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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.

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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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