1968 pully diameters:
Standard car no AC, no PS- crank pulley 7-1/4", water pump pulley 8";
car with PS no AC- crank pulley 7-5/16", water pump pulley 8";
car with AC no PS- crank pulley 8",water pump pulley 5-11/16";
car with AC and PS- crank pulley 8",water pump pulley 5-11/16".

Seems obvious the Pontiac engineers wanted the water pump turning faster with AC than without AC.
Since the alternator pulley is pretty much the same dia in all applications it will be turning a bit faster with AC as well.

My question is, since the AC cars have a single groove pulley between the damper and the two groove pulley are the non AC crank pulleys shallower dished than the AC pulleys to make up for the 1/16" or whatever the thickness of the single groove pulley is? Or to put it another way, if I wanted to put a smaller diameter AC water pump pulley on my non AC car, in order to get the alignment would I need to shim out the crank and alternator pulleys?

I dusted off the old slide rule and did some rough calculations.
For argument sake I used the ouside dia measurement for calculating the circumference and that measurement for belt travel and am not considering belt slip.
Dia. of the PS crank pulley is 7-5/16", circumference is 22.96",
Dia. of the PS water pump pulley is 8", circumference is 25.12",
Dia. of the AC+PS crank pulley is 8", circumference is 25.12",
Dia. of the AC+PS water pump pulley is 5-11/16", circumference is 17.86".

Ratio between the PS pulleys is 0.914/1,
Ratio between the PS+AC pulleys is 1.410/1,
Ratio between a PS crank pulley and a PS+AC water pump pulley is 1.29/1.

If I put a PS+AC water pump pully on my PS only engine the water pump would turn 1.29 times for every revolution of the engine. That's not as high as the PS+AC equipped engine but more than the PS only equipped engine.

At 750 RPM idle the PS crank pulley moves the belt 17,220", that belt travel will turn the PS water pump pulley at 685 RPM,
At 750 RPM idle the PS+AC crank pulley moves the belt 18,840", that belt travel will move the PS+AC water pupmp pulley at 1,055 RPM,
At 750 RPM a PS crank pully will move a PS+AC water pump pulley at 964 RPM.

Now you're most likely thinking, why in the **** would someone tell us this and who the **** cares?
My non-stock engine generates a bit of heat. Going on the assumption Pontiac engineers had a larger pulley ratio for their AC cars than for thier non-AC cars is to handle more heat created by the addition of AC, I also want to turn my water pump faster. The only after market overdrive pulleys I can find are for the 69 + up 11 bolt water pump. That's why I'm wondering about putting an AC waterpump pulley on my non-AC engine. I presently have a 7-1/8" OHV six water pump pulley in front of a fan spacer installed, it gives me enough overdrive but it was dented and try as I may I cannot staighten it enough to get it to quit squeeking. That's why I'm asking about the dish of the AC water pump pulley.
I measured the dish of an 8" water pump pulley #8786819 from the inside pump mating surface to the back of the outer rim at 1-15/16". Does anyone out there have an AC water pump pully #9788886 they will measure for me?
Thank you.

Wow, I'm not even sure I read all that. Pretty sure I lost consciousness after "68 pulley" but I added an A/C pulley behind my two groove crank pulley and am still using my stock WP pulley. No alignment issues to speak of. I have yet to charge up the system so I can't speak to any additional heat issues.

Someone correct me if I'm wrong but I have always used a spacer in place of a AC pulley. I have taken the AC pulleys out and found s factory spacer shims that go between the harmonic balancer and the crank pulley.

Thank you,
Makes sense if the AC pulley is removed something would have to shim the two groove pulley out.


Just looked at it again and almost went to sleep myself.
How do I delete this post?
I woke up at 2 this morning and realised the dish in the water pump, power steering and alternator would have to stay the same and the crank pulley is the only one that may be affected by the AC.

I will try to find an AC water pump pully to bolt on, then match belts.

I have PS & AC and I want a smaller Pully!