Yanmar alternator loosens way too often

Jun 17, 2012
Hunter 356 Mackinaw City, MI
I have the 3 gm30 f engine. The engine is a jewel. It has been totally maintenance (repair) free and 100% leak free for nearly 20 years. The only complaint is the alternator loosens up periodically. Maybe every 15 hours at constant 2800. I have a brand new belt on it and the oem alternator bracket. I’m from the automotive world and in observing the adjustment I notice to get the proper tension on the belt I am at the very end of the adjustment slot. I shopped slightly smaller automative cross over applications with no luck. Has any other of you experienced this with the 3gm30 f engine?
Jun 17, 2012
Hunter 356 Mackinaw City, MI
Yes the bolts are super tight. I think I will reconfigure the bracket a bit (extend the adjustment slot a bit via welding) and the use some real good spider lock washers. More on that later thanks for your comments
Feb 6, 1998
Canadian Sailcraft 36T Casco Bay, ME
Aug 12, 2018
Hunter 26 Carter Lake, Colorado
Just to confirm: the alternator literally moves on the bracket, thereby loosening the belt? Or the belt starts slipping without bracket movement?
Jan 22, 2008
Hunter 34 Herrington South, MD
I had the same problem then discovered that over the last 30+ years someone had mounted the alternator incorrectly. not at the swivel but at the point where the alternator can be adjusted to tighten the belt. On my 3 GMF engine there is already a nut on one piece (forget which one) and all you need to do is tighten the bolt between the alternator and the metal strap. someone had added another nut to the bolt so that when I had tried to tighten it in place by tightening the new nut, I was merely tightening the two nuts against each other. once I removed the second nut and just tightened the bolt against the one nut it became very snug and never moved again.
Jan 5, 2021
Hunter 41 DS Saint Petersburg
The second nut was added as a locking nut. You break the 2 nuts free, tighten the inner nut, then tighten the outer nut to hold it in place and to keep it from vibrating loose.
I have also used the double nut method to remove bolts when the head broke off.
Oct 10, 2019
Signet 20 107 Ithaca
Thought I posted this yesterday but don't see it to fix a typo, if I did, sorry for the second time around, but this version has a link to the video...

I have been preaching (I'm sure annoyingly) for years this message:

Lock. Washers. Do. Not. Do. Anything.

There are good videos on YouTube (here's one:
. It is admittedly an ad for Nord-Lock, but their point is valid, and well illustrated, plus their product apparently actually does work, so more power to them.)

Split lock washers are actually worse than no washer at all, and a split lock with a flat washer is the worst combination of all. Nylock nuts are decent but can be a pain to work with. I don't believe Locktite works on oily hardware, so new or solvent cleaning is required.

A castellated nut and a bolt with a through hole, seized with wire, is the only way I know to really secure a nut to a bolt and still have it be removable.

Maybe the solution in your case is to add "tighten the alternator nut" to your morning engine inspection.

FYI: NASA doesn't allow lock washers on a single thing built for them...
Nov 6, 2006
Hunter 34 Mandeville Louisiana
From my engine.. No problem with it loosening.. plenty adjustment left to take up slack.
Jan 7, 2011
Oday 322 East Chicago, IN
I saw this thread a while back, and liked the idea…

On my 2GM20F, I had trouble holding the belt tight when tightening the bolt. I was using a pipe to try and lever the alternator over while I tightened the bolt, but it seemed to loosen (alternator moved) every time. Then I realized that I was pushing it with the force on the ratchet. I reset the ratchet so that I was pushing the alternator the other way (tightening the belt) as I tightened the bolt, and boom! No more loose belt. Seems so simple when I say it, but never occurred to me before that.

Jan 22, 2008
Hunter 34 Herrington South, MD
I was thinking of getting a variety of Nord-Lock but need to first learn how to create a "go fund me" page. THEY ARE EXPENSIVE.


Oct 21, 2009
hunter 38 Monaco
You could buy a belt that is 1” shorter and you would be in the middle of the adjustment slot.
May 24, 2004
CC 30 South Florida
Are you sure you have the correct belt size. New belts will stretch some and if you are at the end of the slot in the bracket you might need a shorter belt. Oops, had not read prior posts mentioning shorter belts.