Austin 7 - Engine and Transmission

Engine and Transmission

The original 1922 four cylinder Austin Seven engine had a bore of 2.125" (54mm) and stroke of 3" (76mm), giving a capacity of 696cc and RAC rating of 7.2 hp. From March 1923 the bore was increased to 2.2" (56mm) giving 747cc and 10.5 hp. The side-valve engine was composed of an aluminium crankcase, cast iron cylinder block and cast iron cylinder head. Cooling was by thermosiphon, without a water pump, and the generator was driven from the timing gears.

The big end bearings were lubricated by jets from an oil gallery in the crankcase, the oil striking the crankshaft webs which were drilled accordingly. Originally the shaft diameter was 1.25"; this was later increased to 1.3125". The three bearing engine used a white metal centre bearing.

The splash lubricated crankshaft (pressure fed on some sports models) at first ran in two bearings changing to three in 1936. An electric starter was fitted from November 1923. The early cars used magneto ignition, but this was changed to coil in 1928.

The 3-speed (4-speed from 1933) and reverse gearbox was integral with the engine, and had a variety of ratios depending on application. A four-speed gearbox was introduced in 1932 and in 1933 synchromesh was added to third and top ratios extending to second gear in 1934.

The back axle was of spiral bevel type with ratios between 4.4:1 and 5.6:1. A short torque tube ran forward from the differential housing to a bearing and bracket on the rear axle cross member.

Reliant used a derivative of the Austin Seven engine for their early three-wheelers, before developing an aluminium alloy engine (used as the replacement for the Austin Seven engine by the 750 Motor Club in their Formula 750 motor sport).