parts interchange manual

[Fred Oliva]

By the markings you describe for this clutch, It's probably an over-the-counter coil type as Joe says.

There were several fan clutches listed in the assembly manual for 1971 Corvettes. They are as follows:

3916139 - Small block w/o a/c. This was an Eaton designed clutch

similar to what you have but with several significant cosmetic

differences such as the shaft length & geometry. It has a coil

type thermostat on the face, & is 6" in diameter.

3916140 - Small block w/o a/c. This was a Schwitzer designed clutch.

It was used interchangably with 3916139. It has a flat bar

type thermostat on the face, & is 6" in diameter. A date

is stamped into the edge of the flange along with the initials

'SC' for the manufacturer.

5329100 - Small block w/o a/c. This was a Delco designed clutch &

was used interchangably with the 2 listed above. It is almost

identical to the Eaton version. In fact, It's a direct copy of

the Eaton clutch. An interesting story behind that but I won't

get into it here.. This clutch has the part number & a juilan

date stamped into the face of the flange (the part that mates

with the water pump pulley).

3937771 - Small block with a/c & big block. This is a Schwitzer designed

clutch. It also has a bar type thermostat but is 7.25" in diameter.

This clutch, as well as all others have what is characterized as

a small flange. 1971 was the year Corvette made the changeover

to a large bolt pattern water pump. These clutches were for

any interm clutches that had a small bolt pattern water pump.

3991426 - Big block clutch, This is a Schwitzer designed clutch that is the

same as the 3937771 clutch but with the large flange for the

large water pump pattern.

All this according to the assembly manual part numbers. If you're still awake after all this, the next part is the important part: From the cars I've seen, typically the 3991426 clutch (bar type, 7.25" casting, large flange) was installed on most 71 cars. Now, I'm sure there were exceptions, but this is what I've been able to determine as 'typical'.

Remember that if you find one of these bar clutches, it should be dated correctly for you build date.

Hope this helps.

[Patrick Hulst (16386)]

Re: Fan clutch info needed

Fred,

Do you (or does anyone else) know when the 71 changed from small to large shaft? I have 1971 cars with serial numbers about 8500 and 19500, and both have the small shaft pump for 350/270 AC cars, as well as small shaft pulleys, etc. The latter, a mid-June car, even has an August 1970 water pump as the original!

In all of my 1971 small block research I have never seen a small block with a confirmed original pump that has a large shaft. If you try to buy a replacement, however, "everyone" tries to say that it should have a large shaft. Might it be that the change did not come until 1972, despite what "everyone" thinks?

Maybe it's like the clear/amber turn signal lens discussion. I have yet to see an original 71, including late (20000-21000)cars, with amber turn lenses, despite the fact that many books state that only "early" 71's had them.

Patrick Hulst

[Fred Oliva]

Re: Fan clutch info needed

Patrick,

The guy to talk to about changeover information for the water pump is Jack Humphrey. He's researched this subject extensively. We've discussed it at one time because of the tie in with fan clutches -- my obvious interest.

I've observed at least 2 1971 Corvettes with large hub clutches & small block engines leading to the conclusions I stated previously. One car was a really well preserved survivor. The clutches all had the correct dates for the the build date of the car. HOWEVER, there is alot of experimental error on basing conclusions on 2 samples! This is why I tried to denote what I've observed with what I've seen documented.

Your observations seem to indicate that there are indeed cars that used the small hub clutches. This is good info -- I'll be sure to keep a lookout for this in the future.

Fred O

[Joe Lucia (12484)]

Re: Fan clutch info needed

Patrick----

This is not an area that I have studied extensively, but I can tell you this: when the small block change-over occurred to the 3/4" shaft pump, GM produced a new water pump casting. This casting, GM #3991399, was externally similar to the 69-70 3782608 casting in that it had NO bypass boss, tapped or otherwise, on the top of the casting. This was a "short" pump design, and since by 1971 virtually all other Chevrolet small block V-8s except Corvettes had changed to the "long" style pump, as far as I know the 3991399 casting was only used on Corvettes. This casting was used "supposed" to have been used on at least some 1971 Corvettes and, I understand, early 1972 Corvettes. I have 3 of the 3991399 water pumps and all have 1971 casting dates. The date on one of them is "H271", so that pump must have been used on an early '72. Unfortunately, the other two that I have are "buried" at the moment and I haven't the time to extract them.

