Idler Pulley Belt

[Andrew S.]

I have a 1968 427 A/C, P/S equipped car. The idler pulley on the car is
grooved, which I believe to be correct. The 68 Corvette assmebly manual shows a grooved idler pulley configuration. I have noticed 69-72 cars with
smooth idler pulleys.
I am having trouble locating the correct belt for the idler pulley setup. The assembly manual lists the GM part number for the idler belt as 3940990. Well I ordered that belt, and it is way too small. I can't even get it over the water pump pulley and the crankshaft pulley. Does anyone know the correct part number for the belt that goes over the water pump pulley, the cranskaft pulley and the idler pulley for my car? All the other belts, which I ordered using the assembly manual as a reference, fit fine, including the A/C, A.I.R., alternator,and P/S, so I am pretty sure the car has the correct pulleys.

[Joe L.]

Re: Idler Pulley Belt

Andy-----

You're correct; your 68 does use an idler pulley with a groove. This idler pulley assembly, GM #3925592, is unique to 68 Corvettes with big block and AC. It's also quite rare and not reproduced, so you don't want to need one. The 69+ big block with AC idler uses a completely different assembly (bracket and pulley), with a non-grooved pulley. This assembly is reproduced.

As far as a belt goes, every reference I've checked shows the required belt to be a 33-1/2" length. In fact, that's what the GM #3940990 belt was. This belt was a 15/32" wide belt. The other belt used for your 1968 application was a GM #3925595, which is a 3/8" wide belt, but also of 33-1/2" length. To be honest, a 33-1/2" belt seems too short to me, too, but I can't find any other specification. So, I guess you'll have to try different length belts until you find a length which works. This should not be too difficult. In fact, what you might do is to set the idler adjustment so that it's in the middle of its range. Then, run a string around the 3 pullies in the system (crank, water pump, and idler) and measure the length of the string. That's pretty close to the belt length that you need.

As far as pullies go, you might want to check yours. Your crank pulley should be a GM #3911013. This is a stamped steel pulley of 7-47/64" OD. Your water pump pulley should be a GM #3876846. This is a 2 groove, stamped steel pulley.

[Andrew S.]

Re: Idler Pulley Belt

Thanks for the advice Joe. I was planning on using a vinly measuring tape to take the measurement. I'll probably do the string and tape both and see what I come up with this morning. I'll post a response just in case any others have this same problem.