Proper Replacement '72 454 w A/C Fan Clutch

[Owen L.]

I am looking for a replacement stock-appearing fan clutch for use on my '72 454 w/AC. Is the Schwitzer CK that Corvette Central sells a stock appearing direct replacement? The forum search results don't seem to address replacements with enough detail other than GM part numbers. I've been unable to positively ID from the AIM and judging manual. The replacement Eaton on my LS-5 has a significantly loose wobble.

Second question: I found the issue when I heard the fan blades intermittently brushing against the shroud. I bought the shroud and replacement radiator 30 years ago from my local Chevrolet dealer but never installed them until this fall; the shroud seems really close to the fan blades now that everything is installed. Are the Schwitzer fan clutches shorter than the Eaton?

Thanks for any help you can offer.

[Mark E.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

Regarding the second question: I believe the service manual specifies the fan to shroud clearance. I check it by placing a finger on a fan blade tip and rotating it along the shroud. A bit less than a finger width is about right. Use the slots in the upper shroud bracket to adjust. You may need to apply tension.

[Joe L.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

I am looking for a replacement stock-appearing fan clutch for use on my '72 454 w/AC. Is the Schwitzer CK that Corvette Central sells a stock appearing direct replacement? The forum search results don't seem to address replacements with enough detail other than GM part numbers. I've been unable to positively ID from the AIM and judging manual. The replacement Eaton on my LS-5 has a significantly loose wobble.

Second question: I found the issue when I heard the fan blades intermittently brushing against the shroud. I bought the shroud and replacement radiator 30 years ago from my local Chevrolet dealer but never installed them until this fall; the shroud seems really close to the fan blades now that everything is installed. Are the Schwitzer fan clutches shorter than the Eaton?

Thanks for any help you can offer.

Owen------


The original fan clutch for your application was GM #3991426. The fan clutch pictured is similar to a 3991426 but not 100% correct. It may, in fact, be a GM #3937771 which was used for many applications during the 1968-70 period and will not work for any 1971 or later. You can most easily tell if it's a 3937771 by measuring the pilot orifice on the flange. If it's 5/8", then it's a 3937771.

In addition, "CK" was NOT the broadcast code for a GM #3991426. It was the broadcast code for a GM #3916141 which is a completely different configured fan clutch and that clutch will not work for any 1971 or later Corvette. I believe the broadcast code for the 3991426 was "AJ" but I'm far from being sure of that.

While it's likely the play you describe is the fault of the clutch, check to be sure it's not in the waterpump rather than the fan clutch.

[Terry M.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

Regarding the second question: There are shims for the engine mounts that will help align the fan to shroud if there is not sufficient adjustment available in the shroud. See the C60 portion of the 1972 AIM for more details.

[Owen L.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

Owen------


The original fan clutch for your application was GM #3991426. The fan clutch pictured is similar to a 3991426 but not 100% correct. It may, in fact, be a GM #3937771 which was used for many applications during the 1968-70 period and will not work for any 1971 or later. You can most easily tell if it's a 3937771 by measuring the pilot orifice on the flange. If it's 5/8", then it's a 3937771.

In addition, "CK" was NOT the broadcast code for a GM #3991426. It was the broadcast code for a GM #3916141 which is a completely different configured fan clutch and that clutch will not work for any 1971 or later Corvette. I believe the broadcast code for the 3991426 was "AJ" but I'm far from being sure of that.

While it's likely the play you describe is the fault of the clutch, check to be sure it's not in the waterpump rather than the fan clutch.

Thank you, Joe. My water pump definitely has the 3/4" shaft. I sent an email to CC to confirm the pilot bore size and suggested they add it to the web page.

So the 460742 listed in the GM parts books is an Eaton service replacement? It only lists the 3991426 for '71 but the Eaton for '72 and up.

The wobble play is definitely in the fan clutch. The water pump is a newly rebuilt original and firm.

[Owen L.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

Regarding the second question: There are shims for the engine mounts that will help align the fan to shroud if there is not sufficient adjustment available in the shroud. See the C60 portion of the 1972 AIM for more details.

Thank you Terry. It seems the engine would have to drop downward; the fan blades are very close to the upper right of the shroud. The shroud wasn't an exact match for the old broken one and took some work with a Foredom and cutting burr to allow the lower extension shroud to fit properly. Even though it came from GM 30 years ago, it doesn't seem to have the fit quality of the original. I think the new foam seals are also pushing it engine-ward and compounding the close fit.

[Owen L.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

Regarding the second question: I believe the service manual specifies the fan to shroud clearance. I check it by placing a finger on a fan blade tip and rotating it along the shroud. A bit less than a finger width is about right. Use the slots in the upper shroud bracket to adjust. You may need to apply tension.

Thank you Mark. I used woodworking clamps to pull the shroud as far forward as the slots in the upper bracket allow. It feels like the new upper seal is pushing the shroud's lip toward the engine but I don't see how to do anything about this unless I remove it altogether.

[Joe L.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

Thank you, Joe. My water pump definitely has the 3/4" shaft. I sent an email to CC to confirm the pilot bore size and suggested they add it to the web page.

So the 460742 listed in the GM parts books is an Eaton service replacement? It only lists the 3991426 for '71 but the Eaton for '72 and up.

The wobble play is definitely in the fan clutch. The water pump is a newly rebuilt original and firm.

Owen------


The GM #460742 did not even exist in 1972. It came along later for PRODUCTION and SERVICE. I don't know what edition of the P&A catalog you're looking at but it must be a later edition. The GM #3991426 is shown in 1972 and 1973 editions of the P&A catalogs as the correct one for the 1972 application and it is the correct one for 1972-73 applications for all engines. As a matter of fact, the 3991426 remained available in SERVICE until October, 1984 when it was discontinued and replaced by the GM #14063640. The latter was discontinued without supersession in November, 1988.

