Need Help With Water Pump Pulley Problem

[Dennis H.]

Hi Guys

I have been working through the pulley setup on my 65, 327, 350hp w/ AC & PS today using part numbers provided by a NCRS member for the correct L79 setup.

Crankshaft is a 2 groove, 3850838 BG

PS Add On is a 1 groove, 3751232 GM

PS Pump is a 1 groove, 3770509

Alt pulley has no part number but is a left over from one of my 427 projects that from what I see is the same GM part number (3846180) for both the 66 427 and 65 350hp. My pulley is 3" wide, heavy steel which is 1/4" thick on the back side.

AC pulley is 5 3/4" with compressor p/n 6555302

My water pump pulley is off track by at least 1/4 to 1/2 a track and the bolt pattern is the large style and my new water pump is the small style. This pulley has no part numbers on it and when I pulled it off the car bubba had used washers on the backside of the crank pulley to line everything up. This pulley is a 2 groove, 6 1/4" wide by 2 3/4" deep.

Old Water Pump was a 3782608, dated C 26 4, no bypass and has the large bolt pattern for the fan clutch.

New Water Pump is a NAPA 58-403, with bypass and small bolt pattern for fan clutch.

I'm 99% sure I have all the correct pulleys now except the water pump pulley that if I'm correct should be p/n 3890419 @ 6 5/16" wide but I'm not sure how deep (I hope deeper than the one I have).

I didn't think to check my new fan clutch to see if it was a small or large bolt pattern but did buy it from Long Island Corvette for this engine so I'm going to assume until Tuesday anyway that it is correct.

Should I have a small or large bolt pattern water pump?

What is the correct water pump pulley?

Hard to pull a pulley part number from these vendor catologs and I would like to get this right the first time. Sorry this is long but wanted to get all the details out there. Thanks for any help.

[Jack H.]

Re: Need Help With Water Pump Pulley Problem

The A/C option is what changes the water pump pulley from base SB. It's called out in UPC C-60 of your '65 AIM book as 3850680.

The water pump hub incorporated the small bolt pattern on Corvette until '71 and later in Shark era. The '608 water pump you had was correct for the application EXCEPT for its date code (C 26 4)...it'd be a perfect match to a late build 1964 car with hydraulic lifter configuration.

The fact that it had a large bolt pattern hub says it got rebuild somewhere along the line. Once the pump's internal components are overhauled, the drive hub is interference, press-fit, onto the pump's shaft. While there is a 'correct' press dimension, a lot of the rebuilders simply get 'close' and leave it at that!

This could be a source for your problems...

[Joe L.]

Re: Need Help With Water Pump Pulley Problem

Jack and Dennis------

1965 L-79 did not use a '608' waterpump; it used a '326' or '175' casting with external bypass provisions.

The GM #3850680 pulley was used for 250 hp and 300 hp with C-60. The L-76 and L-79 pulley was different. It was supposed to be GM #3850979. However, IF this pulley was ever actually manufactured, it was a PRODUCTION-only piece, never available in SERVICE. I tend to think it was never used. Instead, I believe that pulley GM #3848904 was used. This pulley was discontinued from SERVICE in 1972 and replaced by GM #3995641, which is still available from GM.

As Jack mentions, all pre-71 Corvette waterpumps use the small hub bolt pattern. Many SERVICE or rebuilt pumps have hubs with dual bolt patterns and these will, of course, work just fine. The other important feature is that the pumps all had a 5/8" pilot. So, the pump pulley and the fan clutch have to have 5/8" pilot holes. The 3848904 and 3995641 pullies have 5/8" pilot holes and the small bolt pattern. I'm sure that any fan clutch you get from LICS will have the small bolt pattern and 5/8" pilot-----they only sell parts for 63-67 Corvettes and all used the small pattern and 5/8" pilot.

[Dennis H.]

Re: Need Help With Water Pump Pulley Problem

Thanks Joe and Jack

Now that I know the bolt pattern is correct I guess it all about the right pulley. Do you have any measurement data on the 3995641 pulley?

[Dennis H.]

Re: Need Help With Water Pump Pulley Problem

Joe

I did find the info I was looking for in my GM parts manual.

64-65 (327) w/SP H/PER AC, 3995641, Double Pulley 5 57/64" this pulley is also listed for the 66-68 S H/PER, AC, PS

The crank pulley 3850838 is listed as 64-65 EXC H/PER, FI, 427 or 66-68 S H/PER, AC, PS

So it looks like a trip to the Chevy dealer on Monday. Thanks

[Joe L.]

Re: Need Help With Water Pump Pulley Problem

Dennis-----

The only other pulley you'll be able to get from a GM dealer is the GM #3751232 add-on crank pulley (p/s pump driver). The GM #3850838 is long gone. I don't know if it's reproduced, or not.