Which O Ring?

[Jeff in W.C.]

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.

[Rob Frankham]

Can't think of a way to tell for sure without dismantling, buy both and use the one that fits. An 'O' ring is unlikely to break the bank.

Rob

[Jeff in W.C.]

One O ring is ±$0.50 each and the other is ±$2.50 each. Granted ±$5-$10 (depending on where it's ordered) will not break the bank, but it seems like a waste to spend $5-$10 just in case.

[barryh]

I would be inclined to remove the brake arm and measure the shaft diameter as a clue to the correct O ring size. Even then will you know how many O rings are fitted until the shaft is removed ?

Brake cam.png (71.88 KiB) Viewed 3152 times

[MDchanic]

... it seems like a waste to spend $5-$10 just in case.

You can get them from you-know-where for about $5-6 for fifty of each size, or $8-12 for an assortment box.

Granted, you're still out $5-10, but you'll have O-rings for the rest of your life.

There's also a decent chance that you local real hardware store (not the Low Despot) will have them for a few cents to a buck a piece.

- Eric

[Rob Frankham]

I think I would prefer to go with slightly more expensive rings from a reliable source. Yes, I do have boxes of 'O' rings from various 'less expensive' sources but I don't find them very good. I tend to keep them for non critical (i.e. non automotive uses). Sizing is often variable and the quality of the 'rubber' is often poor. Of course, you can't be sure where your dealership is sourcing it's stock either but I think you are more likely to get a decent product from that source... or at least you have more comeback if it proves to be useless. For $5 (or it's equivalent) I'd rather have a good product.

Rob

[Giles B]

If sourced from another supplier do you go for nitrile or vitron o rings for longevity? I am just in the process of buying new o rings from a local shop for a carb service. The cam shaft ones of course would not be in contact with petrol.

[barryh]

Nitrile O rings are theoretically OK for petrol and oil. I have used them in the carbs and while they do work I'm not convinced that they last as long as they should. There are probably different levels of quality even in Nitrile and the introduction of ethanol in the fuel will be what is degrading them. Vitron are supposed to more resistant to ethanol.