The radial gasoline engines of previous medium tanks M3 were also needed for training aircraft, and an engine shortage was foreseen. To help with the situation, the General Motors 6046 was created. The GM 6046 diesel engine was a pair of GM 6-71 truck engines, each of which could be operated independently if so required. The engines were meshed together on a helical gear which was mounted on a propellor shaft connecting to the tank's transmission. The side and rear armor plates on diesel-engined M3s extended below the level of the track return run, and the rear hull plate was sloped to accomodate the larger diesel engine. The engine compartment was accessed through a single lower rear plate instead of the twin doors on the radial-engined M3s. An exhaust deflector was installed to reduce the dust signature of the tank, and two armored doors with air intake louvres were installed over the tank's engine compartment. The M3A5 was borne out of the dire shortage of tanks which necessitated the installation of the diesel engine in riveted-hull tanks as well as welded-hull vehicles. M3A5 was referred to by the British as Grant II.

The M3A3 was a welded-hull Lee with the GM 6046 diesel engine. The British called diesel-engined M3A3s Lee V and Continental-engined M3A3s Lee IV. This may only be a paper designation, as no M3A3s were produced with the Continental R975. The British, trying to keep pace with the plethora of tank types emerging from the US, reserved designations for expected production models. Lee IV is apparently just such a designation that was reserved but whose vehicle never saw production.

The M3A4's A57 engine was actually five Chrysler automobile engines geared to run as a single motor, and was devised to help alleviate the shortage of tank engines. The size of this conglomeration required extending the hull of the M3A4. There is a distinctive space between the suspension bogies of M3A4s which is about 6" (15cm) longer than that of other M3s. Bulges in the engine compartment floor and roof accomodated the A57's fan and radiator, respectively. M3A4 was called Lee VI by the British.

The A57 was comprised of five L-head engines, each having its own crankshaft, pistons, camshaft, valves, manifold, carburetor, and distributor. They were assembled to a common crankcase. Each of the five engine crankshafts spun a drive gear which drove a larger gear that was keyed to the power unit driven gear shaft. This shaft drove the engine clutch.