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CYLINDER HEADS for MGA -- CH-100

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.

In the chart below, the two part numbers are:
1st = Cylinder head assembly less rocker gear,
2nd = Bare cylinder head with valve guides.

MGA Year Engine type OEM Head Casting Features Part No. No. A 55-56 BP15GB 48G31 12H1670 Dual valve springs 1H1060 (1H967) 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 12H1670 Dual valve springs 1H1060 (1H967) 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 1H967 Dual valve springs 48G222 (12H1670) 39cc chamber volume (change 1.500" inlet, 1.281" exh. of part Type "A" valve hardware number) "15" on top at back 80 bhp at 5600 rpm E 61-62 16GC 48G216 12H432 Dual valve springs 48G215 43cc chamber volume 1.562" inlet, 1.343" exh. Type "A" valve hardware "16" on top at back 93 bhp at 5500 rpm Other UK 1500 commercial engines: Car Year Engine type OEM Head Casting Features Part No. No. BP15J 11G311 48G136 (later part number) 15JC " " " 15JE " " " The following 1500/1600/1622 engines are Australian BMC origin: Car Year Engine type OEM Head Casting Features Part No. No. A misc 1H684 "15" Morris Major, Austin 1H798 Lancer, Wolseley 1500 March 1958-October 1959 B misc 11G216 MOWOG (improved combustion chamber with casting relief on inlet side wall) C misc 11G311 AYH197 (later Australian part no) D 1622cc 16JC 48G222 1620 Morris Elite, released 16JE " April 1962, discontinued E 1622cc 16JD 48G432 February 1964 16JF " 16JK " 16JL " F 1622cc 16U 12H2221

There were several types of cylinder head fitted to the North American market MGB's. These can be identified as follows:

MGB Year Engine type OEM Head Casting Features Part No. No. A 62-64 18G/18GA 48G318 12H906 Plain, Dual valve springs 43cc chamber volume 1.5625" inlet, 1.343" exh. Type "A" valve hardware "18" on top at back B 65-67 18GB 48G318 12H1326 Plain, Dual valve springs 43cc chamber volume 1.5625" inlet, 1.343" exh. Type "A" valve hardware "18" on top at back Improved casting quality C 68-71 18GF/18GH/18GK 48G538 12H2389 Air injection ports Dual valve springs 43cc chamber volume 1.5625" inlet, 1.343" exh. Type "B" valve hardware "18" on top at back D 72-74.5 18V584/18V585 48G644 12H2923 Air injection ports 18V672/18V673 (12H2709 Single valve springs European 38cc chamber volume Spec.) 1.625" inlet, 1.343" exh. Type "B" valve hardware "18" & "L" on top at back E 75-80 18V797/18V798 BHM1062 CAM1106 Air injection ports 18V801/18V802 Single valve springs 18V883/18V884 43cc chamber volume 1.5625" inlet, 1.343" exh. Type "B" valve hardware Single valve springs Water port, top at rear Rear pedestal oil hole offset F 75-80 Replacement type --- 12H4736 Similar to CAM1106 except: Air injection ports (usually drilled). Dual valve springs in most cases. Patent number relocated inside. Water choke outlet hole drilled or not. 43cc chamber volume 1.5625" inlet, 1.343" exh. Type "B" valve hardware "18" on top at back G misc 12H2709 "O" head

There were other types of cylinder head fitted to MGB's not sent to the North American market. For the European market there was a 1968-1970 "18" head which was similar to the North American "18" head but with no air injection ports. For European market 1971 (some late 1970) and later MGB there was an "O" head which is similar to the 1972-1974 North American "L" head, but with no air injection ports.

In general, any Austin B-series engine head will bolt onto any B-series engine block (except for the Twin Cams). Pay attention to chamber volume and resulting variations in compression ratio.

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