Rear Engine Mounts

[kwoolard]

I'm back with another problem on my friends boat. While we had the outdrive removed so we could replace the water hose between the bell housing and the transom, I decided to recheck the engine alignment on the new motor we installed. The first sign that the alignment was off was the outdrive was difficult to remove.

This is a mercruiser transom assembly and outdrive (Alpha One), with a new 350 from Michigan Motorz.

Stuck the alignment bar in the bearing and it entered the coupling only about 1/2"

Tried adjusting the front motor mounts, but ran out of adjustment. Looking through the gimble bearing at the coupler, it looks like the rear of the engine has lowered as if the rear engine mounts have suddenly failed.

Is this a failure mode of the mounts?

What else would cause the alignment to go from perfect, to being way off after 2 hours of engine run time?

 

[dubs283]

Re: Rear Engine Mounts

could be the splines on the coupler are worn

the alignment bar will not go in if the splines are "pointy"

be sure to use plenty of grease in the coupler too

 

[Don S]

Re: Rear Engine Mounts

Is this a failure mode of the mounts?

Absolutely, Could also be a bad coupler, turn the engine 1/4, 1/2, and 3/4 turn and see if the alignment bar goes in at any of the other stages.

PS: If the splines are worn and pointy, it will make the alignment bar easier to go in. Less resistance.

 

[kwoolard]

Re: Rear Engine Mounts

thanks guys,

i bumped the engine with the starter, not sure if i got it exactly at 90 degree intervals, but the bar was still only able to go in about a 1/2".

guess we'll be pulling the motor.....funny thing is we were going to replace the mounts while we had the old engine out, but they didn't look or feel bad at all.

 

[kwoolard]

Re: Rear Engine Mounts

What would fail on the rear engine mounts to cause the engine to lower?

It seems as if the rubber bushing failed the engine still would not be able to lower since it would then be resting on the transom assembly. Looks like for the engine to lower, the mounting boss itself would have to break, either on the bell housing or the transom assembly.

 

[twwabw]

Re: Rear Engine Mounts

Yes- it's possble. If you buy the replacement rear mount bushing kit, it contains (2) SS washers, and in the instructions it states to use them in cases where "insufficient adjustment exists in the front mounts to align using on front adjustment". I ran into this exact scenario, replacing the long block on my 350 mag. It was originally a Quicksiler reman crate motor, and I replaced with a Mercruiser reman long block. The new mounts are the same height as originals + spring washers. The new design eliminates the spring washers. But std. install does not require the additional spacer washers- only if req'd. Wierd but true.

Put the washers in, and with a few turns on the front mount the alignment tools slid in with 2 fingers. Go figure.

 

[Don S]

Re: Rear Engine Mounts

What else would cause the alignment to go from perfect, to being way off after 2 hours of engine run time?

I just thought of something that may explain your problem.
When the alignment was perfect, were all the motor mounts (Front and rear) TIGHT ????? If not and you then tightened down the mounts but didn't recheck the alignment, there is your problem.
Many times I have had new guys at the shop do the exact same thing. Problem is, if you don't have equal weight on the front mounts, when you tighten them down, you can twist the engine and put it out of alignment. If you don't recheck, you never realize it.

 

[kwoolard]

Re: Rear Engine Mounts

thanks guys,

Don, yes, I did recheck after torquing down the rear mounts. I learned that on the motor install for my boat.

I found a good post on the forum that talked about how to check if the rear mounts have failed somehow. It discussed checking to see if the fiber washer was able to rotate around the spring washer. I had my buddy check this, and on the port side the bell housing is tight against the fiber washer, while the starboard fiber washer was able to rotate.

I got to thinking about how his transom assembly mount bosses looked before we installed the engine and remember noticing the port hole seemed to be wallowed out, as if it was slotted from front to rear. At first I thought this was normal, as it could possibly be a way to adjust the engine alignment laterally. Now that I think about it, it's possible that the hole is wallowed out enough that the spring washer has been pressed down into the hole.

We won't know for sure until the engine comes out, probably this weekend.

 

[kwoolard]

Re: Rear Engine Mounts

We've got the engine out and the rear engine mounts removed. Comparing the old mounts to the new mounts they look identical in height and the old ones do not feel like they have collapsed at all.

I looked at the coupler and it looked like the coupler was shifted to one side (eccentric) is the word. I put a dial indicator on the coupler bore and rotated the engine 360 degrees and got 0.070" of total runout. Is this out of the spec limits?

What is the limit?

 

[kwoolard]

Re: Rear Engine Mounts

DonS...? Do you have a tolerance that you go by when checking couplers?

This is the only thing that I can see that is wrong with the set up to cause the alignment to be out.

 

[kwoolard]

Re: Rear Engine Mounts

Anybody have any comments on this?

I am a little concerned about his inner transom plate. The holes for the engine mounts are wallowed out, and I think this may be the cause of the problem. The wallowed holes are allowing the rear of the engine to move a little which could change the alignment while the engine is running.

Is it a good idea to drill the holes out larger and install some aluminum bushings to get the hole back to a nominal size, or should we consider replacing the inner transom plate?

 

[jgrisham02633]

Re: Rear Engine Mounts

Ha, I forgot to look at the post date 2 years ago.