The cylinder head is one of the most frequently serviced areas of the Midget engine, and the differences between the 1275cc A-Series head and the 1500cc Triumph head are fundamental. No components are shared between the two, and even within the 1275cc range there are important variations that affect parts selection, particularly the rocker arm and adjusting screw combinations and the thermostat housing arrangement.
1275cc Cylinder Head
The 1275cc A-Series cylinder head is a cast iron crossflow design with siamesed inlet ports and individual exhaust ports. It is secured to the block by nine studs and nuts, with five plain washers fitted beneath the nuts on specific stud positions. Head studs should be tightened to the correct torque in the specified sequence, tightening in the wrong order or to the wrong torque can distort the head, causing gasket failure and coolant or oil leakage.
1275cc Valves and Springs
Standard valve sizes on the 1275cc head are 1.31 inches inlet and 1.15 inches exhaust. Valve guides are common to both inlet and exhaust positions. Valve springs are a dual arrangement with an outer and inner spring on each valve, and inlet valves are fitted with an oil seal on the stem. Hardened valve seat inserts are available for unleaded fuel conversion, fitting these allows the head to run on modern unleaded fuel without the valve seat recession that would otherwise occur on the cast iron seats of the original head.
1275cc Rocker Gear
The rocker shaft assembly bolts to the head via four pillars. Three distinct rocker arm types were used across production, forged, pressed, and sintered, each requiring its own matched adjusting screw and locknut. Mixing rocker arm types with incorrect adjusters results in poor valve clearance control and potential valve train damage.
1275cc Thermostat Housing
Three different thermostat housing covers were used during 1275cc production. The vertical-flow type (fitted to early 1275cc engines) directs the thermostat outlet upward. The crossflow types (fitted to later 1275cc engines) direct the outlet to the left-hand side of the radiator. The correct cover and gasket must be matched to the cooling system layout fitted to the car.
1500 Cylinder Head
The Triumph 1500cc cylinder head is an entirely different casting with its own valve sizes, port layout, combustion chamber shape, and stud pattern. It uses ten head studs rather than the nine on the 1275cc, and the head gasket is specific to this engine. The rocker gear is also fundamentally different: the 1500 uses individually numbered rocker arms on a four-pillar rocker shaft, with each arm specific to its valve position and not interchangeable.
1500 Rocker Cover
The 1500 rocker cover sealing arrangement changed during production. Early 1500 engines used a cover with conventional studs and a cork gasket.
Later cars adopted a different cover with O-ring sealing and revised mounting studs. The two cover types and their fixings are not interchangeable, and the correct type must be identified before ordering.
1500 Sealed Breather Detail
When fitting a chrome or alloy aftermarket rocker cover to the 1500's sealed breather system, a blanking screw is required to seal the vent hole in the oil filler cap that would otherwise vent to atmosphere. This is a commonly overlooked detail that causes oil mist in the engine bay, oil consumption, and potentially crankcase pressure that can push oil past gaskets and seals if left unaddressed.
Head Gaskets
The 1275cc and 1500cc head gaskets are not interchangeable. A head gasket set is supplied containing the head gasket itself plus the rocker cover gasket, manifold gasket, and the smaller seals associated with the head. For a complete engine rebuild, the head gasket set is used in conjunction with a conversion (lower engine) gasket set, which covers the sump, timing cover, and other lower engine gaskets. The conversion set does not include the timing cover oil seal or the rear crankshaft seal, these are purchased separately.
Performance Heads
Gas-flowed and ported cylinder heads are available as Stage II specifications. These heads feature enlarged and polished inlet and exhaust ports, reshaped combustion chambers, and larger valve seats to improve gas flow and power output. A Stage II head is a worthwhile upgrade for any engine that is being rebuilt, as it delivers measurable improvements in both power and efficiency without compromising reliability for everyday road use.