I have a 1956 Fordson which was shut down for a bad knock, and the seller told me to not start it because a rod was about to let go. I believed him, because I"ve seen Fordson engines in the salvage yard with No 1 rod out the side. (Also an NAA, and a 801, but No 4.) I took the pan off and found the rods good and tight. Noise came from front, so took the engine apart and found that the timing gear had broken its hub. Fordson changed the design shortly after.
It is also conceivable that the compression relief can cause pistons to hit valves.
There were several attempts to solve a headgasket problem. Involves protrusions on sleeves and different materials and thicknesses of head gaskets. Have to be careful with what you have. Wrong set up and piston could hit head. Might be a function of how warm the engine has gotten. Pistons and rods get longer, and oil films get thinner.