1970 350/300 Idler Belt Too Loose

[Steve Nelson]

I acquired a 1970 350/300 Convertible w/PS no/AC last year to use a driver when it's not raining. As one might suspect after almost 35 years its current condition is not quite as it was at the time and point of final assembly. I'm trying to get the engine compartment back to a stock looking (not necessarily with correct date coded parts) and functional appearance.

One problem I'm facing is that the idler belt (middle belt between the water pump and crankshaft - #3941116) is too loose. The alternator belt (inner belt - #3959166) fits properly, and the power steering belt (outer belt - #3874424) fits properly. These are all new belts I ordered from Quanta.

These are the pulleys that are on the vehicle:

Water Pump: #3890419 Dual Groove
Inner Crank: no part number visible, but it's about 7 1/4" OD Dual Groove
Outer Crank: #3751232 Single Groove
Power Steering: #3770509 Single Groove

Since the Inner Crank pulley had no visible part number, I ordered a #3850838 pulley (which is the correct inner crank part # according to page K184 of the 70 AIM) from Corvette Central, and its OD (about 7 1/4") is identical to the inner crank pulley that was already on the car.

I believe the correct OD for the #3850838 inner crank pulley is 7 21/64. The difference of ODs between 7 21/64 and what I have is at most 5/64 or just over 1/16, which I don't think would account for the slack in idler belt that I'm seeing.

I'm confused and pretty much stumped at this point.

What am I missing? Any thoughts, ideas, or suggestions would be much appreciated.

Thanks,

Steve Nelson

[Joe L.]

Re: 1970 350/300 Idler Belt Too Loose

Steve-----

First of all, there is no "idler" belt, per se, used for your application. The belt that you are referring to is called a "captured" or "captive" belt.

Second, you have the correct pullies. Although the GM spec for the crank pulley is 7-21/64", the actual, as-measured OD of GM pullies is often a little different than the "official" GM dimension. For the most part, the only other possibilities for an "incorrect" pulley would be 6-5/8" "official" OD. Obviously, you don't have one of those, so you pretty much have to have the 3850838.

With a captive belt, even a small difference in pulley OD can cause a belt fitment problem. However, I don't think that you have a pulley problem. First of all, I strongly suspect that the pulley that's on the car is original. Second of all, you've essentially confirmed that by obtaining a reproduction of the 3850838 and finding that it's identical to the original in OD. That's pretty powerful evidence that the pullies are correct.

While it is the correct part number, I'd check to be sure that the 3941116 belt that you have is made to the proper length. It should be 35-1/8" long and 3/8" wide.

[Steve Nelson]

Re: 1970 350/300 Idler Belt Too Loose

Joe,

Thanks for the input. Should the 3941116 belt length of 35 1/8 be the length of the inner circumference or the outer circumference?

I may try to get another 3941116 belt and see if it's a bit smaller than the one I have, if a little change in size can cause a fit propblem.

How much play should the "captive" belt really have?

Thanks,

Steve Nelson