What causes the exhaust valve to break at the stem? Ferrea valves, springs, retainers. Bronze exhaust guides. Crower G2 cam measured at .398 lift, duration unknown. 13.5JE piston. Vortex ignition on #1 setting. Previous owner had a 14:1 CP piston with Crower G2 running pump gas(how this is possible I don't know) Installed a Ron Hamp head on tired 07 engine. Installed a new cam chain, manual chain tensioner, and JD jet kit. Tested the AF mix with a 185(JD Jet kit) and was 14:1. Took it out the next day and ran 1 WOT run at 86MPH. Installed 190 main and hit 92. I didn't have the o2 connected next day to verify but I think it was close. Maybe a 195-200. Could the temps from lean jetting cause the valve to break off due to heat stress? How plausable is this? What other causes are there to consider? She ran very well all day... then... click... Hole in cylinder, head smacked, piston has some new valve reliefs. Custom piston for sale, dirt cheap.