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Old 02-01-2013, 05:06 PM   #4952
Semi-reformed Tsotsi
Joined: Nov 2007
Location: South Texas
Oddometer: 1,952
Originally Posted by GreatWhiteNorth View Post

I've read the '08 model year had a slightly changed ring design... don't remember the details (maybe someone could chime in with that), but seem to recall the ring type, thickness, hardness all were a factor. Am personally looking forward to 685 kit upgrade with the Thermo-Bob installed - slightly more power, less vibration and decreased oil consumption - all good. Haven't heard too many guys with aftermarket piston kits complaining about excessive oil consumption. I still maintain operating at higher rpms for extended periods places greater stress on pistons and rings - in my mind, it only stands to reason that the rings flex more and generally have a tougher time sealing. I've read this in car & bike mags, and heard this stated here too - it's on the internet - must be true!
The pre- 08 pistons are known to occasionally break a ring land - the section of piston material between the rings. The piston design was changed for 08 to try to address that. The new and thinner 08 ring design pressue was about 3lbs - not enough. This could cause ring flutter and oil usage at rpm's around 5K. Some 08's had the bores oval too. The late Cary Aspy, of Schnitz, in co-operation with JT pistons, decided on a forged piston (stock is cast) that gave a 685cc displacement and that has a ring pressure of 11 lbs.

I now have 36K on my 08 and the JT 685 piston (47k on the bike). It does not use oil ( about 1/8" on the sight glass) even after a 4k ( no oil changes), 8 day ride with several 7-800 mile days and many, many, miles and hours upon hours of running at or above 5k rpm. I have now done many rides like that.

There are a lot of 'old wives tales', 'tribal knowledge', call it what you like, that are ignorantly spread around on forums about 'all singles use a bit of oil'. Decent piston and ring design, propely bored and toleranced piston and rings will not 'use' oil.

Tsotsie screwed with this post 02-01-2013 at 05:14 PM
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