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How to Replace Bad Isolation Mounts

Replacing isolation mounts

Adjust new engine mounts to the specs of those they replace.

Bad Isolation Mounts?

Feeling more vibration on board than you are used to? A prop issue? Well, unless you’ve hit something recently, or run the old girl aground, it’s possible you have bad isolation mounts. How do you know? Tough to say for sure, although there are some significant indicators. Again, excessive vibration is a biggie; so is the presence of black, powdery dust on or around an engine bearer—you know, like the dust from a worn-out drive belt. And finally, there’s excessive play in an isolator when you lift the corner of the engine it supports slightly via a steel bar or some other prying points. All of these signs point to elastomeric material inside the mount or mounts that is iffy.

A replacement job’s not as wicked as you’d think, however. But before you begin, bear in mind that most engines have four mounts (although some smaller engines used to be marketed with three) and there’s little point in replacing just one or two. Way better to do all four, thereby helping to guarantee equal amounts of flexibility and wear in the future.

Something else to bear in mind—take a pass on an A-frame or some other kind of overhead device to lift your engine. This strategy often makes things more difficult, complex, and even dangerous. Instead, find yourself a small hydraulic jack, of which there are many types and configurations on the market. Check the Web—you’ll find a model that’ll fit your needs.

And one more thing—remember to hit the old mounts with penetrating solvent (one of the best is Kano Kroil, available at a week or so before you crank up your project. This will seriously facilitate removal of lock-down nuts and other paraphernalia. 

The actual work is fairly simple, but go slow as even a small engine is bone-crushingly heavy. For starters, before you put a wrench on any of the old mounts, use a digital caliper to measure the distance from the surface of each engine bearer to the underside of each of the engine’s hangers or mounting arms. Use a Sharpie to mark where each measurement was made and write down the measurements in a notebook. The point here is to log the heights of the old mounts closely when you install the new ones. Now disconnect and slightly separate the flanges that connect the engine’s marine gear to its propshaft (a slotted-type screwdriver works best) and temporarily remove all other constricting parts, cables, or wires.

The next step: Lift one corner of the engine at a time with the jack, replacing each old isolation mount with a new one as you go along. And choose mounts that are either sold or recommended by your engine’s manufacturer. Yanmar, for example, sells Yanmar-only mounts that are specific to right and left sides of its engines, a move that adds flexibility to the mounts that must resist shaft and prop torque the most. Skip buying Yanmar and you may miss this important, longevity-producing feature.

Of course, the final step is a new engine alignment. And unless you’ve done one before (or you’ve got the patience of Job and more time on your hands than you know what to do with), hire a professional. The cost in dollars will be well worth what you’ll save in terms of mental anguish and total frustration.

This article originally appeared in the March 2017 issue of Power & Motoryacht magazine.