MGA cylinder head explained

Early MGA 1500s were 68bhp, then up to 72bhp after the first year (with a little better porting). Original top speed 91mph. The 1600 engine was virtually identical except for the cylinder bore, putting out 79bhp. This used the same "15" cylinder head as the 1500 engine. Original top speed 97mph.

The 1600-MK-II engine (1622cc) was more evolutionary, having a slightly stronger crankshaft, larger wrist pins, one size larger valves and a little better porting in the head, larger volume combustion chamber, and flat top pistons to make the compression ratio come out right, then producing 87bhp. Cylinder head is marked "16" on top at the back. These engines and heads are just a bit rare, having been used for only 14 months of production in the MGA. Original top speed 103mph (after changing the final drive gears from 4.300:1 to 4.100:1 to avoid revving past the red line in top gear). Some early 1622 engines (about 300 units) were apparently fitted with heads marked "15", but having the combustion chambers machined out to the new "16" specs with larger valves.

The MGA Twin Cam engines were always 1600cc, even when used in the 1500 chassis. The Twin Cam head is definitely NOT interchangeable with the others.

The early MGB 1800 3-main-bearing engine 1962-1964 was nearly identical to the 1622 engine except for larger bore and main bearings, flywheel lightened about 8 pounds, and a diaphragm clutch with larger release bearing, all of which can be used in the MGA. The early "18" head used here is nearly identical to the MGA "16" head used on the 1622 engine, except for a minor difference in the shape of the "Weslake promontory" (heart shape in combustion chamber between the valve heads). This head is also in more plentiful supply (and somewhat cheaper). With head and carbs nearly identical to the 1622 engine, the max power output wasn't so much different, about 96bhp in its finest form (1967 spec), but they do have about 7% more torque from low to mid speed because of the larger displacement.

MGA Year Engine type OEM Head Casting Features
Part No. No.

A 55-56 BP15GB 48G31 12H1670 Dual valve springs
1H1060 39cc chamber volume
1.500" inlet, 1.281" exh.
Type "A" valve hardware
"15" on top at back
68 bhp at 5200 rpm

B 56-59 BP15GB/15GD 48G31 _______ Dual valve springs
1H1060 39cc chamber volume
1.500" inlet, 1.281" exh.
Type "A" valve hardware
"15" on top at back
72 bhp at 6000 rpm

C 58-60 BC16GB/16G AEH39 _______ Dual overhead camshafts
AEH29 86.6cc chamber volume
1.59" inlet, 1.44" exh.
108 bhp at 6700 rpm, 9.9-CR
100 bhp at 6700 rpm, 8.3-CR

D 59-61 16GA 48G31 _______ Dual valve springs
48G222 39cc chamber volume
1.500" inlet, 1.281" exh.
Type "A" valve hardware
"15" on top at back
80 bhp at 5600 rpm

E 61-62 16GC ______ _______ Dual valve springs
43cc chamber volume
1.562" inlet, 1.343" exh.
Type "A" valve hardware
"16" on top at back
93 bhp at 5500 rpm