I have an 06YZ450F. The other day at the track the screw that holds the air filter on came off and went into the cylinder. The damage to the valves was bad, both exhaust demolished and 2 intakes bent. I used a dremmel to clean up the piston and I'm in the process of installing new Kibblewhite valves and springs. My question is, can the valves be shimmed with the head on the work bench or does it need to be on the engine. If it dosn't need to be on the engine I'll drop it off at the shop to be shimmed. Thanks,in advance.