There are a few different opinions around.On my original Comet I run .005" bottom skirt clearance, 35 deg. magneto advance and have had no problems and negligible oil consumption.My newest Comet nipped up an original Specialloid so I have gone to diecast with .006" clearance.There is a theory in Australia at the moment that premium fuel (98 octane) burns cooler and is less likely to cause seizures.The evidence so far is only anecdotal; any comments from the ethersphere? I add that my bores are round and the muffs fit properly and are true to the crankcase mouth faces.