My 71 Chevelle (small block) kicks off the alternator belt every time I get on the car. Rev it up, belt goes away. This is VERY annoying.

The pulley alignment seemed ok, but its not.

I have the belt lined up pretty straight now. I had to remove the rear alt spacer to achieve the necessary adjustment.

Now, I have measured the spacer, 3 1/4 inches long.
The distance from the alt to the head is only 3 inches.

I have an aftermarket alt on the car could the case be 1/4 thicker????

Does or did GM have different width spacers?

Any help would be great. Hopefully after this is sorted out, I can stop carrying that 1/2-9/16 combination wrench in my glove box!

I just went through this. I would cut down your spacer to keep everything straight. Do you have the right thickness belt? Are you tensioned properly? A deep groove pulley may be required.

Does a deep groove pulley SLOW down the alt?

once a belt is thrown off....its usually best to replace it (or so thats what i was told growing up)

Does a deep groove pulley SLOW down the alt?
No, it should not.
Yes if the belt is twisted at all it will come off eaiser the next time. Replace the belt with new one and make sure the tension is correct. You should not have any trouble with standard pully throwing the belt off unless they aren't lined up correct or the belt has to loose tension on it. Jb

http://www.chevelles.com/forums/showthread.php?t=227593&highlight=pullies

don't use cheap belts. use a good line of belts like gates. I had the same problem on a 72 had all the correct brackets everything lined up but the cheap autozone belts(kelly springfield) would fly off. switched to gates and even bracket raced the car they never came off again.

Just an idea for you but on all of the race cars we use lawn and garden belts. The ones with fabric all around them. They are made to compensate for uneven pulleys as on tractors and such. We have never lost one and we go to high rpms with our engines.

If your tossin belts your alignment must be off or your really turning some RPMs. Check your alingment, with the belt off, by lying something straight in the grooves....a yardstick, straight tubing, or what ever you have. This will show your alignment. As mentioned, a deep groove pulley helps. The belt tossin was more of a problem with the short water pump motors that had to run much longer belts. The Corvette small blocks have a "rub" shoe that is mounted on the alt bracket to keep the belt from flying off the pulley. Also seen where you run a longer bolt on the adjustment lock to keep the belts from climbing off. Your alternator is mounted in the stock brackets isn't it?

My alt is mounted in the stock brackets. But, it is an aftermarket piece of you know what. I am gonna try a new belt and a 3 in spacer today. We shall see.........

This problem (throwing alt belt) is not totally uncommon on Chevelles. A few years ago, we fought this on my son's 71 BB car. The final, and permanent solution, was a deep groove alt pulley from Chev parts dept. It was kind of expensive because it was a solid, machined steel pulley instead of a stamped steel pulley like most pulleys. Never thrown a belt since.
Yes, we had everything lined up as best as we could. Actually, the complete belt, accessory, bracket and pulley setup came off of the engine in his car and there had never been a previous problem. After teardown and rebuilding the engine, everything that was previously on the engine went right back where it had originally been. So we never determined why it kept throwing belts, but the deep pulley cured it.

When I went to a "real parts store" to get a belt today. I had a conversation with the counter guy. He said his Camaro had the same problem and it was caused by flex in the aftermarket chrome alt bracket.

Well after that I went and looked at the pics from when I first bought my car and sure enough it had a chrome bracket. When I rebuilt the motor I had all the pulleys and brackets powder coated. Apparently I forgot it was not original(earlier I said that the brackets were originals). I dug thru the garage and found a GM bracket.

I am gonna try that with a new belt later.

Thanks for all the advice and suggestions so far!