C3 Drive Belts -Again!

[Michael C.]

Sorry for bringing up this much discussed topic, but after reading many threads on the topic, I'm still uncertain of the "correct" answer. Also, I can't find where (if) the A.I.M. addresses my combination.

I have a 69 427/390 with A/C and P/S - all original pulleys. It's the water pump (to crank) belt that's a mystery to me. My configuration uses the tensioner, not an idler pulley. I know the "secret" to changing the belt, and I know that the length (and related width) is critical.

I've seen some posts indicating confidence in Quanta's identification of belts. Is there a consensus that their listing in the attached image is correct?

Thanks all,
Mike

[Joe L.]

Re: C3 Drive Belts -Again!

Sorry for bringing up this much discussed topic, but after reading many threads on the topic, I'm still uncertain of the "correct" answer. Also, I can't find where (if) the A.I.M. addresses my combination.

I have a 69 427/390 with A/C and P/S - all original pulleys. It's the water pump (to crank) belt that's a mystery to me. My configuration uses the tensioner, not an idler pulley. I know the "secret" to changing the belt, and I know that the length (and related width) is critical.

I've seen some posts indicating confidence in Quanta's identification of belts. Is there a consensus that their listing in the attached image is correct?

Thanks all,
Mike

Mike-----


Your application uses an idler pulley, not a tensioner. A tensioner is a spring-loaded device which maintains proper belt tension automatically. An idler pulley must be manually adjusted to maintain proper tension.

I hate to disagree with Quanta inasmuch as they should have a lot of experience in this area of belts. However, I believe the original belt was a GM #3925595 which is a 33-1/2" X 3/8" belt.

[Michael C.]

Re: C3 Drive Belts -Again!

Thanks for the clarification on the terminology Joe.

When I last changed belts (many years ago) I used a Gates 7320 with specs of 32.58 x .410. That has worked with the idler pulley pushed toward the water pump almost as far as it can go. I suspect the 33.5 will be too long. I guess this will be another one of those trial and error experiences.

Mike

[Joe L.]

Re: C3 Drive Belts -Again!

Thanks for the clarification on the terminology Joe.

When I last changed belts (many years ago) I used a Gates 7320 with specs of 32.58 x .410. That has worked with the idler pulley pushed toward the water pump almost as far as it can go. I suspect the 33.5 will be too long. I guess this will be another one of those trial and error experiences.

Mike

Mike------


I can't find a definitive spec but I believe the 3940990 is about 33" in length. So, maybe Quanta has it right after all.

[Tom L.]

Re: C3 Drive Belts -Again!

A few years ago I went through the same thing on my '72 with the captive belt (I think it's the same belt). Either the idler pulley couldn't tghten the belt enough or the belt was just too short to go on. Didn't get a proper fit until I purchased the quanta belt, none of the FLAPs belts would fit well enough. I can look up the part number if you'd like. Have fun!!

[Michael C.]

Re: C3 Drive Belts -Again!

At this point I think I'll order both water pump belts from Quanta - 3952336 which is supposed to be 31.86 x 15/32 (w/o A/C) and 3940990 which is supposed to be 33.5 x 15/32 (with A/C).

The A.I.M. shows the idler pulley INSIDE the belt (see image), but in my case the pulley has always been OUTSIDE the belt. I can't see it going any other way because the pulley is flat, not grooved. The longer belt might be correct if the pulley was actually inside the belt???

Tom, if you have a BB with A/C, and your belts are routed as indicated in the attached image (idler outside the belt), and you can find the part number w/o much trouble, the number might help clarify the situation. Thanks.

[Tom L.]

Re: C3 Drive Belts -Again!

I'm not expert but I've never seen that arrangement on a vette. The belt in questionis on the inside of the idler pullry on '772's.

[Joe L.]

Re: C3 Drive Belts -Again!

At this point I think I'll order both water pump belts from Quanta - 3952336 which is supposed to be 31.86 x 15/32 (w/o A/C) and 3940990 which is supposed to be 33.5 x 15/32 (with A/C).

The A.I.M. shows the idler pulley INSIDE the belt (see image), but in my case the pulley has always been OUTSIDE the belt. I can't see it going any other way because the pulley is flat, not grooved. The longer belt might be correct if the pulley was actually inside the belt???

Tom, if you have a BB with A/C, and your belts are routed as indicated in the attached image (idler outside the belt), and you can find the part number w/o much trouble, the number might help clarify the situation. Thanks.

Mike-----


The GM #3952336 is definitely the wrong belt for you. That is the belt for non-AC big block applications. It is a special length belt for the applications which use no idler. Therefore, the belt goes between only the waterpump and crankshaft pulleys and therefore requires a exact length belt (that's why it's specified as 31.86").

I believe the reason for the incorrect depiction in the drawing is because of a likely carry-over from 1968. The 1968 with AC application used a unique-to-that-year idler pulley assembly. That assembly included a grooved pulley, just like all the other conventional pulleys. For 1969-74, the idler pulley changed. The pulley used with this assembly was not grooved and contacted the outside surface of the belt, not the "V" surface.

[Michael C.]

Re: C3 Drive Belts -Again!

Thanks for the info Joe. I've got Quanta belts on the way. I'll report back after determining what fits.

[Michael C.]

Re: C3 Drive Belts -Again!

I received the Quanta belts today. Determined that their B-143 (GM 3952336 31.65" x .469") is the correct length for my car. This is the belt described on their site as for 427's w/o A/C (my car has A/C).

If I use their B-144 (GM 3940990), which is described as for 427's with A/C, the idler pulley reaches the end of its travel before the belt is even close to tight. In fact, the idler pulley strikes the water pump pulley near the end of travel. Therefore, I'll return this belt.

Now that this issue is resolved, I have a related question. Quanta describes all of the belts I ordered as "cloth-wrapped." I suppose this is as opposed to "cut" belts. Three belts are like the one on the left in the attached photo. The other belt is the one on the right. I'm certainly no belt expert, but are the three belts (left side) actually cut belts (not cloth-wrapped as advertised)?

BTW - the belt on the right is supposed to be for the P/S to alternator and is .380 (3/8") wide while the others are .469 (15/32"). Does this sound right? The alternator isn't installed at the moment, so I haven't checked its fit.

Thanks all.
Mike