False dawn
The 650cc CSR models from Matchless and AJS could’ve been the firm’s finest hour, though after a promising start, they simply added to the beleaguered business’s woes.
In the era of the ton up boys, speeding from ‘caff to caff ’ in the early 1960s, the Triumph Bonneville and BSA Gold Star grabbed all the headlines, being on the wish list of most leather-jacketed teenagers, but there was a dark horse contender produced at the London, Plumstead factory of AMC – the 650cc Matchless G12 CSR, and its AJS-badged stablemate, the Model 31 CSR. The CSR was an acronym for ‘Competition Sprung Roadster,’ though was often thought by owners to mean Competition Sport Roadster and even nicknamed Coffee Shop Racer. You pays your money and takes your choice. These duplex framed machines, lust objects to the comparative few, were nevertheless a match in power and roadholding to the Bonnie and Goldie, with a top speed quoted at 108mph.
Developed by AMC’s Jack Williams, best known for his work with the AJS 7R, primarily for the prestigious Thruxton 500 mile production race, a blue-tanked Model 31CSR, entered as a Matchless, won the 1960 race, piloted by Ron Langston and Don Chapman. Boosted by the victory, AMC were quick to climb aboard the bandwagon to produce a machine that would appeal to the young coffee bar cowboys with money to spend, aided by access to Hire Purchase agreements offered by the motorcycle dealerships of the day. A pair of 8.5 to 1 compression pistons, twin carbs and high lift cams, all buzz words of the speed hungry, boosted power. Unfortunately, sales stalled, as news of crankshaft breakages began to reach the cafe grapevine. When revved hard in third by these less-than-mechanically sympathetic cafe racers, the cranks were tending to let go on the change up into top gear, with disastrous results to the engine’s wellbeing.
By the time that the Plumstead factory had figured out what had gone wrong (a faulty batch of castings from the foundry) the die had been cast, the reputation set; word was the high revving AMC twin was unreliable. When I was looking to upgrade from my 197cc Francis-Barnet Falcon in 1963 at Pride & Clarke in London’s Stockwell Road, there was a one year old Matchless CSR for sale at only £100. When I asked the white-coated salesman why it was so cheap, he suggested I try to kick it over. “Kick it over and you can have it.” Climbing aboard the gleaming CSR, with its ace bars and rear set footrests, I put my full weight on the kickstarter and forced my foot down. Nothing. It was solid. “Broken crankshaft. The second this month,” he smirked.
We now need to go back to 1948, when, following the success of Triumph’s pre Second World War Speed Twin, AMC (like most British manufacturers) brought out 500cc twins, in Matchless G9 and AJS Model 20 form. While others relied on two roller main bearings to support a steel crankshaft, AMC’s designer Phil Walker went for three bearings, two outer rollers and a plain shell bearing to support the middle, allowing the use of a one piece cast iron crankshaft, with lightweight aluminium
conrods. The theory was to stiffen up the assembly and reduce vibration, which worked well enough on the 500cc G9/Model 20, but when the bores were increased to 72mm in 1956 to create the 600cc G11/Model 30, vibration levels increased. The cast iron barrels were individual castings, sunk deep within the crankcases, while the aluminum heads were also separate, allowing for easier maintenance. All in all, the AMC twins gained a reputation as solid reliable workhorses, often used as ride to work machines, yet easily capable of carrying the family to the seaside at weekends with a double adult sidecar attached.
With pressure building from the strong US market for more cubes, the Plumstead factory looked to increase the engine capacity to 650cc, but with no room in the bores for bigger pistons, Phil Walker had to redesign the crankshaft for a longer stroke of 79mm, while casting a new crankcase and primary chaincase to suit, taking the opportunity to change to Lucas alternator electrics, with coil ignition as an option. Using the G11 as a base for the new engine, the G12 was first marketed in 1958 as a fast tourer, the extra capacity lifting the power to 35bhp (two up on the G11) at 6500 revs, with a top speed quoted at 95mph.
The following year saw further upgrades, including a duplex frame replacing the G11 single down tube version, and new cylinder heads. A street scrambler, capable of desert racing, was demanded by AMC’s Los Angeles importer, and was fitted with a more powerful engine, having the compression ratio increased from 7.5 to 8.5 to 1 and a different, more racy camshaft. Vibration at revs was already a problem on the bigger engine and a stronger, harder wearing crankshaft was cast in nodular iron, to cope with the extra loads. Nodular iron is cast in a complex process, where elements such as magnesium are added to create spheroidal nodules of graphite as opposed to flakes, which produces a more ductile, flexible and strong iron.
The new frame, fitted with Teledraulic forks, was the ideal chassis for this engine, and, now named the G12 CSR, a true ton up machine was available to the affluent leather boys. A test at the MIRA banked circuit saw the CSR run at 100mph for an hour, while journalist tester Vic Willoughby recorded 108mph from a standing start on his factory supplied CSR.
With the CSR winning the prestigious 1960 Thruxton 500 mile production race, AMC struggled to keep up with demand for their new 650s; money was tight and production techniques were in the dark ages at the old Plumstead works, while in second, third and fourth places at Thruxton were the new, duplex-frame T120 Triumph Bonneville, built at the expanding Meriden works. The original Triumph factory in Coventry had been destroyed by German bombing in 1940 and with recovered tooling, a brand new factory was brought back into production at Meriden in Warwickshire. Sales of the Bonneville rocketed, bringing profits from home and abroad to Triumph, while sales at Plumstead failed to cover the development cost of the CSR in 1961, AMC surviving on recirculating profits from its other brands, Norton, Francis-Barnet and James.
Disbelief by the management that the nodular crankshafts were breaking slowed the discovery of the fault at the foundry. Regular quality control would have spotted the problem early, as it was, bikes were leaving the factory with defective items at the heart of their
“A test at the MIRA banked circuit saw the CSR run at 100mph for an hour.”