A rear main oil seal story by Bart Sanders
A couple of weeks after our holiday trip, I noticed some slipping in the clutch. When the engine was hot, it started to rev up without speeding up. Taking the gas down and then slowly increasing delivered power to the rear wheel again.

Coming to think of it, in Paris, after our final kilometers on the Péripherique—the inner circle motorway—in very hot weather and parking the bikes in the hotel parking showed an abnormal amount of oil leakage after I checked the bikes a bit later. There is always the drip from the breather and sometimes a drip of oil from the gearbox, but this time it was much more.

So, now I was in a mild state of alarm. Oil in the flywheel zone? Bad crankshaft oil sealing? Time to open it up!

In parallel, I did some basic reading on the Internet and found out, that basically there are two major classes of oil sealings (aka Simmering). The plain vanilla ones are made of a rubber called NBR (nitrile butadiene rubber, for those who want to know it all). This is a material which may be used below 80-90 degrees Celcius. If not, it will become hard, and lose its elastic properties. And then there is VITON rubber, also known as a fluro-carbon polymer based rubber. It has much better specifications. For instance it can be used up to 200 degrees Celcius in automitive applications. NBR Simmerings are black and VITON sealings are brown. I know that our Chinese standard oil sealings are black. So, first hypothesis is that the very hot engine temperature of an SV engine and oil temps are sometimes well above 100 degrees Celcius—certainly on a very hot day, driving very slowly on a motorway jammed with French traffic—may have killed the main sealing next to the flywheel.
After the usual steps, taking the gearbox out, dismantling the clutch and pulling off the flywheel, I immediately got the confirmation. The main engine seal was hard, parts had broken off and it was completely loose in its housing. The clutch was a few lightyears away from being dry, and it still did its job, more or less that is.
Okay. Time to get hold of VITON engine seals with the size of 50 x 70 x 8 mm. In a well sorted ball bearing shop, I quickly located them and bought a couple of them.
Cleaing the clutch, leaving the plates in water with two tablets of dish washer detergent for a night got the grease off. Putting everything back together was done quickly.
In the pictures you can see the old sealing, the new Simmering and the new ring back in its housing.

I will have to wait some 17000km however, in order to know if the VITON rings are better. The old one lasted 17000km anyway!