Ah Ha moment,,,,
 

Just a note to explain what I found.
I was tearing down my stock 260 Merc, very nice inside.(not using any of it)
Has to be the nicest factory cast crank I've seen in a while. But as I was unbolting the windage tray, one of the bolts was spinning. (would be third main back on the starboard side) The bolt was broken about 1/2 inch in the block.
Makes me a believer in a 4 bolt block.
Jody