Fan won't fit, which part is wrong

[Jeff Y.]

I have a 1972 454, Nov 71 build date and can't get the fan on the water pump. When I got the car it had the wrong pump so I found a rebuilt original and replaced it. Replacement pump is a 3992077. The problem I'm having is the studs on the pump are set at a 1 3/4 diameter and my pulley (3992071BE) and fan clutch have the holes on a 2 1/8 inch diameter. Which part is wrong?

Thanks

Jeff

[Joe L.]

Re: Fan won't fit, which part is wrong

I have a 1972 454, Nov 71 build date and can't get the fan on the water pump. When I got the car it had the wrong pump so I found a rebuilt original and replaced it. Replacement pump is a 3992077. The problem I'm having is the studs on the pump are set at a 1 3/4 diameter and my pulley (3992071BE) and fan clutch have the holes on a 2 1/8 inch diameter. Which part is wrong?

Thanks

Jeff

Jeff------


The waterpump hub is incorrect for your application assuming it does not have dual bolt patterns (i.e. EIGHT total holes). Most rebuilt pumps have hubs with the dual bolt pattern. If yours has only the 1-3/4" bolt circle, then it has the wrong hub for the application. What size pilot shaft does the pump have? It should have a 3/4" OD pilot shaft. I have never seen a hub for a 3/4" pilot shaft which had only a 1-3/4" bolt circle stud mounting pattern.

[Jeff Y.]

Re: Fan won't fit, which part is wrong

Hi Joe

It has a 3/4 pilot. Here is a photo. Is it possible to press the plate off and install the plate from my old pump or do I need to find a new shaft also?

image.jpg

[Clem Z.]

Re: Fan won't fit, which part is wrong

send it back to the pump rebuilders and have them install a new hub with a dual pattern. you have to remove the back plate on the pump to install the new hub to back up the shaft when you press on the new hub

[Jeff Y.]

Re: Fan won't fit, which part is wrong

eBay special, buyer beware. Either I have to get the right parts and do it my self, which I can. Or send it to a new rebuilder. Who is a good person to send it too?

[Joe L.]

Re: Fan won't fit, which part is wrong

Hi Joe

It has a 3/4 pilot. Here is a photo. Is it possible to press the plate off and install the plate from my old pump or do I need to find a new shaft also?

[ATTACH=CONFIG]42322[/ATTACH]

Jeff------

This hub is a "revelation" to me. I have never seen such a hub. I think it may have originated from some non-Chevrolet application. Such a hub was NEVER used on a Corvette.

Anyway, you can remove the hub from the old pump and install on this new pump observing the following:

1) When you press the old hub onto this pump you must press against the rear of the shaft. This means that you have to remove the pump backing plate to access the rear of the shaft. If you press the hub on without doing this you may press the bearing assembly further into the pump OR damage the bearings.

2) You must achieve the proper hub spacing or your pulley will not line up properly;

3) I recommend using green Locktite shaft locking compound when installing a used hub;

4) I recommend checking the runout on the hub after installation on the shaft. This can be done with a dial indicator. Runout on the hub must be very near zero or you'll end up with too much runout at the OD of the installed pulley.

[Clem Z.]

Re: Fan won't fit, which part is wrong

you have to make sure you don't move the bearing assy as this will change the gap between the impeller and the pump housing causing problems.

[Thomas H.]

Re: Fan won't fit, which part is wrong

From my 71 LS5. Original pump.

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[Joe L.]

Re: Fan won't fit, which part is wrong

From my 71 LS5. Original pump.

[ATTACH=CONFIG]42323[/ATTACH]

Thomas-----


This appears to be a hub with a 2-1/8" bolt circle as it should be. Just to confirm, is that what it measures (bolt circle= measured diagonally, center-to-center)?

[Thomas H.]

Re: Fan won't fit, which part is wrong

Thomas-----


This appears to be a hub with a 2-1/8" bolt circle as it should be. Just to confirm, is that what it measures (bolt circle= measured diagonally, center-to-center)?

Hi Joe,

The car is in storage, but I remembered I had the original fan clutch here. That does indeed measure 2-1/8". I remember having a hard time getting the correct replacement fan clutch (GM part). The first two they ordered had the smaller bolt circle.

Tom

[Joe L.]

Re: Fan won't fit, which part is wrong

Hi Joe,

The car is in storage, but I remembered I had the original fan clutch here. That does indeed measure 2-1/8". I remember having a hard time getting the correct replacement fan clutch (GM part). The first two they ordered had the smaller bolt circle.

Tom

Tom-----

1971-82 Corvette fan clutches are somewhat "unique" in that they use a 3/4" pilot in combination with a 2-1/8" bolt circle. Very few other Chevrolet applications use that combination. Your original fan clutch was a GM #3991426. That is a VERY difficult-to-find piece. It was one-year-only and limited to Corvettes and, perhaps, some MD/HD trucks.

You can use a GM #88961763 which is ROUGHLY similar to your original (at least, as close as you are going to find in a NEW clutch) or a GM #88961768. The latter is configured completely different than your original but it (and its predecessors) are my favorite Corvette fan clutches of all time. These things will move some air and make long, slow drives in high ambient temperature conditions, even on grades, a lot less stressful. When fully engaged they are noisy but this mostly occurs when a lot of airflow is needed as described previously and, when that occurs, that noise will be "music to your ears".

ADDENDUM: Forget the GM #88961763. It may have been changed to, basically, an aftermarket-type piece with slotted-style flange mounting.