The MGB cylinder head is an overhead valve design with the valves seated vertically in line, operated by pushrods and rockers from a camshaft in the block. While the basic architecture remained the same throughout production, there were important changes to the head specification across the engine range that affect compatibility, valve sizing, and the oil feed arrangement for the rocker assembly. The most significant distinction is between the 18G-series heads used from 1962 to 1971 and the 18V-series heads used from 1971 to 1980.
Early 18V chrome bumper heads had larger inlet valves than both the preceding 18G types and the later rubber bumper heads, the rubber bumper 18V engines from September 1974 reverted to the smaller inlet valve size originally found on the 18G series. This is a detail easily overlooked when sourcing a replacement head, as the external appearance of the heads is very similar.
Rocker Oil Feed Arrangement
Within the 18V-series heads, a further distinction exists in the oil feed arrangement to the fourth rocker pedestal. Earlier 18V heads had the oil feed hole positioned in line with the two pedestal studs, while later heads had the feed hole offset. The modification is assumed to have occurred around the introduction of the rubber bumper models, but factory-rebuilt 18V engines can be found with either type regardless of whether the car is chrome or rubber bumper.
This means the rocker assembly must be matched to the cylinder head, in-line feed heads require an in-line rocker assembly, and offset feed heads require an offset type. Reconditioned cylinder heads are available for both the chrome bumper and rubber bumper 18V specifications, built with hardened exhaust valve seats to enable running on unleaded fuel. New cylinder heads are also available. For performance-oriented owners, Stage II cylinder heads are offered, these are polished and ported to improve gas flow, fitted with new valves, valve springs, and bronze valve guides alongside hardened exhaust valve seats.
Stage II heads can be built on the customer’s own cylinder head or purchased as a new outright assembly. Individual valve train components are available separately. Inlet and exhaust valve guides are available for owners carrying out their own head rebuild. Valves, valve springs, and associated retainers and collets changed during production, the 18GD engine introduced new valves with chrome stems and a narrower notch at the top of the stem, together with revised valve springs, cups, and cotters without circlips.
The 18V engines used single valve springs throughout.
Roller Rockers & Valve Train
Roller rocker assemblies are available as a performance upgrade, offering lower friction and more precise valve clearance adjustment than the standard pressed steel rockers. They are offered in both in-line and offset feed configurations, and in standard and high-lift ratio options, the high-lift ratio type must be used with the correct combination of valves, valve springs, and camshaft to avoid valve-to-piston contact. The head gasket should always be renewed when the head is removed, and the head bolts and all associated washers should be inspected for stretch or corrosion. The rocker cover gasket, thermostat housing gasket, and manifold gaskets should also be renewed during a cylinder head service for a complete and reliable reassembly.
The MGOC Spares range includes reconditioned and new cylinder heads, Stage II performance heads, rocker assemblies, valve train components, gaskets, and all associated fixings to cover the full range of MGB engine types.