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Joined
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71 Mach 1 429CJ
is it the original alternator? The pulley has a small diameter, it makes the alternator rotate much faster than the engine.

I've restore mine last year and changed the main bearing. Its quite a large one for such a relatively small device, may be yours is damaged/corroded and need be replaced.

it's easy and inexpensive to replace it and can't hurt, even if the cause of the failure would be something else.

The shaft is pretty heavy, and if the bearing locks somehow, the weight/inertia could at the speed it turns snap your belt just like that..

 
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Make sure the pulleys don't have any nicks, gouges, or worn out spots in them that can damage the belt. Make sure everything is turning so the belt doesn't stop moving and burn through. In addition to being too tight, if the belt is too loose it will flop around and possibly climb the side of a pulley or catch on something.

 
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Ok I checked everything . Every think looks correct installed new belt yep it broke that's number 4. Going to look for a larger pulley for the alternator what y'all think. Thanks

 
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I agree with David, missing something.

How about a bent fan blade nicking the belt? Alternator or water pump locking up after it runs for a while.

Have you watched it while it's running? No flopping or vibrating belt? No wobbly pulleys?

 
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I'm glad you figured it out and thanks for post what the problem was, it helps others in the future. Chuck

 
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