 From 1924, FN single-cylinder engines changed from semi unit construction (as seen in the last semi-unit single, the 1922 FN 285TT, in its last year of sale in 1924,) to unit construction engines (as seen in the new-for-1924 M.60).In 1928, BSA made their first and only two-stroke, a 175 cc unit construction bike, for only one season, otherwise four-stroke twins became unit construction in 1962.The 1930 Triumph 175 cc Model 'X' two-stroke, two-speed is their first "all-unit construction" two-stroke single-cylinder engine.