My knowledge of the numbers on the parts is quite limited. I assumed that 51 referred to .051" but I am not sure. The 58 made it too rich for my 216 but the 51 seems to be just right. Somebody has switched in a 58, probably to make it run better. On another note, the used head that I bought had a thick coating of carbon all over the combustion chambers. My motor would certainly not look like that. I think an overly rich situation would coat everything.
