What is the correct O ring for the rocker spindles  on a '71 Triumph T100R and similar bikes?

According to my parts book, the rocker spindle O ring on my 71 T100R is part number 070-3253. I’ve ordered a few over the years, and since it is sometimes more convenient to buy from industrial sources rather than from motorcycle parts houses, I crossed the part to an industrial number. In the top photo at left, the upper ring is one shipped from any of several British bike parts suppliers. The lower one is an industry standard “-011” size. The two appear and measure to be identical. (Click the pics to see them bigger.)

The nominal dimensions of an -011 O ring are 0.301” ID and 0.070” cross section.

I’ve come to believe that this is not the correct O ring for this application. The first suggestion is the notorious difficulty many if not most people have in installing the O ring without damaging it. The pic below shows a typical damaged ring next to a brand new one. In this case, the shaved off piece was hidden between the shaft and the rocker box bore, so I didn't even know about the damage until I took the spindle out and looked closely at the ring.

More convincingly, some simple arithmetic shows that this ring is not compatible with the O ring groove on the rocker shaft.

The O ring groove on my T100R is about 0.060” wide and 0.037” deep, +/- maybe 0.002”, and it has a root diameter (at the bottom of the groove) of about 0.490”. The first clue that there may be a problem with the -011 ring is that its ID is quite a bit smaller than the bottom of the groove. In fact, an -011 ring will have a “stretch” factor of over 60%, when the general rule for O ring design is to provide something under 10%.

The next warning sign is “gland fill”. The groove (called a “gland” in O-ring speak) in the rocker shaft has a  cross sectional area of 0.060 x 0.037 = 0.0022 in2. The -011 ring has a cross sectional area of π(0.070/2)2 = 0.0038 in2. Even adding a little depth to the gland to account for the clearance between the shaft and the rocker box, the cross section of the ring is bigger than the gross section of the gland. Industry practice is to fill the gland at around 75%, and never more than 90%.

The last and most convincing (to me) way to look at this is to calculate the actual volume of the ring and the volume of the gland. Contrary to common belief, though rubber deforms easily, it doesn't compress appreciably, so comparing the volumes should settle it.  The volume of a torus is the cross sectional area times the mean diameter. So the gland has a volume of 0.527π x 0.0022 =0.0036 in3, while the ring has a volume of 0.371π x 0.038 =0.0044in3. There's about 20% more rubber than there is volume to stuff it in. This ring isn’t going to fit in that gland—not without shaving some of it off on installation

I’m an Engineer, but not a Mechanical Engineer. Information about industry practice in O ring gland design was gathered from half a dozen web sites that all agree pretty well.


So if this is the wrong ring, what is the right one?  I have heard that the parts books from a few years later show a different part number, and that there may even have been a bulletin making the change retroactive.   The later part is 60-3548 and is shown at left next to the -011.  I suspect it may be a metric 12 mm ID x 1.3 mm cross section or something close to that.  That size appears in some industrial listings, but is not easy to source in small quantities.  If it is a M12 x 1.3, the stretch would calculate to a few percent, which is within design standards, and the gland fill is less than 100%, but still higher than standard practice would dictate.  Since this is a static application (no intentional movement), maybe the high gland fill is not a big deal.




Comments to: elhollin1@yahoo.com

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