81 100RS Still Smokin'

[hools100RS]

Seasons greetings all!
After a substantial bottom end rebuild mid year and fitting dellortos, the RS was a persistent smoker and the subject of a post that did elicit some great discussion of possible causes and potential remedies. Thanks!!! I thought the prob had been solved but as the bike was being used infrequently, it wasn't until it began being ridden more often, that the problem obviously remained and that the carbs, fuel contamination, crankcase oil level, leaking head gasket, etc, were not the causes. When the motor was stripped the pistons were put on the bench and other than a cursory check (rings looked fine.....)were put back into the motor without another thought, Engine runs well........ just smokes and it shouldn't. The problem has been a topic of shed conversation and pondering for a while now and recently a mate of a mate mentioned that a bike he rebuilt years ago exhibited the same problems and that the cause was pre 81 rings (for cast iron bores) mistakenly installed with the 81's Nikasil bores. I seem to remember ( hazy recollection!!)the top ring of the pistons on the bench was shiny (chrome??) and that a new set of rings ordered from Motorbins are dull black (cast iron??).
I haven't stripped the motor to investigate whether this may be the cause, though a fresh set of rings won't hurt. Has anyone heard of this (possible) mismatch being a potential cause for oil burning and if so, why? A compression test has both cylinders well within spec. though the plugs oil up around town but appear to be a reasonable color after a fast run. Clouds of smoke accelerating from idle, after the motor is warm and pretty high oil usage.
Any thoughts on the matter? hools.

[Zombie Master]

Did you mike the bores to see if they had ovaled?

[hools100RS]

Hi ZM! Last time the motor was apart we checked the bores and they were unmarked. Those nikasil jugs are tough!! The way the motor is running at present doesn't suggest that lack of compression is an issue, though I'll know more,ie, if running with the wrong rings has damaged anything. Merry Xmas!!

[SteveD]

The chrome ring you describe doesn't sound right. The cast iron ones are the correct rings for nikasel bores.
Are you absolutely sure they're nikasel? I once had a 1981 R100RS that had the earlier cylinders, swapped out to replace the nikasels. Can't remember why that was done, but unless the bike has been yours since new then I guess it's a possibility.
mx.

[hools100RS]

Thanks for the reply Steve and Merry Xmas!!! Yep, they are Nikasil for sure and I'm sure you are correct re: the cast iron rings. I suppose only an inspection will give me the answers. The symptoms only become apparent when the motor is warm and I'm guessing that the chrome rings are letting the thinner oil past the chrome rings as they aren't seating properly against the harder bores? Sound reasonable? Regards, hools.