Austin 25/30

Each cylinder is a separate casting.[1]

The carburettor is an automatic diaphragm Krebs type. The automatic throttle is coupled to the centrifugal governor which is mounted on the same shaft as the fan and can only be controlled by the accelerator pedal. The magneto is driven from the inlet valves' camshaft as is also the commutator and the pump for the engine lubrication system.[1]

On the right hand side of the engine there is dual ignition at each cylinder first from the magneto supplying low-tension power to complex ignitors inside the inlet valve caps and again from conventional high-tension sparking plugs through a distributor from the accumulator (battery) dynamo and coil. The low tension igniter and the high tension sparking plug are mounted on the inspection cover above the inlet valves.[1]

A low tension make-and-break contact spindle rises diagonally from its camshaft through the main casting so it makes or breaks contact with the insulated contact pillar on the inlet valve cap. The necessary quick-break action comes from a coil spring at the bottom of the spindle. The high-tension sparking plugs are close alongside the contact pillars.[1]

The exhaust poppet valves are on the other side of the engine. Arranged symmetrically at the top of the engine the inlet and exhaust valves are interchangeable.[1]

The valves have a detachable cover over each valve giving easy access for adjusting seating or complete withdrawal. Between the cams and tappets there are fingers which can be set with the engine running. There is a similar arrangement for the ignition tappets. However the timing of the ignition can be varied with a lever above the steering wheel. The lever moves a timing shaft which moves the tappet pivot-pins inward or outward. The axis of the commutator shaft rocks at the same time.[1] This arrangement brought about oil sealing problems when pressure lubrication became more complicated.[3] The spur wheels that operate the two camshafts are enclosed in a neat dust-proof casing.[1]

There is a centrifugal water pump mounted on the front left of the crankcase casting to draw cooled water from the bottom of the radiator. Independently formed jackets surround each cylinder to carry the cooling water. When cast the lower end of the jacket is completely open. A dome-shaped fitting is inserted at the top for hot water to return to the radiator and the bottom is closed by the exterior of the cylinder casting. This form of construction is to eliminate any chance of overheating from undetected air pockets.[1]

T-head engine

A rectangular cross-section aluminium casting holds the five main bearings and the bearings for the two camshafts. This makes the sump below the crankcase independent and its removal does not disturb the engine's crankshaft or its bearings. Either camshaft can be removed sideways at any time through large doors which also give access to the small rock-levers through which the cams operate the pushrods. Another set of detachable doors on each side gives access to each big end bearing. The aluminium casting forming the crankcase rests on two cast-steel cradles which firmly hold the engine in the chassis.[1]

Oil is drawn from the sump by the oil pump and delivered to the three inner main bearings and to the other bearings within the crank chamber. Oil is led into the centre of the crankpins by oil scoops which lubricate the big end bearings from inside. The end main bearings and each of the four cylinders receive measured doses of oil through a mechanical automatic lubricator. This lubricator, driven by chain, contains positive pumps to force the oil through each of six feed-pipes. Mounted on the dashboard the lubricator's sight glasses give visual confirmation oil is circulating.[1]

Key to engine diagrams[note 2]