A walk power mower has a mower deck that encloses a pair of rotary cutting blades with the grass clippings from the blades being discharged through a rearwardly extending grass discharge tunnel in a rear bagging mode of operation. The blades are powered by a timed drive system that is carried by a horizontal apron that is part of a cast aluminum engine mount. The engine mount and the apron are bolted to a stamped steel deck shell that forms the cutting chamber in which the blades rotate. The mower has a side discharge mode in which a side discharge chute is releasably mounted over a side discharge opening. The chute carries a flow cutoff baffle having an inwardly protruding inner end that sticks through the side discharge opening to intercept and direct the clippings out through the side discharge opening and through the chute in the side discharge mode.