However, the 3991426 ceased to be cataloged for the 1972-73 applications effective with the 1974 editions of the P&A catalog. For 1974, the 72-73 fan clutch was changed to GM #343720 for non C-60 and GM #343721 for C-60 applications. The 3991426 remained cataloged for 1971 applications until the time it was discontinued. The 343720 was discontinued in November, 1978 and replaced by the GM #460742 which was discontinued and replaced by the GM #88961763. The latter was discontinued several years ago and I can't find a new number

This remained the same for 1975-76 except that a different fan clutch, GM #372502, was introduced for L-48 with C-60 applications. This is as far as I'm going to take it now.

By the way, I consider the GM #343721 and its supersessions GM #12529342 and GM #88961768 to be the best fan clutches ever installed on a Corvette and I would not be without one, correct for the application or otherwise. These are Eaton-style clutches and are considerably different in configuration from all other Corvette fan clutches. They are also noisy (because they pull a LOT of air) but they COOL. The 343721 cannot be used on pre-1971 Corvettes because it has a 3/4" pilot, 3-1/4" fan bolt circle, and 2-5/8" hub bolt circle. The 88961768 can be used on pre-71 if a 1971 fan blade assembly is also used. The 88961768 has a dual pattern hub and can be used with a 3/4"-to-5/8" bushing for the pilot. However, I do not recommend this fan clutch for pre-1971 small blocks because the 5/8" shaft waterpumps just are not stout enough.

The GM #88961768 which is virtually identical to the 343721 except for the dual pattern hub remains available from GM.

[Owen L.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

Wow! Thank you Joe -- so much detail of changes over time. Yes, I have '78 or just slightly later parts and illustration books I got from one of my pestering visits to the local dealership back in the day when they changed out to the newest version.

I'm going to look fora comparable exchange for a 3991426 or 14063640 if I want to stay with the Switzer style or 343721/12529342 for the Eaton (not thrilled with the dual-bolt pattern hub of the xxx1768). Would this correct?

[Joe L.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

Wow! Thank you Joe -- so much detail of changes over time. Yes, I have '78 or just slightly later parts and illustration books I got from one of my pestering visits to the local dealership back in the day when they changed out to the newest version.

I'm going to look fora comparable exchange for a 3991426 or 14063640 if I want to stay with the Switzer style or 343721/12529342 for the Eaton (not thrilled with the dual-bolt pattern hub of the xxx1768). Would this correct?

Owen------


The GM #88961768 (or predecessors) would not be a correct fan clutch for any 1972 application and it would obviously greatly deviate from the correct 1972 fan clutch configuration. So, the fact that it has a dual pattern hub is not going to make it significantly "more incorrect". I offer it as a cooling performance improvement for Corvette applications not originally using it or its predecessors. For Corvette applications originally using the GM #343721, the GM #88961768 would be generally configurationally correct except for the dual pattern hub.

Also, I don't know that the 14063640 fan clutch would be a Sweitzer-style fan clutch. It REPLACED a Sweitzer style fan clutch but that does not mean that it is that style. Probably very hard to find, anyway.

[Tom L.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

If you have the "tipped" fan and you're experiencing ANY contact with the shroud or anything else, I'd be sure to eliminate ALL possible causes. These fans are almost impossible to find.

[Owen L.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

If you have the "tipped" fan and you're experiencing ANY contact with the shroud or anything else, I'd be sure to eliminate ALL possible causes. These fans are almost impossible to find.

Thank you Tom, yes, I have the 7-blade tipped fan with the right part # and date to the car. I'm well aware of the rarity of the fan and am taking pains not to damage it in any way.

[Mike E.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

What date is stamped on the fan? Those definitely were batch-built, and certainly not every month. I have seen H70 and F71, but have only been paying attention recently.

Thank you Tom, yes, I have the 7-blade tipped fan with the right part # and date to the car. I'm well aware of the rarity of the fan and am taking pains not to damage it in any way.

[James G.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

Perhaps I missed it, however I didn't see it mentioned whether or not you have the original fan clutch for the car?
Nor did I see a reason for not wanting to purchase an original and rebuilding.

With that said and unless there were a compelling reason I would recommend finding an original dated pump and having it rebuilt.

I had my original Schwitzer rebuilt by Kirkconnell cost was $265 March of 2019. - it went from wobbling and overheating in February - to running straight and true at 180 in the middle of the summer.

[Owen L.]

Re: Proper Replacement '72 454 w A/C Fan Clutch

Perhaps I missed it, however I didn't see it mentioned whether or not you have the original fan clutch for the car?
Nor did I see a reason for not wanting to purchase an original and rebuilding.

With that said and unless there were a compelling reason I would recommend finding an original dated pump and having it rebuilt.

I had my original Schwitzer rebuilt by Kirkconnell cost was $265 March of 2019. - it went from wobbling and overheating in February - to running straight and true at 180 in the middle of the summer.

The fan clutch on the car, since I bought it in the late 80s or 1990, is an Eaton that isn't a factory application. I am refurbing my car to be period appropriate in appearance for replacement parts with the major components being original to the car or date appropriate to the build. I replaced the non-correct water pump with a number and date correct pump ("C" month for an early-May build). I am seeking and installing these kinds of dated components right now if changing them out later causes more difficulty - the antifreeze and mess for the water pump R&R. The fan clutch (alternator and starter) is more easily changed out later with a simple swap. Part of this is financially driven and part impatience; I can get the car running now while taking the time to search out the correct fan clutch later.

Additionally, if I don't get to it and leave it to the next owner, at least they have an easier time with minor components rather than major and I can be proud that its appearance is accurate. That's my justification and I'm sticking to it!