Quote Originally Posted by 2manymustangs View Post
Here Bob, I will give you the factory tolerance list and let you figga it all out...

427 block "cylinder grade D" upper tolerance limit is 4.2364" diameter. This is a standard 427 bore, the tolerance range of which goes from 4.2328" to 4.2364". From 1958 through about 1967 there were 12 cylinder bore grade steps (12345678ABCD) of .0003" each, and from about 1968 through 1978 there were nine cylinder bore grade steps (123456ABC) with .0004" increments. I believe the number of steps was decreased because newer pistons used better thermal expansion technology.
So where does a non-Ford guy look on an old 390 to find out what grade it is? I have a couple laying out in the barn. If I remember right, one is a 68 bock and the other is an early 70s.