Rear engine mount breaks bolts

[geedee]

I have a 3 cylinder 32hp Ford "Dexta" motor in my boat. it is on rubber mounts. the front mounts is a 1/2 inch thick plate bolted onto the front of the engine with lugs onto the front rubber mounts, at the rear there are mounting lugs on the bell housing.. from these lugs there are Z plates up onto the rubber mounts. My problem is on one mount where the Z plate is connected to the lug on the bell housing it has broken the 1/2" bolt off... the bolt is through the lug and threaded into the Z plate. the bolt has a plain and a spring washer.. and it breaks off just under the hexagon head of the bolt! this has happened 3 or 4 times over a 10 year period.. have had the alignment checked etc. Has anyone have any idea why this should be happening?
Thankyou you

Geedee

[transmitterdan]

Do you have a picture of the broken bolt where it separated? What kind of bolt is it in terms if material and hardness? Usually there is writing and/or marks on the head of the bolt.

[leftbrainstuff]

Pics would help.

If the same mount is always failing then it is likely experiencing a disproportionate percentage of the load. Bolt failing is unusual. Are you using cheap hardware. Ie fence bolts from the hardware store?

How is the engine torque resolved through the mounts? You could have good alignment but configured so that the reactionary forces are uneven.

Pics of the failed bolt would also be helpful. Fatigue can be strain (low cycle) or stress (high cycle) related. Understanding the failure mechanism can help pinpoint the cause.

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[geedee]

Thank you for your reply,

Sorry but I did not keep the broken bolt when it was taken our. I could take and post some pics of the Z plates and rubber mounts etc .
The bolt/stud is fully threaded and is screwed into the Z plate as there is not enough space under the Z plate for a nut. the bolt is probably 1/2" whitworth threaded and would doubt it would be of a high tensile type... It just totally puzzles me why that bolt breaks .. I was thinking that the rubber mounts would take up any vibrations?

[transmitterdan]

My best guess is that the bolt that is failing is put into tension by the prop turning. The other bolt that doesn't fail isn't in tension. Further I would guess that the bolt in the tapped hole is not staying tight. If it gets just a little bit loose then the motor will hammer the heck out of the head of the bolt and break it off. For starters I would loctite the bolt into the tapped hole. Also, make sure it is torqued correctly but that may be hard to get info on. Some people don't want to over torque a tapped hole for fear of ripping out the threads. What is the thickness of the tapped hole? Does it allow at least 4 full threads to engage the bolt? If not, that could be a problem.

[vlathom]

Our engine mount bolts broke because of the vibration of the boat. At least, that's what the mechanic thought. Vibration corrected. We'll see.

[W3GAC]

I want to second that you should use the proper strength bolt and given the history... using lock-tight is probably a good idea. If there has been movement over a long period it may have worn an elliptical hold on your Z. If so replace it if new. Try to avoid 'hard' shifts. Check your idle rpms to make sure it's at recommended rpm. Not sure I'd totally rely on boats tack... it gets it's feed from alternator which could have been changed/ different pulley ratio that would cause significant error.

Try to be in eng


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[W3GAC]

Hit wrong key...

... Try to be in engine area with someone else shifting (N-R-N-F) at present idle setting and carefully watch the suspicious engine mount to see just how much it moves and if rubber absorber is taking it all or is there play at the bolt and Z. Also carefully watch the Z to rubber interface... it might be old mounts with dry/ weakened rubber and too much freedom allowing a sudden jerk. (Usually when you replace one you replace all engine mounts, especially if they are old.)

There is another technique I use once in awhile to sort out vibrations. I clamp or use a small about of marine-tex or epoxy to secure a laser pointer on what I am studying. Same on the other part (in your case the bell housing on engine & the mount). Takes some pre-aiming and thought, but the idea is to setup the two lasers to point on a wall as far away as you can. That distance can act like an 'amplifier'. You can watch the two dots move/ wiggle as an 'amplified' proxy of the relative movement of their two parents. It's a lot of trouble to set up but have solved some interesting 'sympathetic' vibration issues.


