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 a 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 strokes be considered unit construction as most used premix or a separate oil supply to the crankcase