I don't know what's going on with my pulleys, but it's driving me crazy! Plus the parts counter guy thinks I'm nuts! Coming in day after day buying another belt. Here's the deal: BBC W/ AC, PS. 3 groove -bottom, PS has 2 grooves, water pump-2 grooves, AC & alt has one. Just recently, without changing anything else, my alternator belt keeps coming off 1st rev over 4,500 rpm. They are LINED up correctly. straight as an arrow. Belts are new. Nothing seems to be loosening. I have a brand new Edelbrock water pump and a GM clutch fan. Funny thing is I've put 8,000 miles on this exact same set up without trouble. Any ideas?

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Triple Black 69 SS 396

Make sure your alternator bearings are good. Loosen the belt and see how it spins. It should turn freely and smoothly.

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Elcaminos are special!

I'd rather walk around with a Chevrolet hubcap in my hand than drive a Ford


work in progress
Big James Elky(AKA Ol Paint) (http://www.chevelles.com/showroom/Big%20James4XL-Ol%20Paint1.jpg)
another shot of Ol Paint (http://www.chevelles.com/showroom/Big%20James4XL-Ol%20Paint2.jpg)

Jimmy P, most people don't realize it but the harmonic balancer is what throws the belt off. Its job is to dampen crank twist and reflex. When it fails to do this job it creates belt slap as the motor increases or decreases rpms. This belt slap or vibration makes it way out to the alternator pulley or whatever and then walks off the pulley. You can actually watch and see this happen, but don't let the belt slap you in the face! The more the pulley is out of alignment the easier it comes off. I run an ATI balancer on my motor with stock pulleys and make blasts past 8000 and have never thrown a belt.

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Chevelles forever....Member #401
67SS w/dual-quad 509
71SS454 LS5 (http://chevelles.com/showroom/71SS454-1.jpg)
97 C5

Jimmy,

I too had this problem. Was about to go nuts! Everybody said " get a deep groove pulley for the altenator." Which is an option. I made a bracket that mounts on the front alternator brace that keeps the belt on. The bracket does not actually touch the belt until you rev it up and that "wave" int the belt is produced. Then it keeps it from going too far and jumping the alternator pulley. I will go into more detail if you are interested. I have not had a belt jump off since. Can't see the bracket unless you look real hard.

Dave

I'm going to go NUTS!! I put the belt back on and ran it to 5000. Ba Boom! Off I took the fan off thinking it might be bent. Same thing, only this time, the belt landed 30 ft away and I had a tough time finding it! I ran the motor without the fan & belt. Lower pulley looked fine to 5,500 rpm no wobbles no nothing. The Alternator pully had a little play in it. Bought a brand new one and a new belt. 5,500 again and Bam! Off again. Hey 454 Malibu, how do make that bracket? I guess a Moroso deep groove is in store too. The Balancer looks fine, but how do you know for sure?

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Triple Black 69 SS 396

Just a thought... Can you tell if the belt is climbing up a particular side of the pulley(s) consistently or whcih pulley it jumps off first (don't loose an eye). When lining up the belts consider that the pulleys need to be aligned not only in a straight line, but also co-planar. good luck.

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Ron Cook
'69 SS 300 Deluxe HT
(Bow Tied's 2¢ worth.)

OK, what's co-planer? Please explain

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Triple Black 69 SS 396

We had the EXACT same problem with my son's 71 BB. I was totally prepared to spend the bucks to replace all the pulleys with deep groove pulleys. I thought I would start by doing it one pulley at a time, starting with the alternator pulley first (it was the cheapest) I ordered 3829387 pulley from my Chevy dealer, installed it, AND THAT'S IT! NEVER, NEVER, NEVER has the belt tried to jump again, not even at 6500rpm (why buy 3 pulleys when one fixed the problem).
One additional note. The new pulley sticks out just a tiny bit more than the original, so I had to shorten the alt spacer just a tiny bit. Too easy.
I think my cost for the pulley was about $22. It is NOT a stamped steel pulley, it is SOLID steel, very tough.

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Tom Parsons

Hey DZ, Is that a 7/16" pulley? My 74 year old Hot Rod buddy told me to use 7/16" pullys with a 3/8" belt. I didn't know of such a thing sounds pretty simple though.

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Triple Black 69 SS 396

Well, I never did measure it and my son's car is not here at this time (he is going to OU). But, for sure, the pulley is PLENTY deep. It also is a little larger diameter and I fully expected to need a bigger belt, but there was enough adjustment in the alt bracket that we kept the original belt. Like I said before, just installing the deep alt pulley totally cured the problem.

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Tom Parsons

when pulleys are in check, the most common reason for loosing the belt is the altenator fan blades bend out at higher rpm, contact the belt and away it goes. The cure is a late model "plastic" altenator fan, or the deep groove pulley that is a larger diameter and hides the fan blades from the belt.

don't know what kind of belts u run but if there those cheap ones(about $4)toss them.......get some good dayton's(about $15 each)they can make all the difference in the world .......

Jimmy,

Tom (DZAUTO) mentioned a deep groove pulley for 22 bucks. A deep groove pulley will solve the problem. I was looking for a deep groove pulley, but could only find one for around 40 bucks. That was too much for me.Although I would also take redbuick's advice and get a good belt. The two together will fix it. I will do a drawing of the belt guide I fabricated and email it.
I used 1/4" x 3/4" crs. The bracket is about 1 1.5" long with a slight bend on one end so when the belt ripples there is a nice ramp on the bracket for the belt to ride up. Belt does not touch the guide until it ripples from the G forces. Belt hasn't come off since and there are no noticable wear marks on the outside of the belt.

I just sent you an email w/a sketch of the
bracket I made. Not to scale, but gives you
an idea.

[This message has been edited by 69454Malibu (edited 02-05-2001).]

My small block has just started throwing belts too. Must be the way the planets are lined up. Why does it just all of a sudden start?

Planes are 'imaginary' flat surfaces. Co-planar means that two items are in the same plane. In this case, the two pulleys' grooves must be on the same imaginary surface.

Most commonly we look down on the pulleys from above an ensure that they are lined up ..but consider the axis of rotation of the pulleys (imagine the shaft that the pulleys mount on are super long, like 3 feet) Are these axis (or shafts) parallel? If these axis aren't parallel, from a top perspective AND a side perspecitve, then the pulleys aren't totally aligned. Typically, it isn't that critical because the belts can flex through some mis-alignment. (reference: In industrial machinery, there are tools that clamp to the bases of machines and use lasers to align drive shafts/pulleys/sprokets and such to reduce wear & increase lifespan in chains, belts etc.)

In a high perf application, this type of thing may come in to play. I am not suggesting getting a laser alignment or anything like that, but be conscious of this when aligning your belts - using thin washers to shim the bottom of the alt mount for example. If the pulleys are the same thickness and # of grooves, use long straight edge across one pulley face and measure how far away the face of the other is or if it is at an angle, in several positions.
<sorry for the long post, not sure how to better describe it - all comments welcome>

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Ron Cook
'69 SS 300 Deluxe HT
(Bow Tied's 2¢ worth.)