Which O Ring? '88 RT Rear Brake Cam Lube

I was talking with someone who helped me with my winter project, and he suggested I take out the brake cam to clean and re-lube, which sounds like a good idea. Looking at the BMW parts fiche, there are two different O Rings indicated for O ring for the cam shaft (both are listed as part #20. FYI, this is not the felt ring, that's listed separately). One O ring is 10x2, while the other O ring is 12 x 1. Is there a way to determine which O rings I need prior to disassembly? The bike 1988 R100 RT.

BMW changed the rear brake cam pre 8/89 or post 8/89. I think the post 8/89 is the 12 x 1 as it lists the Mystic. for that part as well.

Check the production date on your bike. The 12 x 1 was started 1/26/87.
Best,
DeVern

I would order both O-Rings. Just in case.
OM

Adding to title for more clarity in future searches.

Omega Man said:

I would order both O-Rings. Just in case.
OM

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I don't mind spending a bucks or so, but, BMW charges about $5 each for the 12x1. The 10x2 is $0.50 each. I have no idea why the 12x1 is 5 times the cost of the 10x2. Granted, EME has them for $2.50 each, which is better, but still a bit much for my taste. I may change my mind though.

GTRider said:

Check the production date on your bike. The 12 x 1 was started 1/26/87.
Best,
DeVern

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Bike was built in December 1987.

88bmwjeff said:

I don't mind spending a bucks or so, but, BMW charges about $5 each for the 12x1. The 10x2 is $0.50 each. I have no idea why the 12x1 is 5 times the cost of the 10x2. Granted, EME has them for $2.50 each, which is better, but still a bit much for my taste. I may change my mind though.

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Ordering the wrong size takes up time and, in this case, probably cheaper than the replacement shipping cost.
This is how I deal with O-Ring woes-



Both round and square edge style.

OM

88bmwjeff said:

I don't mind spending a bucks or so, but, BMW charges about $5 each for the 12x1. The 10x2 is $0.50 each. I have no idea why the 12x1 is 5 times the cost of the 10x2. Granted, EME has them for $2.50 each, which is better, but still a bit much for my taste. I may change my mind though.

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I get a lot of my o-rings from McMaster-Carr or my local hydraulics service and supply shop, for two reasons. First is cost, obviously, tho the difference between the two isn’t much on a $0.50 o-ring. For me, the bigger reason is I can select the material composition or durometer hardness I want. For example, the o-rings BMW used in brick-K clutches were buna-n as shipped, and they all tended to last around 65k miles before turning hard enough from heat to crack and start oiling the clutch disk. Replacing that o-ring with a Viton ring, with higher heat tolerance, resolves the issue. Same thing for the o-rings on the cam end covers on 90s oilheads.

It would be a cold day in a hot place before I’d pay $5 apiece for those o-rings, but that’s just me. And, my advice is worth exactly what you paid for it.

https://www.mcmaster.com/products/o...-of-measurement~metric/width~1-000mm/id~12mm/


https://www.mcmaster.com/products/o...-of-measurement~metric/width~1-000mm/id~12mm/

Best,
DeVern