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Replacing engine mounts

Oct 17, 2012
Engine mounts play a key role in preventing engine vibration from passing to the boat. These mounts were not aligned properly prior to installation.

Engine mounts play a key role in preventing engine vibration from passing to the boat. These mounts were not aligned properly prior to installation.

If your engine mounts are more than five or six years old, or have been contaminated with seawater, engine oil or coolant, change them. Even if the mounts look perfect, the tough elastomer (rubber-like) material in the engine mount work-hardens over time and ceases to isolate the boat from the engine’s vibrations. Seawater rusts the metal parts of the mount and can cause delamination of the elastomer and metal parts. Rust also weakens the mounting bolts and in extreme cases can cause bolt failure. Engine oil and coolant deteriorates the elastomer and also hastens delamination.

Poorly aligned engine mounts can also cause bolt failure. And, if you have noticed screws backing out on your boat, or if your oil pressure sensor has failed recently, then the root cause may be increased vibration due to bad engine mounts (or a misaligned shaft.)

This unit has a fractured mounting stud.

Engine mounts are designed specifically for each family of engines. The physical dimensions are an obvious requirement, but the dynamic response of the mount itself is not so obvious. While an aftermarket replacement mount will fit your engine, it may not necessarily damp the engine vibrations over the operating range. Specify replacement mounts from the engine manufacturer, or after-market mounts that have given satisfactory service on an engine identical to yours. Avoid experimentation as it is both costly and aggravating.

While waiting for your new engine mounts to arrive, measure the heights of each mount above the engine bed. Using a felt tip marker, place a mark on the engine bed and on the engine mounting arm so you can duplicate the measurement later. Make a sketch of the engine and write the dimensions down on the sketch. A digital caliper makes this job easy.

Repeat the measurement several times to determine an average reading. These measurements will be of great help in aligning the engine on the new mounts. Take the time to squirt some Liquid Wrench or other rust-breaker solvent onto the bolts, holding the engine mounts to the engine beds, and to the nuts above and below the stud that fastens the mount to the engine proper. Getting all of these fasteners free is important.

Inspect the engine carefully for water, oil, and coolant leaks. Repair these now before installing the new engine mounts. Inspect the exhaust mixer elbow carefully as it might be leaking. It is just above one of the mounts and any leak will drop onto the mount. Fix it now — it will probably take longer than you think!

Yanmar mounts are manufactured specifically for each side of the engine. That is, either the starter side or the oil filter side, as engine torque will change the effective loading on either side of the engine. Make sure that you get the mounts positioned on the correct side of the engine.

With a felt tip marker, make a “witness mark” on the two halves of the shaft coupling before removing the coupling bolts. Using a flat-blade screwdriver, separate the coupling halves an inch or so to allow the engine to move free of the shaft. Soak the bolts in some rust remover. Do not try to raise the engine without first disconnecting the shaft coupling.

Digital calipers are used to measure the mounting height.

The next step is to raise the engine off the mounts by using the adjusting nuts on each mount, and a series of wooden blocks of various thicknesses. Be very careful and think your way through each step. Replace one mount at a time, as you surely do not want the engine to fall. A pair or two of wooden wedges is very handy in blocking up the engine. Cut these on a power saw before you start this project.

Before removing each mount, note the position of the enlarged mounting hole or slot on one end of the old mount. Install the new mount accordingly.

Clean the engine bed of rust and debris before installing the new mount.

Install all of the bed bolts in each mount, but do not tighten them yet, as you will need to move the mounts to align the engine.

After all four new mounts are installed, and the engine lowered onto them, let the engine sit for a day or so to “settle” the mounts. The elastomer will compress a bit. If you align the engine immediately after replacing the mounts, the engine will drop a bit on the new mounts and negate your fresh alignment job.

Using your digital calipers, restore the engine to the original height per your sketch. Carefully clean both faces of the shaft coupling as any grit or corrosion will prevent an accurate alignment. Reconnect the halves of the shaft coupling. Remember to observe the witness marks to return the coupling halves to their original orientation. Install only one coupling bolt and nut loosely — just barely finger-tight. Using a feeler gauge, align the engine horizontally and then vertically. A rule of thumb is to get the alignment to less than one thousandth of an inch per inch of coupling diameter; i.e., to within 0.004 thousandths of an inch for a four-inch diameter coupling.

Repeat the vertical and horizontal alignments a few times, as each adjustment will change the other. Take your time and get the engine aligned as close to perfection as possible. There are several good YouTube videos on how to align a shaft or coupling if you are new to this game.

Drilling an engine bed bolt hole with right angle drill.

After making an adjustment, give the engine a few good shakes to settle it before checking the alignment with the feeler gauge. When you are satisfied with the alignment, tighten all of the engine mount bolts. If one or more of the engine bed bolt holes require redrilling, a right angle drill will make quick work of the job.

