Norton Commando 750

The Norton Commando was introduced in 1967 at the Earls Court Show and the first production bikes were available in March 1968.  The bikes were painted British racing green and the oil tank and left hand side cover were painted silver. The Frame and engine cylinders were finished in black.

Overview

Associated Motor Cycles (AMC), Norton's parent company, had become bankrupt in 1966 and had been purchased by Manganese Bronze Holdings, who already owned Villiers Engineering, forming Norton-Villiers. Chairman Dennis Poore saw the need to produce a new flagship motorcycle to replace the aging 750 cc Atlas, the engine design of which could be traced back to Bert Hopwood's 1947 497 cc (30.3 cu in) Norton Model 7 twin. Pore set a deadline for the machine to be ready for the 1967 Earls Court motorcycle show.

Former Velocette engineer Charles Udall had joined AMC in 1961 and had designed an 800 cc (49 cu in) DOHC twin and a prototype built using a shortened featherbed frame. This was known internally as P10 (project 10). Poore set up a design team to develop the P10 headed by former Rolls-Royce engineer Dr. Stefan Bauer. However the P10 was complex, leaked oil and would be expensive to manufacture; the design team started to redesign the engine, giving the revised engine a new code of Z26.

Vibration is an inherent problem of vertical twins. Bauer believed the classic Norton featherbed frame enhanced the problem and was not designed in compliance with good engineering principles. Bauer designed a new frame around a single 2.25 in (57 mm) top tube to increase stiffness and rubber mounted the engine to isolate the engine vibrations from the rider.

By the summer of 1967 it was evident that the new engine would not be developed enough for the September Earls Court show. In parallel to the new engine, the existing Atlas engine had been developed and was now putting out more power than the Z26. A decision was made to abandon the new engine and get an Atlas engined new model using Bauer's frame ready for the show. Poore wanted the new machine to look significantly different, so the engine was canted forward (Nortons traditionally had upright cylinders) and consultants Wolff Ohlins were commissioned to design new bodywork. Wolff Ohlins designed a one-piece seat and tail unit which blended into the tank, all in fibreglass.

Rubber mounting the engine had caused problems under acceleration, the rear mounts distorting causing the chain to jump off the engine sprocket. This was overcome by Norton-Villiers Chief Engineer Bernard Hooper and his assistant Bob Trigg by mounting the swinging arm on the engine/gearbox plates rather than the frame, allowing the engine, gearbox and swinging arm to move as one.

Engine

1969 750 cc Commando engine and gearbox

The origins of the Commando engine can be traced back to the late 1940s when the 497 cc (30.3 cu in) Norton Model 7 Twin was designed by Bert Hopwood. The twin-cylinder design evolved into 600 cc, then the 650 cc Manxman and Dominator until superseded by 750 cc Atlas before being launched as the 750 cc Commando in 1967.

The canted-forward engine retained the 73 mm (2.9 in) x 89 mm (3.5 in) bore and stroke of the Atlas engine. Compression ratio was raised to 8.9:1 and the engine breathed through two 30mm Amal Concentric carburettors and the power output increased to 58 bhp (43 kW) at 6,500 rpm.  An alternator replaced the previous magneto and dynamo. Points for the coil ignition were mounted on a chain driven jackshaft at the rear of the engine where the magneto had previously been positioned. The engine was designated 20M3, the 750 being Norton's model 20 and this engine being the third incarnation of the engine.

When production of the engine moved from Plumstead to Wolverhampton, the engine was updated with the points moving from the rear of the engines to the end of the camshaft, being accessible behind a plate on the timing cover. The tachometer drive was moved from the end of the camshaft to a position more inboard. The rocker covers, which previously had a sandcast finish, were polished on the revised engine. These engines were designated 20M3S.

Transmission retained the pre-unit 4 speed Norton gearbox with a new diaphragm clutch and triplex primary drive. These were accommodated in a new alloy cover that had a single bolt fixing.  The gearbox was strengthened and the clutch plates were changed to sintered bronze when the 850 engine was introduced.  A hydraulic primary chain tensioner was introduced for the Mk3 850 and the chaincase cover changed to 12 screw fixing in an attempt to cure oil leaks.

The Commando had suffered from premature main bearing failures and in late 1971 a stiffer crankcase was introduced and the drive side main bearing changed from ball to roller.  Having roller main bearings on both sides necessitated the use of shims to control endplay.  The crankcase breather was also move from on the camshaft to the rear of the engine.

