fresh top end, in my bike this weekend. new piston, rings and intake valves. we didn't replace the exhaust valves cause they were good. i had about 45 minutes of ride time on this motor before the exhaust valves snapped. the dome is completely destroyed and has a crack in it as well. the opposite side of the cam chain, the 2 bolts that hold the cams in were loose on the intake and the exhaust journals. just on the side opposite of the cam chain. anyone know why this could have happened?

Something similar happened to my cousins race ready 450r last weekend. His exhaust valves snapped off also. He reasoned that it was just because they were old and he was running a 13.5:1 piston on U4.





Was your timing spot on when you reassembled? Are you running a high grind cam?

I think he put it together wrong , possibly didn't have the motor at tdc when he installed the cams or just didn't pay attention. i would have him get you the parts and find someone else to put it back together personally

That is not mine, that is my cousins TRX450r motor, with the engine built by modded motorsports and V&M racing engines. He races A class.

intresting as i stated in another thread i am attending YTI's Motorcycle Technology Program, we actually went over how this happens. It is called Valve float, which is an occurance when the valves are forced open before they have a chance to close causing them to have a collision with the piston inside of your cylinder. this is usually caused by over revving, if you werent ringing the motor out then the only other thing i could think of is that the timing was set wrong and/or the engine was dissassembled and re-assembled when it wasn't at TDC. that sucks, new head, piston, valves, and possible new cylinder in the works for you there.

Also another thing I though of, what was the condition of your timing chain, if it was old and wore it could have caused it to slip on the cam gears while under force. this would definately cause your timing to get out of wack and to have a catastrofic engine failure like that. This is why its always a good thing to replace your cam chain when doing work like that, cheap insurance so that doesnt happen, usually you can get new ones on ebay for around 30 bucks.