Ok - here's our problem. I'm doing this for my husband (at work) and son (at school). So if anything doesn't make sense- just overlook it - confuses me just talking about it. My son has an 01 400ex which he just bought this summer. He decided he wanted to make some improvements on it. It already had a hot cam, noleen exhause, etc. He bought a wiseco 416 piston kit and had it bored. (Now---my husband has had alot of experience working on his own quads for years - but always had 250rs and never dealt with a 4 stroke - so it's kind of a learning experience) Also a friend who has had a bit of experience is helping out. After everything was back together with the 416, it had a little "noise" to it - he rode it around a little (now know shouldn't have done that) and it blew. The valves were hitting the piston, and broke and came out the exhaust. So now -- had it rebored and got the Wiseco 426. Another friend who owns an ATV shop (not honda) checked it out and said that he should have the new piston shaved off 120 thousandths. So did that and also told him to put some clay on the top of the piston, turn it over and see if they were hitting after the "shaving". When doing this - the valves did NOT even touch the clay. All is put back together and when started up, there was some noise again. Took it back apart and the valves are hitting again!! So - what do we do - is there a simple fix for this??? We were at a race yesterday with our other son and a guy told us we should buy a different brand of piston - and that would fix it?? OH--he has a stage 3 hot cam - so after the pistons started hitting this second time, they just put the stock cam back in, but that just kinda puts him back to square one on the power thing. We were also told that it was possibly out of time or there was maybe some stretch in the cam chain. Hubby says their is a tiny bit of stretch in the chain so we ordered a new one of those. Now - need some advice or opinions on this - how to get it going for him without spending a complete fortune. He's saving for a truck and doesn't want to keep pouring money into the quad until he knows exactly what he should do. Thanks for ANY help on this!!

 

I would say the timing is off. It could smash valves even if the timing is off by one tooth on the cam sprocket with a cam that aggressive. When you put clay on the piston to see if the valves were hitting, did you put the head on and time the motor like you would putting it permanetly together??
Kenny