First, thank you to all who offered information and recommendations on resolving my failed transmission.
I have a new transmission. It came from Trans Atlantic Diesel in Virginia.
Finding a dimensional drop in replacement took precedence over all issues of horsepower and gear ratios. I did not want to replace prop shafts, relocate the engine or any other re-engineering activities.
The replacement is (should anyone else need such a thing):
ZF 12M (with oil cooler)
Part Number: 330500200
Ratios: A=2.14 B=1.95
Removal was just as straight forward as everyone has said, except for removing the reluctant allan head bolts attaching the thrust plate. All were stuck hard, but three days of PB Blaster soaking did the trick.
Greg Runkel of Runkel Diesel Marine, Bradenton Florida, 941.751.2886 did the research and installation.
Did a great job; anticipated and headed off all sorts of potential problems.
He also figured out the proper gear ratios and box dimensions. The label was missing from the old box (suspected to be a ZF 10 no oil cooler). Therefore, Greg had to calculate the old ratio by turning the input shaft and counting output shaft revolutions (both directions).
No way I was spending this kind of money without high quality professional help.
The new box fit perfectly. Even shaft alignment was unchanged (and proper).
It works great.
Been living with unpredictable shifting for years. Nice to know the boat will do what I tell it in the future. Lots of dollars but may save crashing into something expensive or being stranded.
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