Perkins AA diesels very similar. Pollution constraints had to be w/in certian limits. So an engine was developed w/ an absolute piston height max (.020 I think) above block. The head gasket made up the difference, while the angle torque settings on the bolts were designed not to allow over squeezing of the gasket. Then a manual rotation was done to insure the pistons were not smacking the head. Perhaps its the same idea.