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[Kerry1]

Originally Posted by geedee

I have a 3 cylinder 32hp Ford "Dexta" motor in my boat. it is on rubber mounts. the front mounts is a 1/2 inch thick plate bolted onto the front of the engine with lugs onto the front rubber mounts, at the rear there are mounting lugs on the bell housing.. from these lugs there are Z plates up onto the rubber mounts. My problem is on one mount where the Z plate is connected to the lug on the bell housing it has broken the 1/2" bolt off... the bolt is through the lug and threaded into the Z plate. the bolt has a plain and a spring washer.. and it breaks off just under the hexagon head of the bolt! this has happened 3 or 4 times over a 10 year period.. have had the alignment checked etc. Has anyone have any idea why this should be happening?
Thankyou you

Geedee

I have no marine experience of this, but I do have experience with 3cyl Ford
engines and do know that they run like a 3 legged giraffe at low speeds. I think it is something to do with the asymetric geometry of a 3cyl motor. Don't get me wrong, I own two Ford tractors both of which are 3cyl motors and one of which I have owned for around 25 years. Great economical engines. Like any engine I suggest not running at any speed for extended periods where there is significant vibration. Find that sweet spot which matches the boat speed you want without the vibration. The destructive power of vibration is I think underestimated. I once set up an irrigation pump on a big trailer that was converted from a truck chassis, I used a 4cyl Ford 500E industrial engine (80hp?) to drive the big pump through a triple drive belt. I got engineers to machine up a flange that bolted directly to the flywheel with a short shaft extending through the bell housing on to a large self centering bearing with a triple cast iron pulley wheel on the external end. The shaft was at least 2" thick. The engineers assured me the set up was true and yet after a few hours the shaft separated from the flange plate. This happened i think a couple of times. Finally we decided to mount an extremely large (and expensive) rubber flexible coupling inside the bellhousing so that the engine was essentially separate from the shaft holding the pulley wheel. Problem solved. To further illustrate vibration problems. I had a 2000litre fuel tank mounted on the chassis as well. The pump used to operate up to 16hrs a day and she did guzzle the fuel like an alcoholic on steroids. I had the fuel tanker delivering direct to the field wherever the pump was situated about once a week. Well after maybe a couple of hundred hours the vibration split the welds on the fuel tank and ultimately I had to mount that on rubber as well.

[Arthur Garfield]

Thought whitworth obsolete.

In any event-Grade 8 bolts plus locktite
Alignment ?
Prop?
engine runs steady?


All the Best

[stevebein]

I had a problem with a different motor, a 4-108. the issue was resolved when the motor alignment was corrected. Do not just tighten the bolts, make sure that the shaft and the motor are aligned as close as you can get. the other suggestions are of note also, but my first check would be engine alignment.
Good luck.

[Rowley]

A common cause of bolt breakage through the radius beneath the head is uneven loading of the head resulting from being tightened on to an uneven surface or the housing surface not square to the thread in the housing.

Fair winds.

[geedee]

Thankyou everyone for your helpful advice.. this last time anyway the remainder of the bolt was very difficult to unscrew from the Z plates... took a lot of effort to unscrew it so was not loose.with vicegrips

[transmitterdan]

Originally Posted by geedee

Thankyou everyone for your helpful advice.. this last time anyway the remainder of the bolt was very difficult to unscrew from the Z plates... took a lot of effort to unscrew it so was not loose.with vicegrips

That may be a telling detail. After the head is broken off the rest of the bolt should unscrew easily if the threads are not corroded. If the shoulder (the smooth unthreaded portion) of the bolt is too long it may seem to be tight even though the head of the bolt is not firmly against the metal plate. In other words the bolt might have run out of threads before it was really tight. This could explain why it was hard to unscrew. If so use a bolt that is threaded all the way to the head (shoulderless bolt).

Another explanation could be the threaded part is yielding and distorting the threads. Possibly because there are not enough threads in the tapped hole (Z plate too thin).

But whatever is going on your description points to a potential problem that may explain why the heads are breaking off. Try to find out why the bolt was hard to remove.