Engine and Gearbox

This is a four-cylinder overhead camshaft unit of 875 c.c. with light-alloy cylinder block and cylinder head with cast-iron cylinder liners. The unit is located at the rear of the car at an angle of 45° and is bolted directly on to the combined gearbox and rear-axle unit, known as the transaxle. The cylinder block, cylinder head, cambox cover and timing case are in aluminium alloy, making the engine exceptionally light. Cast-iron cylinder liners are inserted during the manufacturing process and cannot be removed, but may be bored out to take oversize pistons during major overhaul. Lubrication is by an eccentric-lobe oil pump mounted below the distributor in the front timing cover and driven, at half engine speed, by skew gears at the front of the crankshaft. A full-flow oil filter is used.

All the major castings for the engine and transmission were made from aluminium alloy and are produced in the company's new foundry, which adjoined the Linwood plant, The head was a low-pressure diecasting, as was the cylinder block, but the latter became a high-pressure diecasting. The advantage of using so much light alloy can be gauged from the fact that the complete engine weighs 1701b, or approximately half as much as if cast-iron had been used. Because the block was designed from the outset as a high-pressure diecasting, no sand the internal crank chamber is very smooth to permit the easy withdrawal of the moulding dies and the water jacket is open decked from the top. The centrifugal cast-iron liners are mechanically bonded in position during the casting process and the aluminium cores are used in its construction. Thus overlaps each end of the liner. This ensures rigid location and eases the machining of the top joint face.

There are three main bearings for the forged steel counter-weighted crank-shaft and they are lined with lead indium for sustained high-speed running. Journal diameters are 1.875in. and crankpins 1.625in. The forged steel connecting rods had an angular split for easy withdrawal through the cylinders and the joint faces are serrated for accurate location of the caps, each of which is retained with two set bolts.

The pistons had solid skirts, two compression rings (the top one chromium plated) and one scraper ring, all above the fully floating 0.625in. dia. gudgeon pin. On the front end of the crankshaft there is a spiral gear for the oil pump and distributor tandem drive, the latter being uppermost in a very accessible position. The pump is a Hobourn-Eaton rotor type fed through a large gauze suction filter. On the underside of the engine there is a full- flow filter connecting to the oil gallery, formed with a cast-in tube to avoid a drilled oilway, which was expensive and more difficult for ensuring freedom from swarf.

The drive to the single overhead cam shaft is by a single-stage simple roller chain equipped with a Weller blade-type tensioner. The camshaft operates in a separate aluminium diecast block, having three bearings and containing the inverted piston-type tappets. Valve clearance is adjusted by shims of graded thickness and single springs are used for closure.

The valves are angled at 20 deg from the cylinder axis and operate in a wedged-type combustion chamber without overlapping the cylinder bores. The advantage of this design is its well proportioned quench area, near-centre plug position, and a low surface area to volume ratio which permits the use of a 10-0 to 1 compression ratio when using Premium grade fuel.

Individual ports were used for the inlet and exhaust, the latter being connected to a single short expansion chamber lying beneath the tilted engine. The combined water pump and fan, and generator are belt driven from the front of the crankshaft and mounted on a die-cast aluminium bracket to the left of the engine. The fan and its stator blades, which direct the air through the side mounted radiator, are plastic mouldings.

In answer to popular demand provision is made for a starting handle, inserted through a hole in the bumper. The advantage of a combined gearbox and final drive can be gauged from the fact that this unit on the Imp weighs only 65lb with its oil.

The gearbox is a two-shaft all-indirect ratio type with synchromesh on the four forward ratios; a reverse gear train is mounted outside the main casings.

The synchromesh is of the baulk-ring type and the cone diameters-in fact the same size as those of the Hillman Minx --are very generous at 2.25in. diameter. On the first motion shaft the gears are fixed; first and second gears are integral with the shaft, third and fourth being shrunk on. A splined muff coupling on this shaft overcomes the problems of distortion during heat treatment of an otherwise long component. The hypoid bevel pinion is formed integrally with the second motion shaft and has an 0.625in. offset upwards from the crown wheel axis.

With the introduction of this new model, Rootes made a considerable break with their hitherto conservative design approach; there is, however, sound logic behind these innovations.