68 Fan Clutch

[Mark Stocki]

Looking for a correct fan clutch for my '68 427/400hp. Does anyone know the correct casting or part number to look for?

Thanks,
Mark

[Joe L.]

Re: 68 Fan Clutch

Mark-----

The most likely original fan cluch for your application is a GM #3937771. This is a Sweitzer-manufactured, "bi-metallic plate" thermostat type fan clutch. It has a rather unique configuration, though, inasmuch as it has a "pie-pan"-like configuration which surrounds the center, finned face of the clutch. This clutch was discontinued many years ago. You could look for a core though and have it rebuilt be Fred Oliva.

The other clutch that may have been used for your application is a GM #3916141. This is an Eaton-manufactured clutch with a "coil spring" thermostat. It has no "pie-plate" perimeter. This cutch is still available from GM to this very day, although its configuration has changed a bit over the years. Mainly, the configuration difference involves the face of the clutch and the shape of the silicone fluid resevoirs.

[Joe L.]

Addendum

Here's a photo of the face of a GM #3937771

[Mark Stocki]

Re: Addendum

Thanks for the very informative reply. I'll try to find a core for rebuild.

[Jack H.]

Add on...

As you look for rebuildable 'core', keep in mind there are two similar yet different versions of the Schwitzer clutch Joe showed in his picture. The difference lies in the height of the cooling fins in the center of the face.

The factory original Corvette version has TALL fins that are almost the same height as the surround outer rim of the clutch. The similar version used on many other cars and trucks of the era, have shorter fins. They're easy to detect because a straight edge laid across the outer rim will clear the lower fins by about 1/4-inch. The 'big Schwitzer' clutch fins will almost touch the straight edge...

[Kurt B.]

Re: 68 Fan Clutch

Joe,
What is the correct way to test a clutch fan to determine if it is good or bad(operative or inoperative)
Thanks,
Kurt

[Joe L.]

Re: 68 Fan Clutch

Kurt-----

There's no EFFECTIVE means that I've ever been able to come up with.

[Jack H.]

It's tricky + you need factory specs!

The fan clutch was spec'd in %-slip versus BOTH temperature and RPM. You test by mounting the clutch to the specific fan GM called out in the part spec and spin it up.

Using a stroboscope, you measure RPM of the input shaft and RPM of the fan, compute the %-slip and compare to the part spec. Now, repeat the test this time pre-heating the input air temp and allowing the clutch body to thermally stabilize. This is obviously NOT a test the average home mechanic can execute!

Fred Oliva fabricated a test stand when he and his father began their clutch rebuild business. At one Regional meet, Fred put on a seminar where he'd captured several examples of typical worn fan clutch assys as well as a freshly rebuilt clutch that had NOT been cosmetically restored.

He invited members of the audience to come up, feel and 'play' with the samples and sort them (good bad). NOBODY was able to correctly sort the samples! Bottom line, there's good, bad and marginal and the 'home brew' tests you hear of (count number of revs on engine shut-down, twist the shaft by hand and guage resistance) only suffice to isolate really worn 'basket case' clutch assy's.