I can share some insight. When I pulled my 51 216 apart, there were dropouts in the rod babitt and you could see the height difference in where the babbitt was missing. Now, these rods came from the factory with something like .004" shims on both sides. If you have no shims in there then there must be some wear on those rods. Plastigage will give you an idea of the existing clearance. I would also say that if the clearance is greater than .003" then you probably would hear a knock.
