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Ford Tractors Discussion Forum
Show Parts for Model:

Alternator pulleys

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Ultradog MN

05-21-2014 04:57:59

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I'm converting a 3 cyl tractor to an alternator.
Pulley on the new 10-SI I bought from Napa was for a 7/16" wide belt. I need a 1/2" belt.
So I went back and bought a 1/2" pully ~ $9.
The 1/2" pully is 3 3/16" dia vs 2 5/8" on the original 7/16".
That's about a 30% difference.
I know an alt will charge at idle but am curious how that 30% will affect charging rate on a slow revving tractor engine.
Back when the machine shop was still operating it would have been an simple task to widen the small pully on a lathe. But alas, no lathe now.
Thanks in advance...

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05-21-2014 14:30:15

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 Re: Alternator pulleys in reply to Ultradog MN, 05-21-2014 04:57:59  
It will actually increase the charge rate provided it's got enough adhesion on the smaller pulley to adequately drive the alt at full load. I usually just get the pulley widened...


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Sean in PA

05-22-2014 08:49:36

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 Re: Alternator pulleys in reply to RodInNS, 05-21-2014 14:30:15  
Quoting Removed, click Modern View to see

The new pulley is larger, not smaller.

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05-22-2014 14:01:45

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 Re: Alternator pulleys in reply to Sean in PA, 05-22-2014 08:49:36  
I misread his original post. It's still not going to make any difference.


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John in La

05-21-2014 13:35:58

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 Re: Alternator pulleys in reply to Ultradog MN, 05-21-2014 04:57:59  
I would think the slight difference in speed is made up by the smaller draw and need for recharge of the battery.

You can get a smaller pulley but most I have seen are in the $25 range.
I have even read where someone took a saws all and cut the 7/16 pulley in half and added a flat washer between the two halves making it wider.

I just bought a 7/16" belt of the proper length and let it ride deep in the crank pulley. Been working great for several years that way.

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Errin OH

05-21-2014 10:31:29

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 Re: Alternator pulleys in reply to Ultradog MN, 05-21-2014 04:57:59  
You would need to know the dia of the engine drive pulley to determine the overall increase / decrease.

My "guess" is you will find the tractor's drive pulley is smaller than a Chevy engine. But even if it is the same, the alt will be turning slower due to the bigger pulley on the alt. My ford with auto alt doesn't charge below 8-900 rpms.

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05-22-2014 06:09:08

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 Re: Alternator pulleys in reply to Errin OH, 05-21-2014 10:31:29  
do you do alotta work with your tractor at under 800 rpms?

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Errin OH

05-22-2014 20:28:11

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 Re: Alternator pulleys in reply to soundguy, 05-22-2014 06:09:08  
Nope and that is why I don't plan to do anything about it.

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05-23-2014 08:13:19

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 Re: Alternator pulleys in reply to Errin OH, 05-22-2014 20:28:11  
Yep, mine don't idle much either

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Sean in PA

05-21-2014 10:02:44

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 Re: Alternator pulleys in reply to Ultradog MN, 05-21-2014 04:57:59  
The original pulley on the genny on my 3 cylinder 4000 is 3 1/2". Folks who do the conversion to an alternator with the tach drive have to use the same size pulley as the original generator for the tach to be accurate, and I've never heard of any charging problems from those folks .

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05-21-2014 05:48:53

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 Re: Alternator pulleys in reply to Ultradog MN, 05-21-2014 04:57:59  
once she's excited.. I doubt you will have a problem keeping up with charge rates to refill the battery and run a couple 55w headlamps.

Unless you have an ac and some KC lamps and a huge stereo on that tractor.. should be fine at even less than pto rpms..

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