Originally Posted by motu
Valve train has always been the limiting factor with pushrod engines, that's why ohc is now the prefered option. But plenty of pushrod engines can be made to rev well, some even stock. I used to rev the unit 500cc Triumph twins to over 10,000rpm, completely stock. So if it's happy at 8,000rpm with no valve float issues, that's still safe on these engines - the bottom engine is up to the task ?
The main issue with high RPM in airhead engines has been cavitation at the oil pump. Essentially, the pump is trying to suck too much oil from the sump and it has so much suction that it causes bubbles to form in the pick up tube. The traditional way around this was to drill the pick up out bigger. ( I think it goes from 10mm to 13mm but one of the other guys will correct me if I'm wrong).
After doing this to my racer, it's happy running to 8500 with no problems. I have an oil pressure gauge just in case.
I believe some people have machined the pump thinner so it draws less oil.
I'm not familiar enough with the R65 engine to know if it's a problem with them too.