The engine was an air-cooled 90 hp Armstrong Siddeley engine derived from an aircraft type. Surprisingly the engine and transmission was distributed throughout the hull - with the engine to the left of the driver, the gearbox underneath the commander and final drive at the rear, which Duncan describes as "an unbelievable retrograde step in view of war-time experience". The Medium Mark B and the Mark VIII heavy had introduced compartmentalisation to reduce the debilitating effects of engine noise and fumes on the crew. However, with the Medium Mark I considerations of ease of maintenance took precedence.