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I've have a 3.75 crank pulley and a 2.5 alternator pulley for a 1986 GL 1200 anyone have an idea for a v- belt ?
 

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Wrap a piece of rope or cut an old belt to fit. Bring it to the store and get that size plus one bigger and smalleer. Take back what doesn't work. :) They do that a lot for customers.
 

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Using V-belts, they usually size by width and inside circumfrence ( length), … so I'd pull a good (not stretchy) string around the pulleys in place and mark meeting, remove, measure, and allow for adjustment and try that first



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I was accumulating stuff to do it with three groove serpentine belt, had all parts, then "life" happened = change in plans = sold the 1200 last year, I thought I had a list of V-belts as I had considered them, but then found these alloy pulleys above. Belt list below is for serpentine belts.

NAPA Part No. - Size 3 rib serpentine belts

25030280 - 28.73"
25030290 - 29.72" (Called out in the Pooboy Instructions)
25030295 - 30.111"
25030299 - 30.508"
25030300 - 30.755"
25030310 - 31.68"
25030323 - 32.915"
25030328 - 33.46"
25030334 - 34.024"
25030340 - 34.685"
25030350 - 35.67"

Your 3.75 crank pulley and a 2.5 alternator pulley = 1.50:1 ratio. At 7,500 engine crank rpm, the alternator will turn 11,250 rpm.
 

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No, V belts are sized by the outside circumference. You turn them inside out to use a belt sizer. The width should be the same as the wide part of the pulley. A V belt should never run in the bottom of the groove. Length could be estimated by the distance center to center of the pulleys + 1/2 the circumference of both. 5.89 + 3.93 + the C to C distance.
 

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No, V belts are sized by the outside circumference. You turn them inside out to use a belt sizer. The width should be the same as the wide part of the pulley. A V belt should never run in the bottom of the groove. Length could be estimated by the distance center to center of the pulleys + 1/2 the circumference of both. 5.89 + 3.93 + the C to C distance.
Wish I had a head scratching smilie. Could uses it now. :)
 

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Must be a regional thing, maybe depends on which side of the "Big Muddy" one is on? I've been buying belts for 50+ years, never yet seen a parts man turn one inside out to pull it on gage. After retirement, I worked a few years at our Advance, I just measured their belt by pulling it on the gage & reading the length, pull that belt from rack. Somehow I made them happy, no returns.

Pulling the cord would at most give you a hair short belt, using the cord to get a length on a 1/2" wide V-belt isn't gonna let the belt drop to center of a 1/2" wide V-pulley. You can't make a 1/2" V-belt ride the bottom of a 1/2" V-pulley, until you wear the belt and/or pulley out. If it hits bottom, you need a wider V-belt or narrower pulley.

My method will get you there in the ballpark.

Or ...
... Length could be estimated (by adding the distance center to center of the pulleys doubled ... to 1/2 the circumference of both pulleys) Ctr to Ctr x 2 + PD x 3.1417 (pi) / 2 + PD x 3.1417 (pi) / 2 … where Ctr is center of pulley & PD is pulley diameter.
 

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The commercial gauges take that into account and have lines for each belt width. On a plain measuring gauge you would need to flip the belt. Also there are plenty of online belt calculators from all the pulley makers that will give you a belt size from a center to center, since many pulleys are grooved for more than one belt width and that makes them hard to measure. If you know the part number you can just plug that in and let them figure where the belt rides on that pulley.
 
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