Got a new to us head for the 7520, different casting # that says it fits 7520 (dont have # with me) and junk yard says it came from a 7520. Our old injectors have too small of threads to fit into the new head and the diesel shop says they cant just put a bigger nut on without knowing what its going on. Kind of at a loss of what to do. They said they would send a call tag and get a different head but everything else is bolted onto the tractor and don't wanna pull it again. Injectors are the same as our 4320 has size wise. Parts books don't seem to tell whats the problem either. any one else have this experience?