the innards out to see what you have actually got in there.

On my K75 forks to disassemble them you have to push down on the cap against the springs inside and hold it down while you get a fine tool, like a dentist pick, to remove the metal snap ring (item 6 in robmac's stanchion parts view in post 7) that holds the fork cap in place. Once the snap ring is removed the cap will pop up (hopefully) under pressure from the spring so that you can take it out. The cap has an o ring on it (item 4 in that same parts view) to seal against the inside surface of the fork tube.

I made up this tool to be able to hold the cap down. It clips under the lower triple clamp and I just put a cable tie around it near the lower clamp to hold it closed and allow me to wind down the threaded rod. You might need to put a socket on top of your fork cap for the threaded rod to push against.


In one of your photos with the top ring unscrewed the snap ring might be visible where I have placed the red arrow.


Good luck.

Cheers

I have to pull them apart to change the oil since there is no way to put oil in from the top, so I have had that off. Bad juju.

Should be a small drain plug #7 & 8 in this diagram and be VERY gentle it is easy to strip the thread

Rick,  Getting oil in is the issue. On the R100 and and every other bike I have ever owned there is the ability to refill the forks by removing a bolt or cap or something like that on the top of the fork stanchion.

Although the red plastic thingee is removable, there is no hole that continues through so that fork oil can be put in. Last time I changed the oil I drained out of of 7 & 8 from your diagram. I expected to find something like 16 & 2 fitting into 5 so that I could add oil to the forks. Instead the red cap thingee goes into 5 and there is no hole in 5 that lets me put oil into the forks. The only way I can get oil in is to undo 5 and pull the entire spring out and dribble oil slowly past the springs.