However, since the 3991399 casting was only used on Corvettes as far as I know (although it is POSSIBLE that it was also used on a few HEAVY DUTY trucks), a little further research into casting dates of this pump might indirectly establish about when the 3/4" shaft pump came into use in 1971, if it did at all.

As you are probably aware, the water pump pullies and fan clutches shown in the 71 assembly manual for the base engine are all the "small bolt pattern" type pullies consistent with the earlier 5/8" shaft pumps. Also, there are no changes noted in the "revisions record block". For the LT-1 small block engine, however, on sheet A4 of the UPC, there are several interesting notations. First, in the "revisions record block" it indicates that this sheet was "redrawn and redesigned" on 5-11-70. More interestingly, it shows that the front water pump pulley(A.I.R. drive pulley) is GM #3991425. This pulley is a "large bolt pattern" type with 3/4" PILOT HOLE consistent with 3/4" shaft Corvette-only pumps.

One other point which may or may not be of interest. Mark IV engine water pumps have always used water pumps with a 3/4" shaft. However, from 1965 through 1970, these pumps used a hub with the "small bolt pattern"(or dual bolt patterns) and used a 5/8" pilot. Water pump pullies for 68-70 big blocks use the "small bolt pattern" and 5/8" pilot holes, GM #3889372. In 1971, big block pumps began to use the "large bolt pattern" and 3/4" (shaft diameter) pilots. The water pump pullies and fan clutches, then, were changed to the "large bolt pattern" and 3/4" pilot hole type, GM #3992071 and 3991426, respectively. This change is confirmed in the 71 assembly manual UPC for the LS5 option.

[Patrick Hulst (16386)]

Re: 1971 Fan clutch revisited

Joe and Fred,

My 71 19518 car has the small central hole for the fan pulley as well as the original pump, and this is (obviously) a late car. While I realize that an August 70 water pump of casting number 3991399 on a June 71 car is unusual, I believe it to be correct. The water pump had original orange spray patterns, and it had no evidence of ever being off the block. Most of the motor was, in fact, original paint. The pulleys are all original 71 numbers, and in fact had some original inspection marks on them. The fan clutch is also original, with green inspection paint still on it. As the car is in storage, I cannot tell you the actual mfr or type.

Another reason I believe it to be original is that "everyone" that tries to sell you a rebuilt pump tries to sell you a large shaft pump. Seems to me if someone had tried to replace mine, the wrong one would be in its place.

Our 1971 8500 car also has the small central hole pulleys, but the original water pump (and motor...) were long gone.

One more note, regarding Joe's message, is that I may have a "misdated" but correct pump if GM was trying to use up all the 3991399's at the end of the year if they were a Corvette-only item.

As I mentioned before, another reason not to believe the "common literature" on 71's, if one has an interest in them.

Patrick Hulst

[Joe Lucia (12484)]

Re: 1971 Fan clutch revisited

Patrick----

Perhaps you misunderstood the information in my post. If you have a water pump with casting number 3991399, that IS the large shaft (3/4") water pump casting. There were only two shaft sizes EVER used on Corvette V-8s from 1955 until at least 1991. The early one, used from 1955 through, reportedly, 1970, had a 1-3/16" bearing diameter and 5/8" shaft diameter. There were a number of different castings but all had a bearing bore diameter of 1-3/16".

The second size, used from, reportedly, 1971 through at least 1991, had a bearing diameter of 1-1/2" and a shaft diameter of 3/4". Your 3991399 water pump casting WILL have a bore diameter of 1-1/2". Therefore, it MUST use the 3/4" shaft size. However, if your car uses the "small bolt pattern" pullies with 5/8" pilot and the 3991399 casting, then there can be only one possibility as to why.

