W.E., I still lean to some other explanation.
As you know, my "unneeded" parts list says that I have a '64 544594 water pump pulley from a '64 GP with A/C. Have not found it yet, but believe it will likely show same groove dimensions as all the ones you've seen.
But I do not think a belt description error makes sense.
The A/C belts are generally ALL shown as 1/2" belts thru the '66 MY (and likely beyond) in the '66 MPC.
And this includes numerous belt p/ns issued both before and after 9772317.
It is illogical to me that the '64 would have been detailed with a 3/8" belt when all previous and subsequent applications show 1/2".
And if they are ALL described incorrectly, that also defies logic. Surely they did not keep making the same description mistake time after time.
I did find one bit of info in the Parts History.
When 9772317 was discontinued (I found it in a 1/82 dated History at PartsWiki), it shows to use 9433775.
This belt is 7/16"x61.5".
Belt p/n 9433929 was 1/2"x62". If 9772317 was 1/2", why GM did not recommend this one instead I can't explain.
I found another discontinued 1/2" width belt (9782706) first superseded by 9786985 and then 9786985 was discontinued and shown to use a 7/16" wide belt (9433758).
Andre's post got me to thinking. I'm sure the 1/2" indicated the width, not depth, but I turned to the '64 Tempest AMA specs to find this:
Belt H is the V8 A/C Drive Belt.
36 deg, that checks with the MPC.
62.0 nominal length (SAE), that checks with the MPC.
Width is shown as .47, not quite 1/2". Not sure what to make of that, but in the Belt Chart that shows the later model belt p/ns (ie., 9433775), the 7/16" belts are indicated as .440, the 1/2" belts as .500.
I don't know what to make of that except that perhaps GM preferred to substitute with a marginally narrower belt instead of a marginally wider belt.
Maybe there has been a v belt dimensional change since '64 such that a 1/2" wide belt became marginally wider than a 1/2" wide belt had been in '64.
I'm stretching here and not oblivious to the fact that the water pump groove may not be wide enough for a 1/2" width belt, whether .47 or .500 wide.
The AMA specs show the '64 6 cyl A/C belt dimensions as 55.5" long, .47 width, consistent with the dimensions in the MPC for p/n 9775899.
Also discontinued 1/82, it says to use 9433762, 55" long, 7/16" width.
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