The 'Combat' engine was introduced in January 1972 and was based on the engine used in the production racer. The combat was stamped with 'C' on top of the engine and delivered 65 bhp (48 kW) at 6500 rpm by using a 10:1 compression ratio, an SS camshaft and 32 mm carburettors. The SS camshaft made the engine 'peaky' with little power so to improve acceleration the overall gearing was reduced. This increased the possibility over overrevving, even in top gear when the red line was reached at 108 mph (174 km/h). The engine proved unreliable, with main bearing failures and broken pistons. The 'Combat' engine's quality control problems exacerbated the company's troubles. On the Commando engine the chain driving the camshaft was difficult to adjust and was frequently missed during maintenance and servicing. As the points were driven off the camshaft, this led to incorrect ignition timing that added to the problems. The failures were resolved by fitting 'superblend' main bearings and lower compression ratio pistons, which had oil drain holes rather than the slots of the pistons carried over from the Atlas.  About 1,500 engines in production were re-worked to the revised specification as well as customer machines being dealt with under warranty. The gearing was also raised on the final 750 cc engines. Revisions to the engine and warranty claims led to few Commandos being available to the public in late 1972. The 750 engine was discontinued at the end of 1973.

The engine size was increased to 828 cc for 1973 by increasing the bore size from 73 to 77 mm and was marketed as an 850. The bottom end of the engine was strengthened and used longer bolts to retain the cylinders. The superblend main bearings were retained. Stronger pistons were used and several measures introduced to try and eliminate oil leaks. An anti-drain valve was fitted into the timing cover on the Mk3 850s to stop oil draining from the oil tank to the sump when the bike was standing.

Using a concept designed by Doug Hele for the Triumph twins, the NVT R&D department, under Norman White, developed a balancing system for the Norton engine to resolve the low speed vibrations of the Commando. The system used an extra conrod and dummy piston and cylinder at 90° to the main cylinders. Whilst the system was never used in production, a similar system was later used by Ducati on the Supermono and BMW on the F800.

A short-stroke 750 was introduced in 1974 and used in the factory racers. It used the 77 mm bore of the 850 with a shorter throw, 80.4 mm, crankshaft. The short-stroke engine was offered as an option on the road going John Player Special.

Cycle parts

The Commando used the isolastic frame designed by Bauer, Hooper and Trigg. The Roadholder forks were carried over from the Atlas as were the silencers. An 8 in (200 mm) tls front brake that had been an option on previous models was fitted. A new fibreglass tank that tapered towards rear was fitted and matched with a combined seat/rear mudguard. The front of the seat had two "wings" that extended either side of the tank.

In 1969 reports were received from the American importer, Berliner Motor Corporation, of 5 fatalities caused by frame failures. The failures had occurred around the head stock and the American authorities were threatening for ban the import of the bike. Extensive testing of the bike was undertaken at the Chobham testing facility and the problem identified. Ken Sprayson of Reynolds Tube Company, who made the frames for Norton, designed a modified frame to resolve the problems. The modification replaced the existing steering head gusset bracing with a small diameter tube and the top engine steady and petrol tank mountings were moved onto this tube.

The front end was revised in 1969, fork shrouds and gaiters were dropped and replaced with chromed stanchions with a rubber dust seal.

The centre stand was difficult to use and hung low causing it to ground when cornering. In late 1970 the stand was mounted on the engine plates rather than the frame which was easier to use and gave better ground clearance. The side stand was short and unstable, and was replaced with a longer item with a different mounting. Also for the 1971 model year, sealed steering head bearings and new yokes replaced the previous adjustable bearings. With the introduction of the 850 in 1973, the yokes were changed again to change the steering head angle from 27° to 28°, and a stronger centre stand added.

The isolastic system required correct tolerances to work correctly. The tolerances were adjusted by inserting metal shims. The bushes were changed in 1970 to enable easier replacement and with the introduction of the MK3 850 in 1975, adjustable bushes were fitted eliminating the need for shims and making adjustment easier.

Tyres on the Commando were initially 3.00 x 19 ribbed front and 350 x 19 block rear. Following Malcolm Uphill's 100 mph lap in the 750 cc Production TT at the 1969 Isle of Man TT on a Triumph fitted with Dunlop TT100 tyres, the Commando was shod with TT100s, 360 x 19 front and 410 x 19 rear. This was later changed to 4.10 width on both front and rear.

A front disc brake was added as an option from early 1972 and became standard in mid-1972 for all models except the Hi Rider which continued with the drum brake until 1973. A rear disc was added on the 1975 Mk3 850s.