The piston ring compressor is nearly indispensible for compressing the piston rings on the piston while inserting the piston into the cylinder (without damaging the pistion rings). It is also one of the least expensive tools, in the $6 to $10 range, so you don't have much cause not to buy one (unless you can borrow it). The thumb tab on mine says "Hastings, Made In USA", and it was purchased from J.C.Whitney for about $5 in the early 1970's.

This is basically a thin band of spring steel with a tornequet on one side. Oil the piston well, then use the "L" key to wind up the small shaft to constrict the tool onto the piston. A small ratchet latch will hold the shaft to keep it from letting loose (until you release it). Then drop the piston skirt into the cylinder, hold the base of the spring ring securely against the top of the engine block, and tap the piston out of the tool into the cylinder (a wooden hammer handle works well for this).

Trying to push piston rings into the cylinder bore with a couple of screwdrivers is sheer folly (and not too smart either). If you scratch or chip the edges of the piston ring you can end up with an oil burning problem, and maybe even a scretched cylinder wall. If you break a piston ring you're really out of luck, because there are never any spare pieces in a piston ring set.