Front Fork Rebuild

[fermin]

Decided over the rainy weekend to rebuild the front forks with parts that came w/ the bike when it was purchased (seals, gaiters, stanchions).  After reviewing some posts I noticed that it is usual to have some rubber parts disintegrated and not present.  This is evident with the rubber ring (part#24) not present in the shock assembly (replacemnt at the lower right). However I couldnt find the shim (#20).  It should be a metal part, is it optional?

I plan to install gaiters and from other posts I have gathered the following which I would like to confirm:
1. No need to install the felt ring if I am using gaiters
2. Gaiter vent hole on top
3. Springs were measured to tolerances (19.25 to 19.75 in) I am trying not to replace these, but throwing myself at the mercy of the forum,  I have  knowingly omitted suggested repairs only to find myself doing the work a few months later.
4. O ring at the top of the spring retainer appears to be OK and I dont have top leaks, does it need replacing.
4. Pitting on the stanchion left some slight grooves on the top of the slider do I need even these out with fine sandpaper?. I am replacing the stanchions and the seal seating area is OK.

As always the help is truly appreciated

[Bob_Roller]

Been about a year since I had the damper out of my bike, but I believe the part you are looking for is at the bottom of the damper, not the top .

[Barry]

fermin,

The diagram shows an early version of the fork damper valve. Not sure what year your bike is but there were several changes over the first few years and the history went something like this.

The very first R65 forks did not have shim #20. The shim was only introduced after complaints of the valve body rattling in the stanchion. The first factory mod was to fit that shim which came as far as I remember in 3 different sizes.  If your valve body has any free movement in the stanchion once the circlip is fitted then you need to shim it.

The next factory mod was to fit a spring loaded washer and shorter length valve body which eliminated the need for a shim altogether. If that's what you have then forget about the shim as you don't need it.

The final version was a much shorter valve body with an actual coil spring underneath it. again no shim is needed. Some of the online fiche diagrams show this final arrangement. The diagram for an R80ST which has very similar forks certainly shows it. I've never seen a diagram that illustrated the intermediate arrangement though.

Now that I have seen your photo which took a while to load I think you may have the intermediate spring load washer version although I don't see the washer and circlip in the picture. The thick plastic valve washer tells me it's certainly not the earliest type although later parts could have been retrofitted including a valve body with a deeper recess for the valve washer as the early valve washer was much thinner and made of different material, just a thin steel washer in fact.

Whatever version of the forks you have the bottom line is there should be no axial play once the valve body is secured in the stanchion.

[fermin]

Thanks Barry & Bob, I think I have the spring loaded version, attached is a photo of what the absorber looked like once cleaned and removed form the stanchion.  The ptroblem is that I thought the black (bumper?) ring seen in the photo between the absorber and the stanchion had disintegrated and would be located between the 'holey' washer and the top of the absorber.  However, if installed in this position the valve housing portrudes outside the stanchion.  The white plastic washer in the photo is recessed inside the the valve housing.  So I am thinking that if this bumper if required would have to go to the right or the left of the spring, theres just no room for it anywhere else.

[Barry]

That spring is not the one I was referring to. Looks to me like an aftermarket anti bottoming spring which was never installed in stock forks.

The black topping out bush goes between the perforated valve plate and the damper piston. It is or should be a smaller diameter than the valve body so you push it up beyond the valve body recess into the stanchion proper. In other words the bush should be the same diameter as the main internal bore of the stanchion. Yours looks a touch big to me which would explain your problem. If you need the exact dimensions I have them at home and can post them tonight.

As for what version of damper valve you have. The circlip at least is missing from the photo. If there are no other parts missing then you seem to have a customised valve body. Either there were even more variants than I know about or someone has machined out the valve washer recess in an early type valve body to accept the later type of thick plastic valve washer. Actually that's a good thing if done well. The valve washer should have a bevelled edge on one side which goes to the the top i.e. towards the valve plate so as not to block off oil flow through a portion of the holes.

Edit: The topping bush should measure 28mm OD  18mm ID 8mm thick.

[fermin]

Thanks Barry.  A customized front fork....  and I thought these parts had not seen the light of day since 1983.  I tried to get the bumper in the new stanchion but was not able to get the damper piston in with it. After 4 or five tries I gave up and tried without the bumper, the pistion went in but it wasnt easy... Correct that the circlip was not included in the photo.

As I understand this bumper ring based on its location moves inside the stanchion. The new stanchions were included with the bike, they are not OEM and have laser markings  "forking by frank" the internal diameter (on te top end ) is about 27.5 mm, so they appear to be about 1 mm too large.  The bumpers were also included with the bike and probably date to late 1990s based on some of the receipts for the parts included with the purchase.  Is it possible that they have deteriorated/expanded? They are not brittle.

I have seen some posts with aftermarket bumpers red or translucent, I'll try to find the diameter of these and see if they differ

[Barry]

Looks like the topping out bushes expanded a bit or were made over size. If I remember the new bumpers I bought from motobins were a tight fit in the stanchion but they did go in. I reasoned that the bush should be a free sliding fit other wise they would restrict oil flow coming up through the valve plate. I "turned them down" for a better fit.

I suppose it's possible your replacement stanchions also have a slightly undersized bore. That might explain why the damper piston was a tight fit.

[fermin]

I think  Ill order a new set of bumpers from Max, compare to the ones I have and if the same I'll grind them down a tad.  I am assuming the stanchions are OK, the piston slides in & out perfectly.

Thanks

[fermin]

I forgot, I am inserting the piston from the bottom end, however I think I saw an older post where it was being inserted rod end from the top of the stanchion.  Any difference?.  I have not tried the top route yet.

[Bob_Roller]

The piston ring like seal on the upper part of the damper can get hung up on the lip at the bottom of the inner fork tube .

If it does, try it from the top, I think the groove that holds the wire snap ring helps compress the ring to ease installation, I had problems getting mine back together, until I tried it .

[fermin]

Well, I had a talk w/ Rusty at Max BMW and he indicated that I could leave that part out alltogether.  As far as Ive noticed, other than the leaking seals the front shocks were working OK other than a slight clunk when going over a sharp bump at speed. I think Ill try this and see what happens.  
Other than removing the forks from the triple clamps (the rust spots created a lot of friction and had to use a heavy hammer, and wood blocks , opening the triple clamp with a wedge and finally heating the clamp to slide the stanchions out) the whole operation was painless.  The new stanchions glide over the clamps.

[Barry]

I had a talk w/ Rusty at Max BMW and he indicated that I could leave that part out alltogether.

I suppose you can try it but consider the function of the topping out bush which is to cushion the shock of the forks reaching full extension. Besides the fact that the forks may clunk over sharp bumps I have heard of one instance where the piston was sheared off the end of the damper rod. Whether they clunk or not will depend on the efficiency of the rebound damping and viscosity of oil used. If they do clunk the noise is the piston striking the valve plate in the manner of a slide hammer so you can understand how it might be possible to shear the piston off.

The other small but probably insignificant effect will be that the forks are approx. 8mm longer at full extension which will reduce the clearance when on the centre stand.

[fermin]

Thanks again Barry.  I am attaching the two service bulletins provided by Rusty at MaxBMW.  Though they do not seem to apply to my problem they may help someone else down the line.  I ended up using a thick o ring from my plumbing toolkit, it fit just right though it probably will not last as long.  This is something you should only attempt with farm implements.