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Giulietta 11-21-2006 01:52 PM

Need help YANMAR engine mount
1 Attachment(s)

Brand new Yanmar 3JHE 40HP, SD50 Saildrive, 150 hours only.

The front Port engine mount stud broke, clean between locking nut and engine support, (as if it was cut).

Called Yanmar (complete engine installation by Yanmar also on brand new boat) and they immediately replaced the rubber mount, the stud and the engine support, rechecked all bolts and left. (gave me a free oil change, and filters).

They say never happened before, and the cause was material fatigue !!??

So far new parts are ok. Has anyone experienced this on a Yanmar??

I made a simple drawing of where it broke

Please help.

cardiacpaul 11-21-2006 02:20 PM

It was probably a weak stud that was over-torqued at installation, I wouldn't worry about it any longer.

sailingdog 11-21-2006 07:32 PM

i agree...sounds like it was overtightened.

jones2r 12-02-2006 02:16 PM


I just saw this posting and would like to offer some additional input. If you still have the pieces of the stud, examination of the fractured ends will tell the story (magnification will help).

If the surfaces are rough, then the stud sheared off. More than likely, what you will see is a smooth area and a rough area. The smooth area defines a crack, and the rough area denotes the ultimate failure of the fastener. There are two sources of the cracking. The first would be a flaw in the base metal, such as a carbon inclusion (very common to this type of failure). The second would be the formation of a stress riser in the threaded area.

When threads are formed, they are rolled, a forging process. This process reduces the likelihood of stress riser formation, but it could occur. Another source of stress riser formation in threads would result from the use of a sharp die. If the threads had been damaged during the installation of the engine (very possible) and then a die run over them, the subsequent failure would be possible. If you have the pieces, you should be able to see where a die would have cut sharply into the root of the thread.

I really don't like the design of this mounting. All of the engine weight and vibration are concentrated on the threaded portion of the stud. A better way would be to place spacers (they would have to be machined after taking careful measurements to maintain engine alignment) under the engine block tabs and on top of the rubber mounts (here I am assuming that there is a metal surface on the top of the rubber mounts). Above the engine block tabs, use a heavy flat washer and either a locknut (upset thread, not Nylon) or double nuts in a jam configuration (simply, one on top of the other with their flats misaligned). Use a thread locking compund, such as Loctite red.

You probably will never have any problem with what is in the boat now. The size of the studs are way over-spec'd to withstand the abuse of any compromise. However, if you continue to have problems, you also have the remedy.


ps: Please take a look at my post in this forum regarding jib rigging. I need your input. Many thanks.

Giulietta 12-02-2006 02:34 PM

Bob, Thanks for your help. I realy appreciate it.

The problem is the Engine is new, Yanmar installed it in June, so it is still under the warranty for some more years. And one of my crew is the guy that installed it!! He is the Piano Man, because he is so big!!!

They took the stud with them, and I could almost swear thet they new what happened and didn't want to talk about it, too much.
I alm also inclined towards CP's comment " weak stud that was over-torqued at installation".

I had many options for engines when I had the boat built, and Yanmar was the winner (not cost related). I am sure that they will support their product.

I posted to see if it had happened to anyone else out there.

If I change the studs as you very well suggest, I will void the guarantee. So this is a case of sit back and watch.
The bolt was all smooth at the broken face.

PS Have you been to member galleries? I am posting some photos we did when the running rig was being installed, and of the testing. she still had the EXP logo on the side.

jones2r 12-02-2006 04:01 PM


Really wasn't suggesting changing the studs, just the mounting configuration. I'm sure there's no need to do anything at this time, especially with the warranty situation. If your guy did the installation, then you would know of any use of a die.

If the Piano Man torqued the nuts, I suppose it's possible that he fractured the stud. My other thought is that as tough as studs are, the threads on the nut would have given up before the stud did.


Giulietta 12-02-2006 04:04 PM

Bob, are saying he overtorqued the studs because he is the piano man?? ie very strong piano hauler?

jones2r 12-02-2006 07:01 PM


Yes, "very strong piano hauler". Actually, I don't think he is the cause of the failure. Getting a wrench on the lower nut and being able to resist the amount of torque imparted to the top nut that would fracture that stud just doesn't compute for me. I see the stud as being the strongest part of the whole system, unless it had a problem. I think the stud had a failure; I just don't describe it exactly the same way that Yanmar did.

I have had further thoughts regarding the mount configuration. The rubber mounts probably serve as vibration isolators; my solution would work just fine, except that it could transmit vibration into the mount pads in the boat. In any respect, there should be no reason for you to ever have to do anything further. I just don't like the idea of using threaded areas for structural purposes. They're great for imparting tension into the fastener. Beyond that, they're a liability.


Giulietta 12-02-2006 07:10 PM

1 Attachment(s)
Bob, I am going to get a camera and send you a detailed photo, of the arrangment.

Might take a week or two, but will do.

The Piano Man, normally (over here) is the guy that has his body inside the cabin, standin on the first step in the cabin, and stays next to the clutches, and handles the clutches, boomjack and cunningham!!

Piano Man, because the clutches looke like the keys of a piano!!!:D

I edited later. here are 2 shots of the same engine, however the 1st one is not the saildrive one, but same engine it has 4 mounts. I posted it because you can zoom in on the mount easier.

The saildrive one 2nd photo, only has 2 monts on the front. The rear end is supported by the saildrive (no mounts)

If you copy and paste the photo yoiu can see the mount

jones2r 12-02-2006 08:15 PM


Now I understand why you said the studs would have to be replaced; they are threaded all of the way to the base. As I see it, my method would simply place the loading stress over a longer portion of the threads, increasing the opportunity for the stress to find a riser.

BTW, I didn't know that you are an engineer when I imparted my solution. Anyway, we understand each other. I don't think you need to go to the trouble of taking additional photos, at least not for my benefit. That's a really good look at the saildrive.


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