Just looked back there now. The rubber actually exploded there are pieces everywhere. Same thing that happened last time. The spline still looks new from what I can see of it.

Stringers, motor mounts all very tight and in great shape.

I don't think there is a possibility of teh transom being soft at last survey moisture reading was 5 or 6%. The surveyor said it was one of the driest boats he ever checked. He said a brand new boat will hit 6 on his meter. The boat has only been in the water 30 times since then and only over night once.

The only thing I can't tell the condition of is the rubber and steel bushings that are installed in the bell housing. When I pry on that with a bar it is thight as can be. Those were replaced when I replaced teh motor 2 years ago.

Still searching....

OK, long shot, but I seem to recall reading something about HP couplers requiring somewhat of a different process over stock when putting the motors in. Wish I could remember the thread. That is the only thing I haven't seen mentioned already. Hope that helps, I'm out of suggestions.

Thanks for the info I will search around for thr post you are talking about.

I remember that too but I think it was something to do with the drive shaft bottoming out in certain couplers and pushing on the coupler, in turn stressing the main bearing.

But i could be wrong.

im having the exact same problem! i just blew the second coupler in 30 mins of boating! my motor mounts a offshore as well and the strigners are fine but we did find the top gears in the XR drive were worn and blue, my drive is currently being looked at by a mercury tech as it is still under warranty, my mechanic doesnt understand why this is happening either as alignment is perfect, every one is saying alignment and everything seems fine in my boat, we even tested the stringer by lifting on the motor while it was still bolted to the mounts and it pretty much lifted the boat off the trailer so its safe to say the stringers are fine! dont know whats going on either

the drive/input shaft have some runnout?? a little wobble would do the same thing as a bad alignment...

what do you mean by runout??

very slight bend in the shaft

Please keep me posted with your outcome. I'm pulling my hair out.