My knock on the M-602 did not turn out to good. I was hoping for a stuck valve-broken spring or anything but it was the back rod bearing. I had good oil presure and level and while antifreeze was leaking in oil it still looked like pretty thick oil and should have worked but I guess I pushed it to the limit as all the bearings were showing thier wear. Note - Noise always seem like it was from the top of the engine and when I got the piston out it was smashed clean on top around valve opening so you can't go by what you can't see. Made a second mistake of running it more after knock was noticed and ruined the crankshaft which could have maybe been avoided with a simple tow back home.
A M-602 has a access hole where the Oil filter goes which I could reach in and feel the play at a certain place in rotation. Pulling the oil pan is not all that difficult.
I sure would not run it to much trying to figure it out. I found a good used Crank shaft but things are adding up pretty quick on the repair. The bigger the engine the more expensive parts are. Cleddy