Shorter alternator belt, Mercruiser 140HP

[fishrdan]

I'm putting a new alternator on my 72 Mercruiser 140 HP (SN 2913009) and the old belt is too long, new alt's pulley is smaller, the belt just starts getting tight when the alt hits the end of the slotted bracket. How much will I need to shorten the belt to get the alt centered in the bracket, 1"??? The old Gates 7410 belt is 41 5/8" long, and I'm thinking of getting a Gates 7400, 40 5/8", sound right?

I was thinking of swapping the alt pulleys as the old pulley is a deep groove pulley and the new pulley has the belt a touch above the top of the pulley, maybe 1/16" like the water pump pulley. Only thing stopping me from swapping the pulley (and not having to change belts) is the new alt might have the smaller pulley to spin the alt faster. This is good for me as I troll a lot and having the alt spinning faster will help charge my trolling motor batteries when the main is running. I'm thinking shorter belt, unless you think swapping pulleys would be better.

 

[JustJason]

Re: Shorter alternator belt, Mercruiser 140HP

Trial and error bud. But try this non scientific method and see if it works for ya...

First, measure the length of your belt.
Pull the alternator to 1 side when the belt is the absolute loosest.
With some wire snips... Start cutting your belt in 1 inch increments until the 2 ends just meet pulling it tightly, with the alternator swung to 1 side.
Remeasure the belt.
Buy a new belt 1 inch longer than that.

 

[fishrdan]

Re: Shorter alternator belt, Mercruiser 140HP

Ha! Why didn't I think if that I did pinch it over, to see how much slack there was, but there's only so far you're pinching over a belt... I'll just cut it, pull it tight and see how much too long the old belt is, see what's left over.

Any thoughts on the deep groove pulley and if I should just swap it over to the new alt?

 

[Eric 3.7LX Alpha 1]

Re: Shorter alternator belt, Mercruiser 140HP

I'm putting a new alternator on my 72 Mercruiser 140 HP (SN 2913009) and the old belt is too long, new alt's pulley is smaller, the belt just starts getting tight when the alt hits the end of the slotted bracket. How much will I need to shorten the belt to get the alt centered in the bracket, 1"??? The old Gates 7410 belt is 41 5/8" long, and I'm thinking of getting a Gates 7400, 40 5/8", sound right?

I was thinking of swapping the alt pulleys as the old pulley is a deep groove pulley and the new pulley has the belt a touch above the top of the pulley, maybe 1/16" like the water pump pulley. Only thing stopping me from swapping the pulley (and not having to change belts) is the new alt might have the smaller pulley to spin the alt faster. This is good for me as I troll a lot and having the alt spinning faster will help charge my trolling motor batteries when the main is running. I'm thinking shorter belt, unless you think swapping pulleys would be better.

If the parts store where you're getting the belts from is a short drive, I'd get the 7405 and 7400, one of the two most likely will be the one. Return the wrong sized belt after you find out the correct one, and swap it for an extra belt that works. One of those things that are very nice to keep a spare on the boats anyways.

 

[rlambert]

Re: Shorter alternator belt, Mercruiser 140HP

Ya know, it's funny I had the same problem. All I did was mount a small metal bracket I found layin around to the existing slot bracket and to the alternator allowing it to swing out further and kept the same belt. Works great. I also have a Mercruiser 140. Must be something with the after market alternators.