Replacing Motor Mounts

On my initial purchase survey it was noted the motor mounts needed to be replaced.

I had never done this, it did not seem to bother me, and my boat did not vibrate unless idling, but boy did it vibrate at idle.

Since we had rebuilt the tranny, I wanted to get everything perfect. If the motor was not properly supported and aligned it would cause wear on the transmission bearings and seals, along with the cutlass bearing and stuffing box. So after severe head scratching, I determined it was possible to replace one mount at a time with new identical improved mounts.

Forward original motor mount

Original motor mount, beyond worn out

A call to Brian at American Diesel got the parts moving. Although Brian said no, I think the new mounts have a stainless steel cover. I painted these just to make sure, and used stainless washers and such in some spots.

Comparison new vs original

Before tearing down I measured each mount several times and recorded the numbers. This came in handy, because at some point a boat yard added 3/8” shims under each mount. I loosened the prop shaft hub until it was barely attached.

The new mounts were almost exactly 3/8” taller since they are new. Removing the shims got me at almost the lowest setting on the new mounts. Before leaving the engine was put within .1” of where it was. By removing the bracing on the engine the mount could be lifted straight out easily. Replacing the mount took some gentle prying with a 2×4 to lift the engine to refit the new mounts.

New mount installed

The install took about five hours for all four. Once installed I let the engine sit for a week to settle on the rubber mounts, then aligned it, which took another four hours.

The boat has a shaft collar zinc in front of the prop. A diver removed it allowing the hub faces to mate so I could align the engine.

The prop shaft hub was set to about .001” gap. With the drive saver it is super smooth.

The noise reduction is dramatic at all rpms. My guess a 20db. reduction. There is now almost no boat vibration.