Actually, the engine will run for a while, provided you get a nice carbon ring around and down the piston skirt. This carbon layer will fill in the areas where there is no metal contact, forming a nice compression seal. Leakdown is the killer of any engine. I find some engines have nice compression, but slowly leakdown to TDC. Thats the sign of a worn or bad motor, and they usually can't keep the idle going for long.