A chest compressor includes a piston (14) that moves in downward and upward strokes, with the piston undergoing a smooth reversal at the bottom of the downward stroke. A compression spring such as a wave spring (60), is positioned to engage the piston only near the end of its downward stroke, to smoothly reverse the piston motion, limit downward force on the patient at the end of the stroke, and avoid a downward pulse due to the momentum of the downwardly-moving piston. A stop (90, 92) is latchable in an inward position to allow reduction in the piston stroke by engaging an outward flange (56) on the piston before the piston has moved fully downward.