The ME21/24 engine was Shin Mitsubishi Heavy-Industries' (one of the three divisions of Mitsubishi Heavy Industries until their consolidation in 1964) replacement for the 309 cc single-cylinder overhead valve ME20 engine. Unlike its predecessor, this was a two-stroke, a concept to which Mitsubishi was to prove faithful for its smallest engines until the 1972 introduction of the Vulcan 2G21.