BB vs. SB fan clutch

[Rich G.]

I have a 66 L79 and suspect the fan clutch is tired. I have done the subjective tests I've read about here, and without any real experience, I've been unable to decide. Recently I acquired a 68 L71. This car runs cool even in traffic. My L79 does not overheat, but the temp does rise to 210 in traffic and drops to 180 when cruising. Anyway, when both cars are hot, the BB fan clutch is MUCH stiffer than the SB. Does this mean I should bite the bullet and replace the SB fan clutch, or are the ones in the BB normally stiffer?

Rich Giannotti

[William C.]

Re: BB vs. SB fan clutch

Primary difference in BB vs SB fan clutch is shaft length, if there is noticable difference in stiffness when warm, I'd replace/repair the weak one.

[Joe L.]

Re: BB vs. SB fan clutch

Rich-----

Do you know if the fan clutches on either of the cars are actually original? 35+ years is a VERY long time for one of these things to function properly and be serviceable.

There were quite a few different clutches used in PRODUCTION and available in GM SERVICE over the years and, even, for any given year.

As Bill mentioned, the shaft length on big block fan clutces was usually different than those used on small blocks. In addition, most 68-70 big blocks used an Eaton fan clutch. This clutch has slightly curved fins and a coil-type thermostat on the face.

Many small blocks during the 63-72 period used a Sweitzer fan clutch. This type has the "straight" fins and a rectangular "bi-metal" thermostat. Some small blocks did originally use an Eaton-type coil "thermostat" fan clutch, though.

As I mentioned in a recent post, ALL original fan clutches used integral studs and nuts/washers to attach the fan to clutch. Bolts indicate an aftermarket replacement. Also, all original fan clutches used holes in the clutch hub for attachment to the water pump. SLOTS indicate aftermarket.

Considering the above points of identification, what is the configuration of the fan clutches on the 67 small block and 68 big block?

[Rich G.]

Re: BB vs. SB fan clutch

Bill

Thanks...it goes on the list.

Rich Giannotti

[Rich G.]

Re: BB vs. SB fan clutch

Joe,

The 66 SB has what looks like the Eaton unit. It has rounded fins and I can feel a coil on the front, although I don't have a mirror the right size to see it. It DOES have nuts on studs holding it to the fan. I'd be surprised if it is original.

The 68 BB clutch looks similar, but has a larger diameter. The fins have a "step" about 3/4 out from the center where the angle changes. It is fastened with bolts through the fan from the back.

As far as slots on the water pump side goes, it doesn't look like either one has them, but it could be hard to see when installed. The BB clutch is stiffer both cold and hot than the SB.

Thanks

Rich Giannotti

[Les Jacobs]

Re: BB vs. SB fan clutch

Rich
Can you swap the L71 clutch into the L79 to see if it makes a difference before replacing it? Les

[Joe L.]

Re: BB vs. SB fan clutch

Rich-----

The 67 fan clutch sounds like it could be a GM SERVICE replacement unit of GM #3916141. This unit was originally one of the big block SERVICE fan clutches and became the SERVICE fan clutch for all 60-70 Corvette applications over 25 years ago.

Your 1968 unit sounds like an aftermarket replacement. That doesn't mean it's a bad thing, though. Based on performance, it looks like it works real well. Maybe you out to get another one like it for the L-79? The fan clutches used on all 1960-70 Corvettes are functionally interchangeable, so the one on your 68 would fit the 67 perfectly.

The slots, if present, on the fan clutch hub shouldn't be hard to se with the unit installed on the car. Just look at the flange of the clutch where it bolts to the front surface of the outermost pulley (and through the pullies to the waterpump). If there are slots instead of holes, it will be readily apparent at a glance.

[Rich G.]

Re: BB vs. SB fan clutch

Les, Joe, Bill

Thanks for the info and ideas. Joe, I don't think I see slots in either one of these, but I'll reserve decision until I pull one off. Too cold in the garage right now!

Thanks again

Rich Giannotti

[Roberto L.]

Re: BB vs. SB fan clutch

A few years ago my previous fan clutch got more and more loose, until one day suddenly got totally free, no clutch effect. Not very pleasant in traffic. A good solution was a rebuilt unit from Oliva, which works first rate.

Roberto, NCRS #30019, RMC