A few weeks back, I participated in a discussion on the board which touched on the subject of the 71-72 GM casting #3991399 water pump. This pump was used, reportedly, for at least part of the 1971 model year small block engines and, reportedly, for the first half or so of 1972 production. After that, it was replaced with casting GM# 330813 which was used to the end of C3 production. The 3991399 casting is unique in that it uses the large bearing and associated 3/4" shaft AND this pump DOES NOT HAVE the large, by-pass fitting on the top of the pump as do all 330813 castings which are relatively common. In that regard, the 3991399 casting is similar to the '608' casting used with 70 and older pumps with small shaft and without external by-pass provisions. The 3991399 pump is relatively rare inasmuch as it was apparently used only on Corvette small blocks during a very short time period. It was discontinued from service in late 1973 and replaced by pumps with the 330813 casting. It is POSSIBLE that the 3991399 pump casting was also used on some heavy duty trucks during the 71-72 time frame, but I haven't been able to confirm that as yet.
Based on the experiences reported by some members with '71 small block cars, it appreared to me that, perhaps, early 71 cars used the 3991399 casting but with the shaft pilot turned down to 5/8"(like 65-70 big block pumps)and the small, 1-1/4" bolt pattern hub like all of the pre-71 pumps.
Well, I've finally resolved this once and for all(I think). Today I was fortunate enough to acquire an NOS GM waterpump with the 3991399 casting. This is a complete GM assembly in the original GM box. According to the label, it was originally delivered to a Chevrolet dealer in New Brunswick, Nova Scotia from the Englewood, NJ GM parts distribution center. This pump uses the 3/4" pilot and large pattern water pump hub. Most interesting of all though, it has a casting date of C201, which makes it the oldest 3991399 casting I've seen. I couldn't believe my eyes when I opened the box. Needless to say, this water pump will occupy a hallowed place in my collection.
Based on the experiences reported by some members with '71 small block cars, it appreared to me that, perhaps, early 71 cars used the 3991399 casting but with the shaft pilot turned down to 5/8"(like 65-70 big block pumps)and the small, 1-1/4" bolt pattern hub like all of the pre-71 pumps.
Well, I've finally resolved this once and for all(I think). Today I was fortunate enough to acquire an NOS GM waterpump with the 3991399 casting. This is a complete GM assembly in the original GM box. According to the label, it was originally delivered to a Chevrolet dealer in New Brunswick, Nova Scotia from the Englewood, NJ GM parts distribution center. This pump uses the 3/4" pilot and large pattern water pump hub. Most interesting of all though, it has a casting date of C201, which makes it the oldest 3991399 casting I've seen. I couldn't believe my eyes when I opened the box. Needless to say, this water pump will occupy a hallowed place in my collection.
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