BMW NOT unit construction R and K series have a separate gearbox bolted to the crankcase The term bolt up unit construction is sometimes applied as was the case with the original BSA A7 where the separate gearbox bolted to the crankcase Also consider the Nimbus where the gearbox and engine use a common oil supply but the gearbox is again bolted to the engine Most of the American fours used a true unit construction Given the opening definition can 2 strokes be considered unit construction as most used premix or a separate oil supply to the crankcase