Corvette? pulley 3991423 BW

[G B.]

I'm trying to sort out a pulley misalignment problem on a '71 small blcok Corvette. The water pump pulley appears to be the culprit. It is a double deep groove style, but it looks like a stovepipe hat. The number stamped into it is 3991423 BW. I can't find this number listed in my Corvette reference books. Maybe it's a Camaro or Chevelle performance part. The pump snout hole is 5/8" diameter. The pulley O.D. is maybe 6 inches.

Can anyone help me identify this pulley as to a specific application?

[Joe L.]

Re: Corvette? pulley 3991423 BW

Jerry-----

Well, this is a strange one. GM #3991423 IS a Corvette pulley. In fact, it's ONLY application is Corvette. It was used for most 71-80 Corvette small block applications, except some A/C-equipped cars. The pulley is a 2 groove with a 5.9" OD. It is 3.07" deep from the front surface to the rear surface. It has a 2-3/16" bolt circle, as do all 71+ Corvette w/p pullies. Now, here's the strange part: the 3991423 pulley has a 3/4" pilot hole. ALL 1971 Corvette w/p pullies use a 3/4" pilot hole and they are one of the very few Chevrolet applications that did use this size. So, I can't figure out why yours would have a 5/8" pilot; this was the PRE-71 pilot size for Corvettes.

[G B.]

Secret factor you didn't consider

I make a lot of mistakes, especially when I'm tired in the evening.

I noticed that the center hole was bigger than the earlier pulleys I normally work with, so I just guessed while typing that the diameter was 5/8". DuHUH!

I'm sure you're right and it measures 3/4".

[Ed Jennings]

Re: Secret factor you didn't consider

Jerry, seems like the length of the water pump changed about 70 or 71. Has this engine got the right height pump?

[Joe L.]

Re: Secret factor you didn't consider

Ed-----

Yes, in addition to the change in the mounting hub bolt circle size and pilot size, the "length" of the small block water pumps used for 1971 and after was slightly longer. The pre-71 pumps measure from the block mounting surface to the face of the hub a length of 5-9/16". 71 and later pumps measure ABOUT 5-3/4".

It's hard to get them mixed up, though, because of the difference in mounting hub bolt circle and fan clutch pilot----the pullies and fan clutch for pre-71 and 71+ won't interchange.

[G B.]

Thank you both

The misalignment will probably turn out to have several contributing factors, Ed. Some cars have just been through too much to have only one root cause of any problem. I have to investigate these issues: balancer swap, rebuilt alternator, replacement water pump, headers, missing fan clutch, wrong fan... I'm sure you can visualize this mystery.

Joe thanks for your research. It's amazing how much info you find out based on an obscure part number.

[Clem Z.]

jerry you can move the pulley hub on the shaft

since it is only a press fit. i have done this to get the alignment correct.