This 1972 Volvo MD1 single cylinder diesel marine engine saw service in a 1959 28' Kings Cruiser sailboat. The boat was decommissioned and I was given the engine for my collection.
It appears to be 55mm by measurement. I have never taken it off but if I were to, I would have to find or make a 55mm socket and knock it loose with my 1/2" electric impact wrench. It looks like a clockwise thread.