RE: Effect of a Scratch
I can't find the picture of it, but I have a Fox airplane engine that ate a wristpin G-clip.. Actually, it ate 4 different clips scoring the iron piston and steel liner. I put a new piston and liner in it with new clips and it did the same thing. I guess the wristpin floats back and forth quite a bit and pushes the clips out. Anyway, despite having a scratch that goes from the top of the piston to the bottom and a scratch in the liner the entire stroke of the piston, it does still run and idle pretty well. Running it fast and hot with a lot of castor oil in the fuel sort-of varnishes up the scratch masking its leak-down effect.
My Fox engine bleeds cylinder pressure down pretty quickly if you rotate the engine slowly, but sounds normal and has a nice bounce to it if its flipped over quickly.
I'm not surprised the OP's engine still runs with that scratch in it. I'm curious as to how it happened since the wristpin clips are still in place. Glow plug element wire break off at one time maybe?