Four engine photos here have been procured from
the MGA Twin Cam Enthusiasts email group
with permission. (Click for larger images)

The MGA Twin Cam engine was derived as an extensive modification of the Austin B-series 1500 cc pushrod engine with larger bore for 1600cc displacement and a slightly stronger crankshaft extended at the front end (and forever unique to the Twin Cam engine). The in-block camshaft is retained in rudimentary form to drive the oil pump and the tachometer drive cable, and is generally referred to as the half speed shaft. It is gear driven rather than chain driven, so it rotates opposite the direction of the crankshaft. Using the same oil pump, the half speed shaft and the oil pump and tach drive gears have helical gear teeth spiraling in the opposite direction to maintain proper rotation for the oil pump and tach drive.

The cast aluminum twin cam cylinder head has the valves canted at a 40 degree angle from vertical (80 degrees included angle) forming a cross flow "hemi head" combustion chamber that allows for use of much larger valves, and accompanied by high crown pistons. The valves are driven by overhead camshafts which are chain driven from the half speed shaft by a long roller chain guided and adjusted by two idler sprockets (therefore rotating opposite direction from the crankshaft). The pattern of head bolts is rearranged, particularly on the exhaust side of the engine, to allow four discrete exhaust ports (in the absence of pushrods), and four separate intake ports. Thus we have an eight port cylinder head where the pushrod engine had only five ports (two intake and three exhaust).

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The Twin Cam distributor is at the front left side of the engine, mounted in the new taller cast aluminum timing chain cover, and the earlier distributor drive boss on the right side of the block is left blank, not machined. Dual SU H6 carburetors are mounted on the right side of the engine. Spark plugs are on top down the center of the cylinder head. The heater water valve is different and is mounted on the heater shelf in front of the heater box.

The engine number plate is attached on top of the block at the back of the engine. The oil filter is mounted in the original position on the right side of the engine. The oil sump is enlarged, particularly in width, and fabricated of cast aluminum. The Twin Cam engine rear plate incorporates the high starter position to match the gearbox. Water pump, cooling fan and generator are still belt driven, but the generator has a larger pulley so as not to run too fast with the intended higher engine speed. Early in production the generator pulley was reduced in size for better battery charging at low engine speed, but was still larger than the pulley on a pushrod engine. Picture above shows an incorrect generator with wrong end plates, fan and pulley from a pushrod engine (and misaligned fan belt). See article TC-314 for more information on generator fan and pulley variations.

The Twin Cam radiator is positioned farther forward, requiring complimentary changes to the radiator (shorter height), the radiator mounting diaphragm, and the horizontal air pan ahead of the radiator. There is also a remote expansion tank for the cooling system located high at the left side of the engine and attached with a pair of bolts to the rear branch of the exhaust header casting. A thermostat housing is located between large hoses just ahead of the remote tank. After a short period of production the remote tank got an even more remote pressure relief valve connected by the small overflow hose.