Making Alternator Belt Tension Easy

Jan 5, 2017
2,172
Beneteau First 38 Lyall Harbour Saturna Island
The engine room on our First 38 is very tight. The alt. belt tension was a 4-handed job with only space for 2-hands. This mod. left change from $5. Correct tension should save some wear and tear on the belt.
alternator adjuster.JPG
 
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Likes: Don Crowther
Feb 10, 2004
3,510
Hunter 40.5 Warwick, RI
I have a tensioner that I got from Harbor Freight (I think) that expands between two vee-belt pulleys. Simple to adjust to the right tension and then tighten the mounting bolts. Horribly difficult job without it.
 
Jan 5, 2017
2,172
Beneteau First 38 Lyall Harbour Saturna Island
Just using the 3/8" deflexion by one finger method. Gauge looks like it would be much more accurate!
 
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Likes: Benny17441
Mar 20, 2004
1,662
Hunter 356 and 216 Portland, ME
I use the same tool, from JC whitney I think. Works great and easy to install and use on a 3GM
 
Oct 22, 2014
16,150
CAL 35 Cruiser moored EVERETT WA
I read recently that the way to get 3/8th deflection is to twist the belt on the longest run of the bet. It you can twist it 1/4 turn (90 degrees) you have good tension on the belt.

I think that is correct. Any one else use this twist test for their belts?
 
Feb 26, 2004
21,960
Catalina 34 224 Maple Bay, BC, Canada
Any one else use this twist test for their belts?
I do, too. Plus I have a belt tensioner like Rich mentioned to set it. 1/2" deflection on the long belt length is a good crosscheck.
 
Aug 22, 2017
1,608
Hunter 26.5 West Palm Beach
I have custom machined tensioners in the past, to get around problems like that. I never thought to use a turnbuckle. That's crafty.

The 1/4 turn trick works well if the unsupported span of the belt is within a certain range. I don't know what that range is. If that method has been working for you, stick with it. If you have never used that method on a given boat before, use it with caution the first 1 or 2 times.
 
Jan 5, 2017
2,172
Beneteau First 38 Lyall Harbour Saturna Island
I was down to the boat this morning and tried the 1/4 turn trick. Turns out my 3/8" and a 1/4 turn are about the same. I'd never heard of the 1/4 turn trick before. Thanks.
 

pateco

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Aug 12, 2014
2,207
Hunter 31 (1983) Pompano Beach FL
We're doing two Boat parades over the holidays, so last weekend I decided to do a complete engine service on my 2GM 13hp. She has been running well, but when I popped the cover off, I saw that my Alternator belt had a huge chunk taken out of it.
2017-11-25 18.02.23.jpg 2017-11-25 18.02.30.jpg
I found the chunk itself as a single piece in the engine bay bilge. I replaced it with a new belt as part of the servicing, but what could have caused this type of damage?.
 
Oct 25, 2011
572
Island Packet IP31 Lake St. Louis, Montreal
The engine room on our First 38 is very tight. The alt. belt tension was a 4-handed job with only space for 2-hands. This mod. left change from $5. Correct tension should save some wear and tear on the belt.View attachment 143344
Nice, thanks for sharing. Ditto for the 1/4 turn trick.

Cheers

Matt
 

jviss

.
Feb 5, 2004
4,627
Tartan 3800 Westport, MA
It you can twist it 1/4 turn (90 degrees) you have good tension on the belt.
With all due respect to John and others, the 3/8" thumb trick, and the 1/4 turn trick, are all very subjective, qualitative measures; that is, they are not quantitative, which is what this application demands. The reality is that if you don't severely under-do or over-do tension you won't damage anything, like the water pump bearings, and you won't slip. But I have found that I often had to tweak the belt tension to eliminate squeal with big alternator loads. And, I lost a water pump once, due to a failed bearing. So, without being to measure accurately, you are trading convenience for some degree of belt and bearing life, and perhaps utility (as in the squealing example).
What does "if you can twist it mean?" If who can twist it: me, or my son who's half my age and twice my strength? I guess that within a pretty wide range, with the visual feedback, folks will depress and twist at varying degrees of pressure or torque to satisfy the 3/8" deflection or 90º rotation, and feel good about it. I offer that it's yet another example of the ideometer effect, the same thing that makes dowsing rods, Ouija boards, and Chevreul's Pendulum work. Now, if you put a spring balance on the 3/8" deflection method, or a torque wrench on the 1/4 twist method, you can have a quantitative measure. The Krikit does just this for the former.
http://www.bbc.com/future/story/20130729-what-makes-the-ouija-board-move
 
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Oct 22, 2014
16,150
CAL 35 Cruiser moored EVERETT WA
@jviss has a point. The kirkit tool does measure in pounds or kg of belt tension. So if you knew or could translate the “about 3/8 in deflection of the longest leg” from the manual into pounds tension and you could be assured that all the belts were manufactured to the same tensioning standards, you could be assured of maintaining consistency in your belt tension.

That of course will not guarantee that you won’t have a bearing fail on the water pump or a belt begin to squeal. There are many differing variables occurring on all of these parts that affect their function and performance beyond belt tension.

So for me I choose the old school tools that have worked and continue to work for these old school engines. Trust your setup but verify the performance. I’ll tweek my belt and adjust if it twists too much or not enough. I’ll monitor my water pump for seepage (first sign of a bearing seal problem). I’ll adjust or replace a squealing belt, but also check the alternator and water pump pulleys for bearing play. And then go sailing.

Because if my belt is 10 pounds of tension off I would need to do those inspections any way. The belt twist is quick and accurate enough. Additionally it accommodates that not all belts are made equal.
 
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Likes: pateco
May 7, 2012
969
Hunter e33 Maple Bay, BC
The engine room on our First 38 is very tight. The alt. belt tension was a 4-handed job with only space for 2-hands. This mod. left change from $5. Correct tension should save some wear and tear on the belt.
Michael, am I looking at this wrong or is it the camera angle; but, given that the notches are running on the outside of the pulley, it appears that the belt is inside out,