My m/c is an '87 C-10, my parts m/c is a '99 C-10. The water pump failed on the '87, so I pulled the water pump from the '99, no brainer, get a new seal, and o-rings, and plug-n-play right...not so fast, there's a 4MM difference in the water pump shaft o-rings. Same water pump, the '87 o-ring is 33MM, the '99 is 37MM, with parts numbers of 92055(A) & 92055(C). Me and the dealership parts guy talked to a mechanic, who seemed to think the engine case water pump bore could be slightly larger for the '99, and he recommended going with the o-ring specified for the '87, which we ordered at a list price of some $10.00 more, at $15.59 Part #92055-1266. Has anyone run into this ? Can anyone explain the difference ?