my cosworth had a missfire so after changing the plug that was playing up and still no joy ,i did a compresion test 1-150psi 2 130-psi 3-0 4-150. I removed the head 2 day 2 find that the hardend vavle seat had droped down 3-4-mm and stoped the vavle from shutting. Has this happend 2 any 1 else,or ever heard of it?. is it a broblem that cylinder 2 is 20psi down on the other 2?