Apparently, for 1971 small blocks(at least some of them) Chevrolet used the 3/4" shaft pump with a PILOT TURNED DOWN TO 5/8" and a "small bolt pattern" hub(or a dual bolt pattern hub). This was not uncommon. That's exactly how all big block pumps were done from 1965 through 1970. It's also how MANY, MANY other Chevrolet small block water pumps were constructed after the they changed to the 3/4" shafts(1-1/2" bearing size) in 1971. If they had used the 5/8" pilot and "small bolt pattern" hub, then they could have used the the earlier pullies as indicated in the assembly manual. In fact, I've often considered using a 3991399 pump with 5/8" pilot shaft and "small bolt pattern hub" on my 69. It would be virtually undetectable in appearance once on the car and the 3/4" shaft pump is FAR more durable and reliable---that's why GM went to them.

It is also possible, I suppose, that 1971 used a 5/8" pilot and the "large bolt pattern". That would have neccessitated a 1-year-only pulley design that I have never seen. Check your pulley if you can for me. Measure the center-to-center dimension of the water pump mounting hole bolt pattern. It will be either 1-1/4"("small bolt pattern") or 1-1/2"("large bolt pattern"). Also, if you can find them, I would be interested in the water pump pulley part number.

I'm very interested in this. It might just fill a gap in my repertoire.

[Patrick Hulst (16386)]

Re: 1971 Fan clutch revisited

Joe,

Ok...I think I got that.

What I definitely know is that when I have tried the larger shafts (which would be 3/4", yes?) of rebuilt pumps on either of the cars, the pulleys DO NOT FIT. This would, therefore, mean I originally had the 5/8" size unit, as no sizes other than 5/8 or 3/4 were made, yes again? I purposely went down to the local Pep Boys in early 1996 with my original pump and compared my pump to what they spec'd for 69, 70, and 71 cars. To get one that matched cosmetically, with the lack of top boss (and tap) and a small shaft (term used correctly?) I had to purchase the unit specified for, I believe, a 69 to 70 Corvette. Of course, for the measly $5 core charge, I kept my original.

Joe, I will try to get time soon to go down to my basement at home (if I ever get out of the hospital tonight, as I am on call and lots of people are being admitted) and closely examine my pump for you.

I tried e-mailing Jack Humphrey off-line as Fred suggested, but the mail was returned, so I do not know what his thoughts are, as he also has 2 1971's.

Thanks, Patrick

[Joe Lucia (12484)]

Re: 1971 Fan clutch revisited

Patrick----

Yes, there were only 2 size shafts ever used, 5/8" and 3/4". However, that is the size of the shaft as it goes through the bearing. A 3/4" shaft pump may have a 5/8" PILOT. The PILOT is the part of the shaft that is "above" the water pump pulley hub and the part that one generally sees and assumes as the shaft size. It is the part that goes through the pulley and center-locates the fan clutch hub. All big block water pumps from 65-70 used a 3/4" shaft with a 5/8" PILOT.

Supposedly, Corvettes from 1971 used a 3/4" shaft and a 3/4" PILOT. I was not aware that any Corvette small block ever used a water pump with a 3/4" shaft and a 5/8" PILOT, even though MANY other Chevrolet small blocks did. However, your cars may just prove that theory wrong, if your pump and pullies are original. As I mentioned in my earlier post, if your original pump used the 3991399 casting, you MUST have the 3/4" shaft. No other bearing/shaft assembly would fit in a 3991399 casting. However, if your pullies are original and have a 5/8" pilot hole, then at least some 71s must have used the 3/4" shaft with 5/8" pilot. I know for sure that 73+ used the 3/4" shaft with the 3/4" pilot and I believe that 72s did, also. But, 71s may have been an odd-ball.

It was always my assumption that if 71s used the 68-70 configuration pullies, that they also used the 68-70 water pump, casting #3782608. However, if your cars have the 3991399 casting, that is definitely the 71-early 72 casting. Incidentally, the 3782608 pump casting is probably waht you found at Pep Boys. It is externally similar to the 3991399, except that the 3991399 pump has the 1-1/2" bore size and has reinforcing ribs located around the bore housing. The 3782608 pump has the 1-3/16" bore and, USUALLY, no reinforcing ribs. Actually, some 3782608 pumps DO HAVE certain reinforcing ribs, but that's another story.

