i recently bore my atv .20 over now the piston hits the head what do i do?

 

it hits the head??? it shouldent unless you have a big dome piston. are you sure it isnt hittin the plug?? if it is hitting the head try adding a nother gasket to the head. my 400 did that because of the high dome on the piston. i jus got a SHORTER PLUG AND ADDED 2 MORE HEAD GASKETS!

 

Stacking gaskets is not a very good idea, they were never meant to be used like that, you risk a blowout.They make special compression lowering "thick" gaskets,or even spacer plates, but the bigger question here is why does your piston hit? was the head or deck of the jug milled?where did you get the piston from, and are you 100% sure it is the correct one? who did the machine work, can you ask them what was done for the over bore? Check on those points, get as much information as you can, this sounds a bit weird, I have rebuilt several quad engines, and never had a problem like that, I have have dealt with several over bores on a stock jug. Please keep us updated on what you find, I would like to know what you find out !!!!
I just thought of something after I posted, make sure your timing is correct, what you could be seeing is the piston hitting one of the valves from being so far out of time, if you are coming up on the compression or exhaust stroke and a valve is still open, it will hit, and stop the piston (or go right through if you push too hard!) for the piston to hit the head, it would have to be going higher than the top of the jug, which I doubt is happening.

 

i installed the piston, it hits the valves, i ordered the piston froma yamaha shop. it only does it when it is reved up a litlle idling it usually doesent do it, i havent milled the head, do i need to do that?

 

if the timing is off it would backfire wouldnt it? it didnt hit the valves until i had drove it for a about a week then it started hitting i pull the head seen where it was hitting the piston so i ground it that worked for about a week then it started hitting

 

It is possible your timing chain is so shot it is stretching and allowing the timing to be off enough to make the valve timing out of whack enough for valves to hit, bad thing is, changing the cam chain is a little more involved. Check the chain out, make sure your auto tensioner is working right, and make sure the timing is right on the mark. Milling the head would make it run closer to the piston, that's why I asked about it, too much milling, and it will cause problems.