Yanmar alternator bolts breaking

[ChuckSK]

This is a really wild guess. If both alternators are controlled by the same regulator, then that is a common element and might explain the almost simultaneous failure of both brackets. The regulator might be putting a pulsing load on the alternators, causing fatigue failure of the bracket bolts. Just a thought.

[hellosailor]

"900 hours seems pretty soon to be changing mounts. "
Actually the rubber companies say to replace mounts at five years of age, regardless of hours.

But perhaps you could go over this from the start: Exactly what is "stock" and what modifications (brackets, alternators, etc.) have you made? Presumably "stock" would still be working, it is most unusual to shear off bolts that way.

Which leaves the question of what you have modified, before we get into engine teardowns and failures.

If it is the brackets and alternators that are non-stock, the solution may require more robust brackets and drilling and tapping more bolts to hold them, etc.

[Bash]

Originally Posted by DeepFrz

Also make sure you aren't over torquing the bolts, and as someone above mentioned that the bolts are not bottoming out.

+1 on not over-torquing the bolts. I had the same situation with a Balmar on a Yanmar, breaking a bolt at less than 100 hours, but since then no problems with the replacement bolt.

[Cheechako]

sounds vibration related for sure. single belt 100 amp? I had a lot of issues with that setup in general once I got a regulator that really pushed the alternator... is it possible alignmenet changed? Or did you change belts recently and are the slightly bigger crosseon?

[perchance]

Originally Posted by D Wayne

The alternator is attached to a bracket that is bolted to the block with two bolts. It is the two bolts that attach the bracket to the block that are breaking on both engines!

Check the belt layout to see if it is inadvertently creating a shear force on the bolts. You could possibly have a lever effect depending on bolt placement and belt track. Just a thought.

[Ex-Calif]

Something changed - you operated successfully for a long time. Nothing has "changed" in the alternator mounting system.

Maybe what changed is the vibration levels. What shape are the engine & tranny mounts in?

Worn engine mounts could have changed the vibration signature of the engine.

I also agree that after market 100 amp alternators are more than the existing mount system was designed for and you should consider a more robust bracket design.

[Wotname]

Originally Posted by Ex-Calif

Something changed - you operated successfully for a long time. Nothing has "changed" in the alternator mounting system.

Maybe what changed is the vibration levels. What shape are the engine & tranny mounts in?

Worn engine mounts could have changed the vibration signature of the engine.

I also agree that after market 100 amp alternators are more than the existing mount system was designed for and you should consider a more robust bracket design.

I agree with Ex-Calif (although I am not sure I would say "successfully for a long time").

My take is that the existing mount system was never truly suitable for the aftermarket Balmars but you got away with it while everything was "good".

As Ex-Calif (and others) suggest; the engine mounts are wearing and thus changing the vibration signature and this has accelerated the failure of alternator mounting bracket bolts. The previously dormant problem has been exposed with the wearing of the engine mounts.

Beef up the mounting design, replace the engine mounts, realign everything and report back in a 1,000 hrs .

[D Wayne]

Thanks everyone for chiming in. Today I learned about bolts. I thought that 8.8 on the bolt head meant that it was a class 8 bolt. Apparently not, that's a 5. So today I actually installed class 8 bolts and hopefully that will help. However there is still that question "what changed?". Although I hate to admit it I think it may be the motor mounts. They are really soft when new and they don't improve (what does?) with age. I think the larger alternaters held on with two small bolts just can't stand the shaking. I have also identified a possibility to install another support from the top of the alernator to a sturdy bracket on the block. Hopefully that will help too. Motor mounts at $260.00 each times six is not a pleasant thought. Gotta love those Yanmars!

[Nemo55]

Motor mounts @ $260 ??? your being robbed for sure,,,

[D Wayne]

Nemo
No ****. I found an aftermarket in Lynwood Wa for $130.00
By the way. We are sorta neighbors. I'm in Nanaimo.

[rigormortis]

I have a Yanmar 4j....etc. with an 85 amp Balmar, up from the stock 65 amp. It has double feet for the bottom motor connection (adjusting arm on top with bolt) and the bottom bolt has spacer. Does yours have spacers? Are they in place? Different engine, I know, and I know little about anything. Just asking.
My bottom bolt wore a new oblong hole in the engine original mounting hole. Vibration, obviously, but no breakage.
Pete

[Nemo55]

yeah,,even iso elastic hydraulic mounts are only $120 CDN,,,someone is gouging big time.....
Hey Neighbor,, i'm in Van until the 20th,,then Mexico..

Originally Posted by D Wayne

Nemo
No ****. I found an aftermarket in Lynwood Wa for $130.00
By the way. We are sorta neighbors. I'm in Nanaimo.

[D Wayne]

Rigormortis
Mine has two bolts one above the other and only about two inches apart with no spacers. My feeling is that to support the weight and torque of the 100 amp alternator it needs another support at the top to help counter the torque on those two bolts so close together. Still pretty sure that the motor mounts are too soft and adding hugely to the problem. I am seriously considering sourcing stiffer aftermarket mounts. Yanmar parts are a ripoff and so far I'm am not impressed with either their design or build quality.

[hellosailor]

Hmmm, 260$ for a 25$ part...Gotta admire Yanmar for finding ways to make a franchise profitable. (Oh stewardess, would you have another one of these little bags for me? This one is full now.)

The alternator on the "end" of the bracket is analagous to a weight on the end of a lever arm, with the bolt(s) forming the opposite end. As the mass of the weight goes up (bigger alternator) it will have a disproportionate tug on the bolt. If there's enough motion...snap.

I suppose a good engineer could run some numbers to see if the newer bolts will be good enough, but it might also be a good idea to replace them every year? Two? Rather than see how long they can take the strain. And of course, set them with a torque wrench since over-torquing would also help contribute to a failure.

[DeepFrz]

Can you give us some pictures of the alternator mount on the engine. I may help us come up with a solution.

Edit: I may be wrong but I thought all Yanmar engines came with the dual foot, 3.15" mount. Could someone have adapted a mount to fit the wrong case alternator to the engine?