78 water pump pulley

[Bill Wilkins]

I have been looking for a 3991423 water pump pulley for my 78 for awhile and every time I find one I get outbid on it at the last minute or they want over $100 for it. I can find other two row ones for 20 bucks. Does any one know why this pulley used on nearly 10 years of vettes is so expensive?

Bill

[Joe L.]

Re: 78 water pump pulley

Bill-----

One reason is that this pulley was, for whatever reason, always an expensive pulley. When it was GM-discontinued in January, 1999 it GM-listed for $114.00.

Another reson is this: although this pulley was used for a lot of years on Corvettes, it is a Corvette-only item; it was never used for any other chassis application. Corvettes were one of only 2 GM vehicle applications that used a 3/4" water pump pilot (and, consequently, a 3/4" pilot hole in the pulley). Also, they were one of only 2 that used the "short" water pump after 1971, when the bolt circle size for the water pump mounting changed. All this results in a uniquely-configured pulley.

The other chassis application that used the 3/4" pilot and short water pump were some medium and heavy duty trucks. They didn't use the 3991423 pulley, though. However, they did use the same water pump as 71-82 Corvettes. In fact, that's why all mid-1972 through 82 Corvette waterpumps, PRODUCTION and SERVICE, have the external by-pass fitting on the top which is plugged with an ugly, square-head pipe plug. The external bypass was needed for the truck application, which was the ONLY other application for the Corvette water pump. 1971 through mid 1972 Corvette small blocks used a similar pump, GM casting #3991399, that didn't have the external bypass. However, after mid 1972 GM must have decided that they didn't want to produce 2 different castings for such a limited use waterpump. So, the "one-size-fits-all" casting of GM #330813 came into use for both Corvettes and MD/HD trucks.

Now you know "the rest of the story".

[Bill Wilkins]

Re: 78 water pump pulley

Thanks Sir,

I new there had to be a reason. Someday I'll find one that won't cost a small fortune and mine will get replaced. The one I have now works but isn't the right one.

Bill