ernie55xx

after having the block done at local machine shop and putting together,when i went to start it,it only backfired and wouldnt do any thing,come to find out the exhaust valves where hitting the top of the pistons.not much but enough to leave a mark on the new +40 flat top pistons,i install a comp cam 268 454 454 cam in it with new lifters push rod and rockers all stock except for cam ,lifters and valve springs,the only thing i thought was maybe i used the wrong head gaskets,they were just the thin steel ones over the counter,any ideas would be helpfull ,this is not my first engine so iam at your mercy. i think it bent the exhaust valves, thanks

old dog

it dosent sound like to much lift on the cam ,the only thing i can think of is that the head's were excessivly milled and the block had the deck milled . your probably have to have the pistons relived to clear the valves . are you running the stock ratio rocker arms ?1.5 or hi lift 1.6 they give close to .50 thousand's extra lift .