71 BB Water Pump??

[Sheldon S.]

I am a bit confused about water pumps for the 71 BB. I was under the impression that the 3992077 was just for the 71/72 BB corvettes and had the 3/4 inch shaft in them. I see lots of the 3992077 pumps for sale with correct dates but always the 5/8 shaft. Recently saw a NOS one still in the box and it has the small shaft. Can someone explain this. Also I have one with the small shaft with correct date for my car and have another one with correct shaft but wrong date. Can the large shaft one be put in the correct dated pump housing?
Thanks in advance.
Sheldon

[Joe L.]

Re: 71 BB Water Pump??

Sheldon----

ALL big block waterpumps, Corvette and otherwise, short leg or long leg, have a 3/4" OD waterpump SHAFT. What you're talking about, though, is not the shaft----it's the PILOT. All pre-1971 big block pumps, Corvette or otherwise, long leg or short leg, used a 5/8" pilot. All 1971 and later big block waterpumps used a 5/8" pilot, too, EXCEPT for Corvette and some MD/HD trucks. Those applications used a 3/4" pilot.

As a result of the fact that all big block short waterpumps (i.e. the only kind ever used on a Corvette) used a 3/4" shaft, all of the castings used over the 1965-74 period are functionally interchangeable. So, any casting can be used to build up either a 5/8" pilot unit or a 3/4" pilot unit. As far as rebuilders go, they really don't care which casting they use. In fact, in this day-and-age many will use either casting and use ONLY a 5/8" pilot. Then, for those few applications requiring a 3/4" pilot (Corvette being one of those few), a precision 5/8"-to-3/4" bushing is supplied with the pump. The cost of the bushing is minimal and is, apparently, cheaper than stocking 2 different part numbers.

I've not seen a GM NOS waterpump using casting #3992077 with a 5/8" pilot. I have several NOS examples of these and ALL have a 3/4" pilot. However, there's absolutely no reason that GM could not have used this casting for pumps with either pilot size. While I don't think that any PRODUCTION application of the 3992077 pump casting ever was fitted with a 5/8" pilot shaft, later SERVICE could very well have been made in that configuration. If so, the PART NUMBER on the BOX should be GM #3957982, GM #3990993 (most likely), GM #458486, or GM #14087581.

If I haven't already answered it above, the answer to your last question is that any bearing/shaft assembly for a SHORT LEG big block waterpump can be installed in any short leg, big block waterpump. You can also use the one with the 5/8" shaft, IF you also use the bushing I mentioned above AND the pump has a large bolt pattern or dual bolt pattern hub (which it very likely does have).

You ABSOLUTELY CANNOT USE THE 5/8" SHAFT PUMP WITH PULLIES AND/OR A FAN CLUTCH WITH 3/4" PILOT HOLES UNLESS YOU USE THE ADAPTER BUSHING. Doing so courts disaster.