Series I terminology

SIAMESE BORE The original 1.6-litre engine block was re-worked and bored out to form the 2.0-litre engine. The increase in bore diameter brought cylinder numbers 1 and 2, and numbers 3 and 4 very close together (conjoined, if you like) leaving no space for a water cooling jacket between them.

SPREAD BORE

In late 1953 (for the 1954 model year) a re-designed 2.0-litre engine block was introduced in which the bores were spread further apart, allowing a water cooling passage between each of the bores.

INLET OVER EXHAUST (IOE)

This refers to the valve arrangement of the petrol engines in which the inlet valves are in the top of the cylinder head (as on later engines), but the side exhaust valves are lower down in the engine block. The exhaust valves are inverted, and are seated in recessed combustion chambers formed into the top of the bore in the cylinder block. The exhaust valve stems protrude downwards inside the block where they are worked by the camshaft below them, via rockers. It’s a relatively complex and interesting arrangement carried over from Rover car engines, from which the SI engine was derived.

FREEWHEEL This unit, fitted until 1951, was borrowed from the Rover cars and worked by disconnecting drive from the front axle when driving on engine overrun, and re-connecting drive when accelerating. It made handling on road a tad erratic, but is a great period piece.

PULL RING

The freewheel could be locked (to give full time four-wheel drive) by pulling a ring near the driver’s floor when in low ratio. It was replaced by the more familiar yellow knob.