You did the right thing by retaining your 3991399 casting; these are unique to 71 and early 72. Also, it could be easily rebuilt with the 1-1/2" bearing-3/4" shaft-5/8" PILOT that your car apparently used. These bearing assemblies, which were NEVER available from GM as a SERVICE part, are available from water pump rebuilders(if they'll sell you one) or from industrial bearing suppliers. The German bearing company "FAG", which supplies many water pump bearing/shaft assemblies, manufactures ones with the 3/4" shaft and 5/8" pilot. Incidentally, all of the bearing/shaft assemblies, whether of the 5/8" or 3/4" shaft size, use a 5/8" shaft(turned down in the case of the 3/4" shaft types)on the inner part of the assembly. That's because ALL Corvette water pump ROTORS accept only a 5/8" shaft.

[Jack Humphrey (17100)]

Re: Fan clutch info needed

Interesting, Joe. I've looked high/low for a '71 cast date 399. Both of the two I have are '72 cast dates. One I took off a June '72 build SB that went under a semi trailer with 22K miles on the odo. Needless to say, this Corvette was REAL SHORT!!! I think I was the first guy to wrench the hood open since the car was originally wrecked and totaled out. That's where I got the 399 WP casting and 544 alt core for my '71 that my uncle had serviced by replacement....

[Jack Humphrey (17100)]

Re: Fan clutch info needed

For what it's worth, all 399 cast water pumps I've seen (they're rare birdies) had small hubs with 1.75 inch bolt circles. As I mentioned above, I've yet to see a 399 with cast date prior to '72 so this says the pump casting was used into 1972 with the small/narrow bolt pattern fan mount....

[GREG JAEB]

parts interchange manual

WHAT OTHER VEHICLES SHARED DOOR HINGE PINS AND BUSHINGS WITH 81 CORVETTE? DEALER HAS DISCONTINUED THESE.

I CAN STILL GET THEM FOR MY 82 CHEV. P/U BUT DON'T REALLY WANT TO TAKE MY CORVETTE DOOR OFF TO COMPARE AND IT BE WRONG.

I KNOW CORVETTE CENTRAL STILL OFFERS PIN KITS BUT I WOULD RATHER PICK THEM UP LOCAL IF IT'S A SHARED PART.

[Joe Lucia (12484)]

Re: parts interchange manual

Greg-----

I've got good news and bad news for you. First, the bad news: if hinge pins/bushings from another GM vehicle would fit a Corvette and the Corvette pins/bushings were discontinued, GM would supercede the Corvette pins with any other pins/bushings that would SERVICE the application. So, if GM has discontinued the Corvette hinge pins/bushings without specifying a supercessive part number, the chances are there are no other parts in the GM parts system that will SERVICE a Corvette application.

Now the good news: the Corvette hinge pins and bushings ARE shared with many other GM vehicles and they are NOT discontinued. The hinge pins are GM #20046147 and GM list for $3.10/each. There are two different bushing designs. Pre-80 Corvettes used bushing GM #9721917 for all hinge applications. This bushing has a comparatively thin cross-section. It is still available from GM for $0.61/each. This bushing MAY also have been used for some hinge positions on 80-82 Corvettes.

80-82 Corvettes used bushing GM #20429500 for some or all hinge pin bushing locations. This bushing has a relatively thick cross section and is also still available from GM for $0.62/each. I'd suggest purchasing some of each bushing and replace with the same configuration bushing which you remove from each hinge pin bushing position.

One word of caution here, though: the hinge pins currently available from GM are not exactly like the pins originally used on the cars. The currently available pins are generally longer than is required for most applications. The pins must be cut, as required, for the particular application. Use the old pins as a guide for cutting. I recommend tapering the ends of the pins, as original, for ease of installation.

Most major GM dealers should have all of the above-referenced parts in stock.