I keep the rpms under 6,000 and the others have lasted quite well... had one with a 14cc un-shrouded head, header, s/a carb, filter and it ran like a champ!!! then I have the one im on now... that I went the other way with.. shooting more for flow with the same carb and pipe as be for, un-shrouded the stock head with polished ports, (not ported!!! just polished) 2 nr carb spacers, nocked the gov gear off the crank, 1-3 rockers, and 18lbs springs... im thinking of milling down its head but just don't know if she'll hold???