I misplaced my Micrometer but using a ruler the best I can tell it is 2 and 3/8 inch . A quarter will fit in ring groves and the best I remember a quarter is 0.955 in. The piston pin looks to be 1/2 inch but again I am using a ruler. I have a set of calipers somewhere but have not found it yet. Got to get better organized for sure. Hope this helps some. Tom