Hi there,
Sorry that this is three years too late! I didn't even notice this until now!
I hope you got your problem solved.

I did eventually, and it turns out my problem was actually caused by too large a gap between cutlass bearing and prop, causing torsional vibration. Seems so obvious in retrospect, but a whole variety of mechanics - including transmission company manufacturer staff didn't come to this conclusion. It was a combination of that and my flex mounts being too flexible, or rather flexible enough to aggravate the situation.

It was finally figured out and fixed by the team at Coecle's Harbor Marina where I stay in summers now. They manufacture boats, so they really know their stuff.

Hi Sgt. Pluck, I have just read about your experiences with your M-25XPB. I installed the same motor last year, and have had two transmissions go, same as you, in under 300 hours total. I also find that the damper plate is damaged, with the teeth in the socket worn to about 50%.

How did you rectify the problem? I am negotiating with Westerbeke under warranty, but am a bit isolated, stuck in the Marshall Islands until I can get the motor running again.

I would appreciate any advice; sounds like you've been down the exact same route.

Cheers,
David Fisher.