Abstract: A two-stroke cycle diesel engine having ported cylinders is provided with a piston having an accumulator volume between the two top piston rings, the volume being sized large enough in relation to the piston ring leakage area and operating conditions to limit the build-up of gas pressure between the rings to a value below that of the cylinder pressure during nearly all conditions of normal engine operation. The arrangement substantially stabilizes the position of the top piston ring against the bottom of its piston ring groove, thereby avoiding shock loading of the top ring through cyclic unseating due to differential pressure reversals.