Photo would help.. on a car I've had for years we simply added another bolt with washers and lock washers in the slot and just under the alt.

 

Buy a longer boat and where it pokes through the "ear" on the alt and put a nut & lock washer on or a nylcok nut. After you make your adjustment then snug the nut on the back side to lock it into place....

One of the biggest issues on Yanmars is that the M8 pivot bolt fits VERY sloppily in the engine mount hole and the alternator mount holes. If you are brave you can drill the engine mount to accept a larger bolt the fits more snugly and is not a "sloppy" fit.....

On this engine both the alt bracket and the alternator ear are "tapped" for the adjusting bolts. They always tend to loosen up under high charging loads as they do on many motors.

The simple fix is shown in the pics below. Adjust the alt, tighten the adjusting bolts, then tighten the back nuts to lock everything into place. They will not "work free" again..

 

Buy a longer boat and where it pokes through the "ear" on the alt.........................

There's great wisdom in Maine Sail's post, but I like the "nautical slip"......
"Buy a longer boat"

 

I have the same issue with the alt belt on my 3gmd. Seems after only a few hours the belt is loose. I also have rust on the pulley but that seems to have smoothed out over time. I have used both the Yanmar belt and gates and seemed to both wear. I will try Maine Sail advice, but all my bolts are tight after the belt has loosened up, so not sure if that is the problem. Alt is the original 35 amp so not like I am putting huge load on it. When I sight across the pulleys it looks like they could be out of allignment- (I need to check it against straight edge which I have not done) , but not sure how that could happen as engine is all original.

 

If I understand y'all correctly, the problem is the bolt through the alternator and the adjustment arm allows the alternator to pivot and slip along the slot in the adjustment arm. To test that theory, adjust it tight and draw a line along the adjustment arm where it meets the ear of the alternator (on the outside, so you can see it after slippage). If it is the pivot bolt that moves, the line will stay next to the ear; otherwise, the ear moves away from the line.

Have you thought about something cheap and easy, such as putting a lock washer between the adjustment arm and alternator (and running the bolt through it, of course)? The pressure from the nut and bolt will press the alternator ear into the lock washer and the lock washer into the adjustment arm.

Also, I am presuming the nut and bolt stay tight. If they don't, you'll need a second lock washer under the nut.

Regards,

Tom

 

The threads on my finally just stripped out. So I drilled it t and replaced with a bolt & nut. But that still got loose. So I installed an extra bolt nut in the bracket slot that s slid up next to the alternator to from a stop. The alernator has never slipped since then.

 

Looks like I am not alone on this, which is good in that there are lots of ideas for the fix I am going to try the lock washers, plus a second bolt in the slot, and then add some marks to see if I can tell where slippage aoccurs, if it still slips after the first fixes. Both the second bolt and the added washers and nuts seem so simple, but I never thought of them.

For the rust in pulley grooves: Years ago, I had a BMW that had a problem with belt noises from the pulley (fairly common problem with that series). Someone marketed a piece of stick-on sandpaper, shaped to fit around the three sides of a typical v-belt, only an inch or two long. The idea was to just stick it to the belt and drive. In a while the sandpaper would smooth the pulley, and would then drop off as it wore out. AFter getting tired of trying to fix the noise with adjustments and new belts, I bought a pack of these for a couple of dollars, and never had the noises again on that car , and fixed same noises on another BMW owned by a son a few years later. anyone ever heard of something like that still in the market? shouod be easy to make, except for a glue that works. The paper looked like a waffle or lattice pattern with holes to allow flex.

Thanks for the ideas.

 

I thought I'd report back after re-doing the alternator belt adjustments again this weekend. Mainsail got it exactly right; "On this engine both the alt bracket and the alternator ear are "tapped" for the adjusting bolts. They always tend to loosen up under high charging loads as they do on many motors.

The simple fix is shown in the pics below. Adjust the alt, tighten the adjusting bolts, then tighten the back nuts to lock everything into place. They will not "work free" again.."

While I can't say they won't come loose again, I added new lock washers all around, and also benefitted from his tip that the bracket and ear are tapped internally. I had not understood that, and as a result, I wasn't really tightening the bolt into the tapped threads so much as tightening each end of the bolt/nut at the same time (as I would with a simple bolt in a smooth hole). Now I have the bolt really snugged down, with a lock washer on it, and then a nut on the outer end, also with a lock washer and good force applied. Just for kicks, I also added a second bolt with lock washers and nut just as a back-up, as suggested by others.

I am wondering if some of the others with belts that loosen too fast might be mis-handling the tapped ear as I was. Thanks to all.