Norton Model 7 Dominator
The Norton Model 7 Dominator is a 500 cc vertical twin motorcycle that the Norton Motorcycle Company made from 1949 until 1955. It was the first of the Norton Dominator series of motorcycles. The engine was designed by Bert Hopwood, and departed from Norton's previous practice of making only single-cylinder machines. The Model 7 was used in Japan as a police motorcycle.
Background
In the United Kingdom after World War 2, motorcycle manufacturers resumed production of civilian machines after making military machines. The UK Government had launched an "Export or Die" campaign to aid economic recovery from war. Triumph's 1938 Speed twin had been well-received, and other manufacturers started to design their own 500 cc twins.Having worked under designers Val Page at Ariel and Edward Turner at Triumph, where he assisted with the design of the Triumph Speed Twin, Hopwood was recruited by Norton to design a new twin engine.
The new Model 7 Dominator, using Hopwood's engine in adapted Norton ES2 cycle parts, was launched at the November 1948 Earls Court Motorcycle Show.
Technical details
Engine and transmission
The engine has a 360-degree built-up crankshaft, with central flywheels. This meant that a central main bearing could not be fitted. The outer main bearings were a ball bearing on the timing side and roller bearings on the drive side. Alloy connecting rods were split at the big end, and used a bush for the big end bearing. The crankshaft and conrods had been tested in an earlier Norton twin, which was designed by J.E. Moore, but did not going into production. The camshaft is in front of the engine, driven by a chain from a half-speed pinion. For ignition, a Lucas magneto is fitted at the rear of the engine, and to power the lights a dynamo is fitted at the front of the engine. Both are driven by chains.The cylinder block is cast iron and has an integral push rod tube. The cylinder head is also cast iron, and has cast-in rocker boxes to eliminate potential oil leaks between head and rocker boxes. A shallow combustion chamber, combined with low included-angle valve configuration and flat top pistons gives a good swirl effect. The head has widely splayed exhaust ports to aid cooling. A single Amal carburettor fuels the engine via an alloy manifold. On the show model, the manifold was part of the head casting.
The engine has a traditional long stroke, with a bore and stroke of x, and produced at 6,000 rpm.
An alloy version of the cylinder head was available to special order in 1952. This had been developed for use on Norton competition machines from 1950 onwards, including the Norton twins used in the 1950 ISDT Competition.
Primary drive is by chain to a wet clutch, and is enclosed in a pressed steel chaincase. To fit in the ES2 frame, a new 4 speed "lay-down" gearbox was fitted. Final drive is by chain.
Cycle parts
The Model 7 has the same cycle parts as the single-cylinder ES2, which comprises Norton's "garden gate" plunger frame and long "Roadholder" telescopic fork. Brakes are single drum brakes front and rear. From 1952, the Model 7 engine was fitted in the swinging arm "featherbed frame" to create the lighter and better handling Model 88. However, production of the Model 7 continued, as the featherbed was thought unsuitable for sidecar use.A new rear sub-frame was designed by development engineer Bob Collier in 1953, giving the bike swinging arm rear suspension. A larger front brake was fitted in 1954.