We used a coating on the big bore CR500 pistons, lasted about 2 meetings. Later OE Honda CR500 pistons came coated already but they didnt last either (all the holes is the barrels dont help things you know). I think there is more friction to be lost in not using twin ring pistons rather than coating pistons in teflon or what ever you choose to use.

However, the trouble honda went to on some of their race bikes to lose friction was awsome. On the last of the RC45's the gear box output shaft was plated to reduce friction on the oil seal!
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If I have to take the carbs off once more...