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Alternator belt tension video advice

Jan 15, 2014
62
Morgan 323 Portland
Trying to determine if alternator belt is too loose? I just installed new one, my first time, with this result:


1984 Yanmar 3GMD. Thanks in advance.
 
Nov 6, 2006
8,872
Hunter 34 Mandeville Louisiana
Yup it is too loose. A good push with your thumb at the mid point should only move the belt about a half inch.. Usually, a lever is needed to get enough tension on the belt.
 
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Apr 8, 2011
291
Hunter 36 Deale, MD
Or a screw. Tensioner.

View attachment 186740
There's really no good commercial tool I've found to do this. I own the tensioner pictured above - perhaps the most common. It worked well once, then failed at a weld the second time I used it. You'll see that's extremely common with this tool when you look at the reviews. when it works its VERY handy. I had someone re-weld it for me and am hoping that fixed it. We'll see. I'd much rather use something like this than a crowbar or screwdriver which could damage something if I crank too hard. Breaking a casting on an engine that you couldn't see when you thought you were levering safely would be a bad day.
 
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Jan 15, 2014
62
Morgan 323 Portland
In the one of the articles mentioned above the DIY tensioner is 2 blocks of wood, 2 nuts and a 3/8" threaded rod. You can likely picture the way that's all put together to make a tensioner.
 
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Jan 11, 2014
6,330
Sabre 362 113 Fair Haven, NY
There's really no good commercial tool I've found to do this. I own the tensioner pictured above - perhaps the most common. It worked well once, then failed at a weld the second time I used it. You'll see that's extremely common with this tool when you look at the reviews. when it works its VERY handy. I had someone re-weld it for me and am hoping that fixed it. We'll see. I'd much rather use something like this than a crowbar or screwdriver which could damage something if I crank too hard. Breaking a casting on an engine that you couldn't see when you thought you were levering safely would be a bad day.
You can make your own tool for pennies. A piece of ⅜" threaded rod, a few washers and nuts, and some scrap hardwood. See the article I posted earlier. If you to get fancy, use a turnbuckle instead of the threaded rod.
 
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Likes: tfox2069
Jan 11, 2014
6,330
Sabre 362 113 Fair Haven, NY
In the one of the articles mentioned above the DIY tensioner is 2 blocks of wood, 2 nuts and a 3/8" threaded rod. You can likely picture the way that's all put together to make a tensioner.
In the article, if you click on the one picture you'll get this photo. The blocks are cut to roughly match the curve of the pulleys. There is ⅜" hole drilled through the blocks to accept the threaded rod. Specific dimensions will depend on your engine. When tensioned the end of the threaded rod should be about half way or a little more through the blocks on both ends.

Place the tensioner between the pulleys and turn the nuts to push the blocks apart and tension the belt.

IMG_0402.jpeg
 
Oct 22, 2014
13,458
CAL 35 Cruiser moored EVERETT WA
I borrowed one of the tensioners I posted. It worked as advertised. It was a great 3rd hand that held the alternator in place while I tightened the securing bolts. The Alternator has stayed in place and the belt tight. Much better than I experienced using lever skills from my car fixing days.
 
Jun 7, 2016
247
Catalina C30 Warwick, RI
I have the tensioner pictured above. It has worked very well so far. That belt is definitely too loose, the hard part is finding the balance between proper tension and overtightening to the point of putting stress on the bearings. The 1/2 inch rule is pretty decent, but you want to find the tension where it is tight enough to not flop around/slip but not so tight that it is straining/pulling the alternator out of alignment. Basically it should have a little play but not be a tight rope.
 
Jan 5, 2017
1,891
Beneteau First 38 Lyall Harbour Saturna Island
I replaced the slotted bar with a turn-buckle. Works very well and you get one handed adjustments.
 
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Jan 5, 2017
1,891
Beneteau First 38 Lyall Harbour Saturna Island