Alternator Belt Tensioner Home Made

[Hugh17]

I've read several post about using an Alternator Belt Tensioner with links for these tool from various sources. Every review I've read states that the T shaped end is prone to breaking off and would have to be welded back on if it breaks. I'm not opposed to spending $20 or so on a one purpose tool, but I have no desire to do so if it's likely to break the first time I use it.

I've come up with a homemade belt tensioner that is not going to break and it cost me $5.25 to make. Your cost may vary a little depending on location and retailer.

Using a 3/8"x8" galvanized turnbuckle, and a little modification on a grinder this little tool is a jewel. The 3/8" bolt is plenty strong enough to spread or expand the distance between the V belt pulleys on the engine and the alternator. 

I found that the belt can be tensioned sufficiently by hand turning without using a wrench or other tool to adjust the turnbuckle.  Link to the turnbuckle used is https://www.acehardware.com/departments/hardware/chain-and-rope/rope-and-chain-accessories/51623

The minimum distance between the pulleys is about 7-1/2" on my spare M25XP using a #7395 belt measuring 3/8"x40-1/8". Your distance may be slightly larger if you are using the #7400 belt that measures 13/32"x40-5/8". I've used both size belts on the engine in my boat, which is a M25, depending on the pulley size of the alternator and stretch of the belt. As you are aware there is very little room to tighten the belt.

I hope this is of use to others. Pictures are below.

[Ron Hill]

Jim : Great Idea.  With a bit of practice on the grinder you can even concave the ends!!


A thought

[Stu Jackson]

James, I added a link to the Critical Upgrades tension post to this one.  Thanks.

[Hugh17]

Jim : Great Idea.  With a bit of practice on the grinder you can even concave the ends!!A thought

Ron, that was the first thing I did, it just doesn't show very well on the picture. My main concern was the end of the eye bolt bottoming out in the pulley before it might bring pressure against the sides of the pulley and spread them.

I'm not a metal worker, and have few tools for working with metal, but with a bench grinder and some patience I "eyeballed" the modification. It also helped that I had a spare alternator on the table so I could keep testing the shape of the flat eye.

I'll have to say that I was very pleased with the result. There's no indication of the tensioner slipping in the pulleys.

[Ron Hill]

Jim : You can do the same "concaving" with a half-round file with the turnbuckle in a vice!!


A thought

[Hugh17]

Jim : Great Idea.  With a bit of practice on the grinder you can even concave the ends!!A thought

Ron, that was the first thing I did, it just doesn't show very well on the picture. My main concern was the end of the eye bolt bottoming out in the pulley before it might bring pressure against the sides of the pulley and spread them.

I'm not a metal worker, and have few tools for working with metal, but with a bench grinder and some patience I "eyeballed" the modification. It also helped that I had a spare alternator on the table so I could keep testing the shape of the flat eye.

I'll have to say that I was very pleased with the result. There's no indication of the tensioner slipping in the pulleys.

I installed a new alternator and changed the belt on my M25 and the belt tensioner worked great. There is some difference between my old M25XP and the M25 in my boat, but I had plenty of adjustment available in the turnbuckle/tensioner.

[Ron Hill]

Jim : If you want to buy a super tool get a Dremel tool.  After awhile you find it is invaluable!

I see that you have the old original engine mounts!!!  If you really want to treat yourself, change them and you'll think you have a new engine!!  Many posts in WiKi, Mainsheet tech notes and this message board on new Engine Mounts!!

A few thoughts

[Hugh17]

Ron
I have a Drimmel. Agree that it's a great tool. As for the engine mounts they are new. I had this engine installed when I purchased the boat and bought new engine mounts for it.
James

[captran]

does anyone know if this would fit/ work on the 1997 35 engine???

[Ron Hill]

Randy : I have a tool and never used it (don't think it would fit) on my new M25XPB engine (same alt. set up as the M35BC engine).  Never had a problem getting the alternator belt tight by hand!!!

A thought

[KWKloeber]

Randy

The M25 and M35 have the identical front ends.
Check out the parts manual for both engines on the wiki site. Under "manuals."

[captran]

Thanks.  Last year toward the end of the season had a brief squealing noise.  One time I shut down and restarted 3 times before it stopped.  It reminded me of the squeal that occurred when my water jacket pump went out 10 years ago, only that squeal did not stop and I got towed in. But when we were putting the boat to bed, it seemed like the belt tension was looser than it should be, though I didn't measure the belt deflection.  I recently asked the boat yard mechanic to check it out, in case it is the water jacket pump.  will head over to the boat next Monday and thought I could pick up the ace turnbuckle just in case it would make the process easier, as I recall when I changed out a belt a few years ago, I wasn't sure I got it tight enough.  (but it has been running great until the brief intermittent squeal the last few weeks).

[Jon W]

FWIW - NAPA Auto sells a handy tool for measuring the belt tension. NAPA part number is NBH-KR1 and costs about $20.

[Ron Hill]

Randy : The "rule of thumb" for belt tension is that you cannot twist the belt (between finger and thumb) more than 180 degrees! 

A thought

[captran]

I am getting old and forgetful, and at the risk of looking like the total idiot that I am, do you loosen both the bolt at the top of the alternator arm and the bolt at the lower end of the alternator arm.  from the picture, I only see the loose bolt in the picture on the right bottom being loose on picture number 1.