Abstract: A motorized lawn mowing machine has a generally horizontal deck with an engine releasably mounted thereon. A vertical drive shaft driven by the engine protrudes through an opening in the deck to mount a grass cutter blade thereon below the deck. A releasably mounted adaptor plate is mounted on the deck between the engine and deck, the plate having an opening for passing the shaft and having radially extending extension arm mechanism with parallel arms projecting radially at a sidewisely forward angle. The plate houses a first pulley fixed to rotate with the shaft at the level of the arms. The arm mechanism slideably supports a vertical flail assembly having an upper second pulley in alignment with the first pulley. An endless belt drive member is trained around the pulleys, and manipulatable mechanism is connected to slide the vertical flail device on the arm mechanism to engage and tauten the endless member to rotate the flail assembly.