Debris from a foreign object in a cylinder has a habit of wedging itself into the gap between the piston and the cylinder wall, on top of the top piston ring. If it's metallic, it can create score marks in the cylinder walls, leading to compression loss and blowby. I'd suggest pulling the pistons and checking for debris imbedded around the piston rings. I'd also reccomend a full teardown of the cylinder head, and an inspection of the valve seats, etc.
Jerry
