I recently purchased a (second hand) brand new set of 30 thou ACL race series flat top piston & rings 8MKRY9115 because they cost $200.00. For my 253 rebuild. I simply thought they were a better piston than standard but are obviously different. When I rang their listed dealer in NSW ( not the performance one.) they could not tell me if these pistons allowed bigger valve heads to be used, which would be a great excuse to tell my wife that I need some.