1971 fan clutch

[Frank V.]

I have a 1971 350 base engine w/auto and a.c. This car also has the copper radiator and seven-blade fan. My question is, the fan clutch failed, and I can not find a replacement for it. Nobody lists a fan-clutch for this application. Has anybody else encountered this problem? Or no where I can find a replacement?

[Joe L.]

Re: 1971 fan clutch

I have a 1971 350 base engine w/auto and a.c. This car also has the copper radiator and seven-blade fan. My question is, the fan clutch failed, and I can not find a replacement for it. Nobody lists a fan-clutch for this application. Has anybody else encountered this problem? Or no where I can find a replacement?

Frank-----


1971 did use a one-year-only fan clutch, GM #3991426. It's very difficult to find. The part number is not stamped on these units, but the photos below depict a 3991426 on the RIGHT SIDE of each. If your goal is 100% originality, this is what you're looking for. There are similar fan clutches out there, also rare, but they have a different hub and fan blade mounting pattern.

If functionality and not originality is what you're after, then that's a whole different story. In that case, I HIGHLY recommend you use the GM #88961768 pictured on the LEFT SIDE of the photos. It will be a 100% bolt-on installation.

If you want something that looks closer to original (but certainly not something that's going to fool many judges), obtain a GM #88961763. While it will appear CLOSER to original, it won't perform anything like the 88961768, but it will be adequate. The 88961768 (and its predecessor part numbers) is ABSOLUTELY the best performing fan clutch EVER installed on any Corvette. Alas, without significant modifications to the fan system it can't be used on pre-1971 Corvettes. The 88961768 is somewhat noiser than the original 71 fan clutch but it cools BIG TIME.

[Frank V.]

Re: 1971 fan clutch

Thanks for the info!
Living in Florida, I need all the cooling I can get! Sitting in traffic with the a.c. on, the temp gauge climbs quickly down here.

[Joe L.]

Re: 1971 fan clutch

Thanks for the info!
Living in Florida, I need all the cooling I can get! Sitting in traffic with the a.c. on, the temp gauge climbs quickly down here.

Frank-----


One other thing I should have mentioned. It's POSSIBLE that early 1971 base engine used the 1970 configuration. I've never confirmed that this ever occurred, but it's possible. To check for it you can do one (or both) of 2 things.

1) Check the fan blade. If one of the blades has the part number 3955182 stamped on it, then it's the 1970 configuration; if no numbers are found or if the number 3991420 is found, then you have the 71+ configuration;

2) Check the water pump casting number. If it's 3991399 or 330813, then you have the 1971+ configuration. The 330813 would not be an original pump for 1971 but it would be the 1971 pump configuration.


If you have the normal 1971+ configuration, then you can use the 88961768 fan clutch.

[Jack H.]

Re: 1971 fan clutch

I had my original restored ('71 SB A/C) and I've put +65K miles on it since... Whatever you do, I wouldn't discard your current core!

[Frank V.]

Re: 1971 fan clutch

The only numbers I have found on the fan assembly are
on the fan blades 3991427 and 914685 H7O. Does that mean anything to you? My assembly manual shows a clutch part number for the base engine and another one for the big block. Both have 5 blade fans. In the C60 option (AC), it shows a 7 blade fan, but no part number. Just a footnote saying "assembly per RPO LS5".

[Jack H.]

Re: 1971 fan clutch

Check your copy of the '71 AIM and you'll see the 3991427 PN IS the factory original PN called for on the fan! That's good, they're tough guys to find...

The 'H 70' emboss is the 7-blade fan's date code (August 1970). That's another positive check!

Now, Joe and I have had talks about the AIM's call out for the fan clutch... The same C60 page in the book says to use a 3937771 assy. But, that CAN'T be...

Why? That's the PN for the fan clutch that was used on BB engines in 1970 and earlier.

What's wrong with that? Well, the water pump changed in 1971 to a 3991399 which sported a larger front shaft/bearing and used a WIDE vs. NARROW bolt circle pattern.

There's no way a 3937771 fan clutch will bolt on to a 3991399 water pump! The best I can fathom is this is an error in the AIM and the correct fan clutch for that fan blade was PN 3991426.

But, you report the correct fan PN and that leads me to believe you've got the correct/original fan clutch since they apparently bolt together properly. As Joe said, your water pump could be the correct original 3991399 (housing cast number), or it could be the later 72L+ version of that pump with a housing cast number of 330813...

[Frank V.]

Re: 1971 fan clutch

I thank you for the info. If the 88961768 clutch will bolt up to my pump and fan, I'll get it, and have mine refurbrd. I was just making sure that I got a clutch that would work right in an AC car. I assumed there must be some difference in the clutches to compensate for the extra heat load and 2 extra fan blades.

[Joe L.]

Re: 1971 fan clutch

I thank you for the info. If the 88961768 clutch will bolt up to my pump and fan, I'll get it, and have mine refurbrd. I was just making sure that I got a clutch that would work right in an AC car. I assumed there must be some difference in the clutches to compensate for the extra heat load and 2 extra fan blades.

Frank-----


You have the correct fan blade assembly for your application. The 3991420 part number I mentioned in my previous post is the part number for non C-60 applications. The 3991427 is the correct, original part number for C-60 small block applications. Either blade assembly would confirm that you have the normal 1971 fan and drive configuration.

So, the GM #88961768 should bolt up and work perfectly. The predecessors to the 88961768 (which were virtually identical) were used for late 70's-early 80's Corvette applications with C-60 and/or heavy duty cooling. Fan clutches were not specific to the number of blades used for the fan.

By the way, does the fan clutch you have on the car now look just like the 3991426 I have pictured in my previous photo? If you could post them, photos would be nice.