My favorite Corvette waterpump is and has always been the GM #3998205 with casting number 3991399. This was a Corvette-only waterpump although it's possible it was also on some MD/HD trucks with small block. It's also a waterpump that was only used for a very short period of time----1971 and early 1972. It was superseded by a similar pump but with one important difference. That pump, casting #330813 used on Corvettes from later 1972 through 1982, had a big, ugly, unused external bypass boss and fitting on the top. It was plugged with a big, ugly, 3/4" NPT, square head pipe plug on Corvettes. I hate seeing such plugs underhood and, actually, it's quite "un-factory-like". But, for some reason, GM made an exception here.
The GM #3998205 pump appears like the earlier GM #3957981 (casting #3782608) when seen installed on the engine and its snout hidden by the pulleys. However, it has the large bearing and shaft assembly which is far superior to the "spindly" bearing and shaft used in pre-1971 small block waterpumps. Consequently, it's far more rugged and long-lived than the earlier pumps.
As some may know, beginning with the 1969 model year, only Corvettes and some MD/HD trucks used the short leg waterpumps, both small block and big block. All other Chevrolets went to the long leg variety. However, the 1971+ short leg pumps had a different hub spacing than the earlier pumps and used a 3/4 pilot as compared to the 5/8" pilot used for all earlier pumps.
Below are pictured an NOS GM #3957981 (cast 3782608). This is the 1969-70 Corvette small block waterpump. Note the small diameter snout. An interesting thing about this pump is that it was cast the same day as the original pump on my 1969 Corvette small block (May 16, 1969) and from the same pattern (GM27).
Also pictured is an NOS GM #3998205 (casting 3991399). Note the large snout with buttress-like reinforcements and the reinforcing ribs on the left side of the pump (like earlier Tonawanda-cast small bearing pumps). This one was cast September 17, 1971. I have several NOS examples of this pump and several rebuilt examples. I've never seen one with a date after about mid-1972. Note also the pattern number of 2. They didn't need many patterns for this pump since its use was so limited and its tenure so short.
The GM #3998205 pump appears like the earlier GM #3957981 (casting #3782608) when seen installed on the engine and its snout hidden by the pulleys. However, it has the large bearing and shaft assembly which is far superior to the "spindly" bearing and shaft used in pre-1971 small block waterpumps. Consequently, it's far more rugged and long-lived than the earlier pumps.
As some may know, beginning with the 1969 model year, only Corvettes and some MD/HD trucks used the short leg waterpumps, both small block and big block. All other Chevrolets went to the long leg variety. However, the 1971+ short leg pumps had a different hub spacing than the earlier pumps and used a 3/4 pilot as compared to the 5/8" pilot used for all earlier pumps.
Below are pictured an NOS GM #3957981 (cast 3782608). This is the 1969-70 Corvette small block waterpump. Note the small diameter snout. An interesting thing about this pump is that it was cast the same day as the original pump on my 1969 Corvette small block (May 16, 1969) and from the same pattern (GM27).
Also pictured is an NOS GM #3998205 (casting 3991399). Note the large snout with buttress-like reinforcements and the reinforcing ribs on the left side of the pump (like earlier Tonawanda-cast small bearing pumps). This one was cast September 17, 1971. I have several NOS examples of this pump and several rebuilt examples. I've never seen one with a date after about mid-1972. Note also the pattern number of 2. They didn't need many patterns for this pump since its use was so limited and its tenure so short.
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