 The 500 cc Triumph 5TA followed, and the 650 cc models were made unit construction in 1963.The 1963—1969 unit construction 650 cc Triumph Bonneville has become sought- after models, partly as the 1970-onward oil-in-frame chassis was considered inferior.The BSA Bantam range of two-stroke engines introduced the unit construction concept to BSA since its introduction in 1949.