If you know the exact o-ring size (metrics are also available) you can substitute ones from an o-ring supplier like McMaster-Carr.
Several things besides just size are also critical. The material - must be heat and oil resistant - vitons were used on hot plastic hydraulic machines, but there also is teflon, and other materials like buna - read MCarr page and it explains uses for various materials. Also critical is the durometer rating - this is how hard the oring is. 70 durometer is common, but at the plastics factory, they liked 90 better - less deformation under pressure.
One other thing, when determining oring size, be careful as some of the methods are a little "iffi." Also, try to use the proper oring for the location and measure a NEW one. Some orings get distorted after use, and sometimes expand, and sometimes shrink or get hard and brittle which makes measuring more difficult.
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Thanks Jimmylee. I guess I had better just be patient and wait for the parts to arrive.
Thought I should jump in here and add some comments to JimmyLees. When I originally posted the question, I honestly thought O-rings are a standard size both in the Imperial Measurement and Metric. After spending some time at the local o-ring supply house here in Calgary, I came away being more confused than when I went in. I agree with the term "iffie", in that JimmyLee is on to something. Thought about it hard and long and decided to just wait and order the BMW OEM supplied material, cause I hate redoing things a second time because of either being in a rush or using something that I shouldn't have.
Just to add to the Durometer information, in that the harder the material the less likely you will have absorption from outside medium or gases. I have seen this first hand in my world were Viton Durometer 70 O-rings were exposed to H2S (Hydrogen Sulphide) and the o-rings absorbed the media and expanded 10 times its original size. That is a no no when working around piping and transmission equipment especially when lives are at risk, as one whiff of H2S can knock you down and send you to the gates of heaven.
Bottom line is, use BMW OEM supplied material, as you know your getting the right material and size.
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1968 BSA Starfire, 1976 BMW R75/6, 2009 R1200RT