The previous owner of my boat found me and we were discussing the boat and found out he had the same problems (prop spinning in neutral) as I'm having with the starboard transmission. My prop spinning has become worse thru the years and now is to the point where I need to fix it. The trans fluid is always bright red and does not smell burnt. He said that he had it rebuilt and it only lasted 10 hours before the plates were warped.
He had a well known guy rebuild the trans the 2nd year he had the boat. The trans didn't last more that 60 miles before the plates were warped again.
He had it rebuilt again then wound up selling the boat.
He suggested that something in the trans was out of spec and that I should start off with another trans.
Now that I know some more history of the boat. I wonder if my bellhousing is suspect too. When I pulled the motor several years ago I noticed that the bellhousing had been welded up apparently with a brazing rod or something. The welds look very good. The cracks are where the trans bolts to the bellhousing. When I slid the trans into the bellhousing, everything went together smoothly and the trans bolts did not hang up in the threads. Now I wonder if the bellhousing is suspect too?
Is this common for the bellhousing to crack? Considering that the parts are original from 1980. The engine is a factory 454 twin turbo Gale Banks Mercruiser SSM II. The port trans seems fine and does not spin in neutral.

Here is a bad picture of the crack under the starter.

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Those cracks can happen when a drive locks up. Or of course flexing in the hull. I wouldn't think that's causing the clutch plates to warp. Usually the plates warpbwhen ran low on fluid, overheated, etc.

Maybe Marc from BAM will chime in, as he's the man to talk to on these trannys.

Was the trans ever put on a run stand and checked for volume of flow and pressure? Bad sealing rings, worn bushings or even a worn shaft can significantly reduce pressure and flow.

I don't know much of the history of the trans. I am the 4th owner of the boat. The second owner was the one who told me about the trans problems and this was in the late 90's when he owned it.

Incorrectly adjusted shift linkage could also cause issues. It is mandatory in these transmissions that the shifter goes fully into gear (fully seated in the detent on the tranny) or you are not getting full pressure to the plates and that will cause premature failure.

Just a thought!

Btw I have a couple BW bell housings and working BW transmissions that I am selling for cheap if interested.

Stick some gauges on it and make sure you have zip on the f and. R. Ports when in neutral .