1966 L79 with A/C and Power Steering - correct pulleys

[David P.]

I'm trying to confirm the correct pulleys for a '66 L-79 with power steering and A/C. I have the following pulleys:

Crankshaft - 3850838 BG 7-1/4" diameter
Power Steering add-on - 3751232 6-3/4" diameter
Alternator - 3846180 3" diameter
Water Pump - 3890419
Power Steering - 3770509

From my research, I believe these are correct. However, my main concern is whether or not the 2 groove crank pulley is suppose to be a larger diameter than the power steering add-on pulley?

[Joe R.]

Re: 1966 L79 with A/C and Power Steering - correct pulleys

I'm trying to confirm the correct pulleys for a '66 L-79 with power steering and A/C. I have the following pulleys:

Crankshaft - 3850838 BG 7-1/4" diameter
Power Steering add-on - 3751232 6-3/4" diameter
Alternator - 3846180 3" diameter
Water Pump - 3890419
Power Steering - 3770509

From my research, I believe these are correct. However, my main concern is whether or not the 2 groove crank pulley is suppose to be a larger diameter than the power steering add-on pulley?

Hi David:

My knowledge about pulley combinations is mostly focused on the 1967 configurations. However, I think the 1966 configurations were most likely the same as 1967. For 1967, all the part numbers you list are the same except for the alternator pulley. The 3846180 alternator pulley that you list would be correct for a 1967 L79 with A/C but not PS. In 1967, when the L79 was equipped with BOTH A/C and PS, the alternator pulley was typically the standard-groove 3909817 or sometimes the standard-groove 3875968 (both were used in 1967).

It's important to understand that, in general, the pulleys used on the L79 were wider "deep groove" pulleys, while the pulleys used on the 300 hp engine were "standard-groove" pulleys.

However, when the L79 was equipped with BOTH A/C and PS, it used the standard-groove pulley set from the 300 hp configuration. The reason was that with the PS belt in the forward-most third groove, the L79 deep-groove pulley set had insufficient frame clearance for putting the PS pump pulley in the forward-most third groove. So, this specific configuration of the L79 used the 300 hp standard-groove pulley set.

Functionally, it's okay to use the deep-groove 3846180 deep-groove alternator pulley with the other pulleys you list, because the mis-alignment with the standard-groove pulleys will be only 1/8 inch and the belt is very long. However, the correct alternator pulley is a one-piece (fan and pulley combined) standard-groove pulley.

BTW, another little-known factoid is that the L79 with BOTH A/C and PS used a different water pump than all the other L79 configurations. The difference was that for use with the standard-groove pulley set, the pressed-on flange for attachment of the fan clutch was placed for use with the standard-groove pulley set, 1/8" farther back than the deep-groove position. This subtle difference was the reason for having the separate "HP" code for an L79 that had BOTH A/C and PS.

So, if you experience mis-alignment between the 3890419 water pump pulley and the 3850838 crank pulley, it is likely due to the water pump flange being in the wrong position.

Lastly, to directly answer your question about the difference in diameter between the 3850838 crank pulley and the 3751232 PS add-on pulley, that difference is to be expected. For A/C applications, GM wanted to spin the water pump and fan faster than the non-A/C confogurations. So, they made the crank pulley diameter larger and the water pump pulley diameter smaller than the non-A/C configurations. The result was a 22% increase in the speed of the water pump relative to the speed of the crankshaft.