Brake Cam O-Rings

[Mike V]

I've noticed I'm getting a bit of weeping at my final drive. My suspicion is it's the O-rings on the brake cam letting some fluid pass by. Just a suspicion at this point since I haven't disassembled the rear brake assembly. But I've always been curious about this cam and its grooves regarding the O-rings.  My cam has four grooves and when originally going into this area during my restoration there were only two O-rings. Doing some research last night after ordering new O-rings the only place I could find information was on Bum's web site and a search here on the forum. Some cams have two grooves, mine has four. I saw some historic threads here mentioning some of you placing O-rings on all four grooves, while Bum's site specifically mentions ... as Bum is known to do with colors and font choices, to only place O-rings on the cams with four grooves to the two grooves that are SQUARE cut. I remember replacing two O-rings in their original position on the cam without a recollection of the specific grooves and their profile.

Before I pull things apart on my 81 R65, does anybody have a close-up picture of the 4-groove brake cam?  I'm thinking 4 O-rings are better than 2. Unless my thinking is wrong. 

[montmil]

Just so happens I do...

This is the brake cam shaft for my '81 R65.

Only additional tip I might add is to use extreme caution when refitting the shaft with the new o-rings as the edge of the shaft bore is pretty danged sharp and could possibly damage the new ring/s. I put a very small bevel on the edge. Leaks be gone.

And you n' the Snowbum are correct... the ring grooves are flat-bottomed.

[nhmaf]

I believe (perhaps incorrectly) that BMW had a number of reference/marker grooves on the rod to be used for possible/future milling to the proper square-bottom channel for o-ring use.   As they tended to 'work things out on the fly' and/or tweak things until a problem was solved.   I've got 4 shallow, curved grooves in my '82s rear brake rod - they look like the shallow ones closest to the middle of the rod in Monte's picture and are too small to fit o-rings without being properly done on a lathe.   Depending in when the brake rod in any given bike was assembled, it may havae more or fewer grooves in it, or alternatively, the other grooves could be added by an owner ordealership with access to a small machine shop.

I have just 2 o-rings on mine on either end.  And, for grins I tucked one under the felt washer that goes on the outside under the brake lever.    I've considered getting the rod machiend for more o-rings, but (knock wood) I've been doing OK thus far..

[Barry]

No experience with this because the early models didn't have this problem but Bob Porecha reckons that BMW didn't ever completely cure that problem even with 4 rings. His suggested solution was to just lower the oil level a touch.  

From an engineering standpoint you would have expected two O rings to work just fine never mind needing four. What with the breather it's not as if there can be much in the way of oil pressure.  Something strange must be going on like the braking forces pulling the spindle away from being concentric in the bore.

[Mike V]

Barry, Your depository of documents is amazing.  I've come to rely on your library.
Monte, great pic, thanks.
Mike, thanks for your reply also. I think I already may know what's going on now.
Barry, I couldn't agree with you more.

I've already decreased the volume of oil.  If my memory serves me correctly I think I only installed one o-ring at each side, as it came apart originally.  Not the first assembly and fitment contradiction I've found on this bike. After I can examine my particular situation more closely I'll report back - hopefully with some pictures and useful information.

As a footnote; the leak is a minimal amount.  A weep at best but enough to drip a nickle size spot below the final drive. A nuisance at worst.  But like most of these situations it won't solve itself or magically go away on its own.

I'm wondering what the purpose and function of the "other" grooves were intended to perform.

[Barry]

For what it's worth the early model brake cam shafts also have those shallow grooves. Lubricant reservoirs ?   A bit small perhaps but I can't think of anything else.

[montmil]

There are actually three stages of unwanted exterior oil dispersal within the Airhead family:

Weepy, Seepy and Leaky

They are The Three Stooges from Motorrad.  

[Mike V]

Interesting how the REAL OEM Fiche site shows only a Qty. of 2 for the o-rings, part no. 07119906328 (#23).
Another lesson learned not to use the REAL OEM site as an assembly manual. I haven't checked Max or others.

I'll have to re-read Bum's web site. I thought I remember the section mentioning only 2 o-rings and not 4, or did he mean per side?

Just sharing a thought while thinking out loud about this ...

[nhmaf]

For what it's worth the early model brake cam shafts also have those shallow grooves. Lubricant reservoirs ?   A bit small perhaps but I can't think of anything else.

I think that BMW had them made with those grooves in most of the rods, and then decided which ones were to be used for o-rings - the machinists would then just mill out the proper square-bottom channels in the rod per instructions, using the shallow grooves as the reference locations - that's my theory anyhow

[Mike V]

Makes sense, in a BMW non-sensicle kind of way. I just re-read Bum's site, he states there were 4 square grooves that are to receive o-rings in 1981 to 1984.

I can't seem to get away from brake assembly o-rings for some reason.  I'll fill all the square grooves with new o-rings when I get back into the unit.

Thanks guys.

[Bob_Roller]

I've removed the brake cams on all three of my R65's, 81, 82 and 84 and all of them only have two square grooves for o-rings, the other grooves are too shallow to accomodate o-rings .

Not to say there aren't cams out there with more proper grooves .

[montmil]

As I withdrew the brake camshaft from the '81's rear drum, I was expecting just the two o-rings on the shaft as noted on the fiche. Surprised when four showed up and darned glad I ordered extras. Don't have a clue if the camshaft is from a later model or if a PO had additional grooves cut. Happy with four.