A fluid pressure operated piston engine apparatus generally includes a piston unit, a valve configured to selectively direct pressurized fluid into the piston unit, a valve shifting mechanism, and a magnetic detent device. The magnetic detent device includes a valve actuating member coupled to a valve element in the valve. The valve actuating member is movable between first and second magnetically held positions respectively holding the valve element in first and second positions. The valve shifting mechanism includes a biasing device operatively connected to a shaft associated with the piston unit. As the shaft approaches the first and second ends of its stroke, the biasing forces are used at least partially overcome magnetic attraction holding the valve actuating member. In this manner, the valve actuating member moves between first and second magnetically held positions to cause the valve element to shift and redirect pressurized fluid into the piston chamber of the piston unit to effect reversal of the direction of travel of the shaft.