Throwing pulley/alternator belt

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Revoke36

Revoke36

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Hey guys. I keep throwing the alternator belt when I hit high rpm’s. I have the skinniest v belt I could find but doesn’t matter. The belt is slightly off alignment. I’ve shimmied a few bolts but it’s still slightly out of alignment. Tips please? Hope pics explain it better than I can. Thanks.

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  1. Get it propperly aligned.
  2. Get it propperly tensioned
  3. Get correct belt for the pulleys.
My bet is if you setup a go pro camera you would see the belt whipping around and with it not aligned correctly walking up the side of one of the pulleys.

Your photos look like you have a 1/2" wide belt in 3/8" wide pulley.

Also look at the pulleys for bent areas

Also if you are using an aluminum water pump with brackets for a cast iron one or visa versa, that induces an offset issue.
 
Last edited:
Oldmanmopar said:
Get a single pulley alternator. The off set is in the center of the AC dual pulley .
Click to expand...
That’s exactly what it looks like to me; that the belt would be straight in the middle of the dual pulley. So you’re saying a single pulley alternator has a different offset?
 
Going to a larger pulley can affect the voltage you get which can affect the ignition systems ability to get the fire started.
 
Revoke36 said:
That’s exactly what it looks like to me; that the belt would be straight in the middle of the dual pulley. So you’re saying a single pulley alternator has a different offset?
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Yes the pulleys are different neither of the dual V's will line up with a single. The rear is back to far and the front is front to far
 
I just checked a double pulley alternator against a single pulley and the best I could measure the difference of the alignment was about 0.100" or less than 1/8" probably negligible.

67 dart 273.

cast iron water pump
I could not find any better photos (sorry)


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Dicer said:
I would buy a new belt and try it, once a belt flies off, they get crooked.
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I’ve been through several belts. Of different sizes. They all get tossed in the high rpm band. I’m certain it’s walking off since the alignment is offset, albeit by very little.

** Ah, I see what you’re saying. Get a new belt when I get it straightened out cuz any tossed belt has been compromised. Thanks!
 
Revoke36 said:
tossed belt has been compromised.
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Yes. Also use a straight edge from water pump pulley face to alternator face to check alignment.
Gotta ask, how high is high rpm ? Had a customer with a Big Block Chevy that would throw the belt at 7200 rpm.
His problem was Crank Walk. The thrust bearing was the problem.
 
I had the same issue, went with an March aluminum under drive crank pulley and problem solved. Before that super tight worked.
 
Here is something else to think about..............what is the diameter of your crank pulley, what is the diameter of your alternator pulley? Years ago I was having the same problem of throwing belts, it came down to 2 things:
1. the drive ratio of crank pulley to alternator was 1 to 2, so at 6000rpm crank speed, the alternator was spinning 12,000 rpm AND;

2. the alternator was built with bushings instead of bearings causing significant drag (cheep Autozone alternator ), replaced alternator with one using ball bearing construction, problem solved. I would have swaped out pulley on alternator to a larger one if needed to slow alternator down but was not needed.

In solving this problem, working in the driveway, I could watch the belt stretch like a rubber band as rpm climbed; didn't make any difference how belt was adjusted to begin with.

Again something to think about.
Good luck!
 
Does the alternator have the bushing in this ear? If it is missing the alternator will not hold alignment.
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Alan
 
to newer denso alt . made new brackets ,alined as good as i could. started to throw belts @ 6k +. bought an 09 GM belt tensioner fabbed a mount . works perfect. cant provide a photo as car is in storage.
 
JBurch said:
Here is something else to think about..............what is the diameter of your crank pulley, what is the diameter of your alternator pulley? Years ago I was having the same problem of throwing belts, it came down to 2 things:
1. the drive ratio of crank pulley to alternator was 1 to 2, so at 6000rpm crank speed, the alternator was spinning 12,000 rpm AND;

2. the alternator was built with bushings instead of bearings causing significant drag (cheep Autozone alternator ), replaced alternator with one using ball bearing construction, problem solved. I would have swaped out pulley on alternator to a larger one if needed to slow alternator down but was not needed.

In solving this problem, working in the driveway, I could watch the belt stretch like a rubber band as rpm climbed; didn't make any difference how belt was adjusted to begin with.

Again something to think about.
Good luck!
Click to expand...
Yep, you're onto it !!!
 
I have a 340 with A/C, belt alignment is perfect, you can lay a round steel rod in the groove of each pullet & see where the true aligned point is... This 340 even with perfect alignment would toss the belts every time I spun it past 6300 rpms... I tried different brands of belts, I tried extra tension.. Didn't matter, it tossed the belts...

Then I tried an old hotrodder trick, I added a long bolt to the lower alternator bracket, it doesn't normally touch the back side of the belts but it's less than 1/4" from them... When the rpms go up it touches not allowing the belt to grow away from the alternator... Haven't lost a belt since then & I had a 6800 rev limiter on the engine for a few years... Oh, 4.30 gears & Edelbrock heads so it breathes decently...
 
1WildRT said:
Then I tried an old hotrodder trick, I added a long bolt to the lower alternator bracket, it doesn't normally touch the back side of the belts but it's less than 1/4" from them... When the rpms go up it touches not allowing the belt to grow away from the alternator
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Minor improvement on the design...

Add a piece of tubing to the bolt to make a bearing of sorts. Copper tubing even better.
 
I just reduce the belt load by increasing the leverage on the Alt.
 
You could remove the load of/on the alternator by disconnecting the field during high rpms.

Yes you will have lower voltage to power your ignition etc. But there are ways around that through electronics


Questions

  1. How are you using the car. Drag only, street only, combo
  2. What rpm are you going to
  3. What dia are your crank and alt pulleys
  4. What engine
  5. What year
  6. Cast iron or aluminum water pump
  7. What is the part number of the belt you are currently using
 
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