Check the alignment once more with the feeler gauge, clean the faces of the shaft coupling, and replace the coupling bolts after cleaning the threads with acetone and applying a drop or two of blue Loctite on the threads of each bolt.

Check all of the engine mount bolts for tightness before starting the engine. Run the engine in gear, over a wide range of revolutions, listening and feeling for any vibration. Shut the engine down and feel the coupling. A properly aligned coupling will not be hot.

Harry Hungate and his wife Jane replaced their engine mounts in Gaeta, Italy, last year before crossing the Atlantic Ocean. They are now cruising the U.S. East Coast aboard their Corbin 39 Cormorant.

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Feb 10, 2016 04:04 pm
 Posted by  Avocet

In a twin Yanmar 4JH4E engine installation, the propellers rotate in opposite directions. This means that the torque resistance is generated in a clockwise direction on one engine and in a counter-clockwise direction on the other. So, my question is, should the installation of engine mounts on one engine (e.g. 200 on starter side and 150 on oil filter side) be mirrored/reversed on the other engine? Or do the transmissions eliminate the effect of the different propeller rotations and therefore the engine mounts should be installed in the same location on both engines?

Nov 23, 2017 02:39 pm
 Posted by  YYY

Hello. I've got a Mercedes OM636 engine on a Westerly Conway and it needs new mounts. The current mounts are 100mm center holes, the new ones are 110mm center holes. I'm thinking of using one existing holes on the brackets and drilling only a single new hole for each new mounts. That way I need to drill only 4 new holes instead of 8 new ones and the new holes will not have a figure 8 shape (if i drill new holes for each, since the holes are 10mm wide, adding 5 mm to each in order to accommodate the longer mounts, will create a figure 8). This will require to mount the engine 10 mm back (aft) on the brackets than it is currently mount. There is extra 26mm length on the rear end of the brackets aft of the existing holes so this can be done, and, there's enough space for the propeller to move back with the shaft 10 mm so it does not hit the hull/rudder. Im wondering if this is a feasible thing to do or am I better off keeping the existing engine placement on the brackets. What other repercussions might be if I do that? The coupling on the shaft are simple to adjust as they have I believe no place for a feeler gauge. Thanks in advance for your opinion!

Feb 25, 2018 11:22 pm
 Posted by  Bruce

This part of the article: "Specify replacement mounts from the engine manufacturer, or after-market mounts that have given satisfactory service on an engine identical to yours. Avoid experimentation as it is both costly and aggravating." should be heeded!

We tried R&D mounts for our Yanmar 4JH2-TE and they were terrible. The vibration at idle was unbearable. A costly experiment indeed!
s/v Migration

Jun 18, 2018 09:52 pm
 Posted by  Cam from Isoflex

Good people, there are many after market mounts that are available and in many cases far superior to the original ones supplied. I must declare that I am a manufacturer, however, I know what efforts we put into improving performance of marine engine mounts. We manufacture thousands. We are aware of manufacturers that cater for the let us say "economy" end of the market and indeed make mounts that are incredibly stiff. Vibration transfer will occur, especially at idle. The other extreme, too soft, will often exhibit vibration at high rpm, where thrust creates misalignment. Selecting the right mount is critical leading to improved vibration isolation and longevity. As our host above pointed out, installing them is critical too. One point he may have missed was that the mounts need to be equally loaded. ie the weight on each mount needs to be the same, or in some cases both front and both rear. Failure to do this creates "soft footing" or "tripoding" like a rocky table in a restaurant. Critical! You need to measure the deflection or "squash down" on each mount to make sure they are very close to the same. This is not the height of the adjuster nut, but the squash of the elastomer or "rubber". I make the comment we use polymers for their superior resistance to oil, fuel, water and rubber and consistency in stiffness. Whilst we are considered aftermarket, we supply to most OEM engine companies for their more demanding commercial installs. I am disappointed that using ineffective after market mounts was a fail for you.

Jun 23, 2018 12:07 am
 Posted by  ed f.

Great posting... thank you! After exceeding my patience in waiting for a mechanic to do this, I'm about to attempt it myself. So thanks for the input and vote of confidence!
1) Does anyone know where/how to find what side mount goes on which side? I'm certain mine are factory installed (13yr old boat) so I "should" be able to go by what I take off, but I'd like to KNOW for sure as well. And resources/references are alway good to have.
2) In regards to Cam, this "squash" you talk of makes sense. My question... wonderment... is, if the mounts are worn, which I'm sure mine are deflated (time and excessive vibration), why would I measure where they are NOW? Wouldn't that be an artificial setting? I'm guessing... assuming... there's a drive line from out of the engine to the tip of the prop shaft, which effects the shaft in the cutlass bearing, and through the packing gland. I'd think it'll never be back to that manufactures "drive line" it originally was. So should the engine sit freely on all 4 mounts, with no pressure on the adjuster nut... will that give it a natural evenly distributed weight bearing on the mounts? Then... start alignment?

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