Plumstead’s Posh Parallel: 1920 AJS Model 20

Memories of riding an AJS Model 20 as a teenager drove Chuck Thompson to have one shipped to the U.S. from Australia to restore.

| September/October 2018

  • ajs spring twin
    Chuck Thompson’s 1952 AJS Model 20.
    Photo by Robert Smith
  • ajs spring twin
    The Model 20 uses a single Amal carburetor.
    Photo by Robert Smith
  • ajs spring twin
    Chuck Thompson’s 1952 AJS Model 20.
    Photo by Robert Smith
  • ajs spring twin
    The fat rear shocks became known as "jampots."
    Photo by Robert Smith
  • ajs spring twin
    Chuck Thompson’s 1952 AJS Model 20.
    Photo by Robert Smith
  • ajs spring twin
    Chuck Thompson’s 1952 AJS Model 20.
    Photo by Robert Smith
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    George Rowley at the Isle of Man TT in 1936 aboard the AJS V4.
    Photo by Robert Smith
  • ajs spring twin
    The 498cc air-cooled parallel twin runs a 7:1 compression ratio, putting out 29 horsepower, good for an 87 mph top speed.
    Photo by Robert Smith
  • ajs spring twin
    The 498cc air-cooled parallel twin runs a 7:1 compression ratio, putting out 29 horsepower, good for an 87 mph top speed.
    Photo by Robert Smith
  • ajs spring twin
    AJS 20B models are fairly rare, so owner Chuck Thompson takes special care of his.
    Photo by Robert Smith
  • ajs spring twin
    The AJS badge on the engine case, below the cylinder barrel, is a neat touch.
    Photo by Robert Smith
  • ajs spring twin
    The front fender "pedestrian slicer" is another neat touch.
    Photo by Robert Smith

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1952 AJS Model 20 Spring Twin

  • Engine: 498cc air-cooled OHV parallel twin, 66mm x 72.8mm bore and stroke, 7:1 compression ratio, 29hp @ 6,800rpm
  • Top speed: 87mph
  • Carburetion: Single 1in Amal 76
  • Transmission: 4-speed Burman gearbox, chain final drive
  • Electrics: 6v, Lucas K2F magneto ignition
  • Frame/wheelbase: Steel tube cradle frame/55.25in (1,403mm)
  • Suspension: AMC Teledraulic fork front, dual shocks rear
  • Brakes: 7in (178mm) SLS drum front and rear
  • Tires: 3.25 x 19in front, 3.5in x 19in rear
  • Weight (dry): 394lb (179kg)
  • Fuel capacity/MPG: 4.5gal (17ltr)

When Chief Designer Phil Walker started work on a parallel twin for Britain's Associated Motor Cycles, he seemed determined to better the competition.

By 1949, every major British motorcycle manufacturer had announced a parallel twin, including BSA, Ariel, Norton and Royal Enfield. (Velocette and Vincent, as usual, went their own way.) Edward Turner's Big Idea had become ubiquitous in the industry, and represented modern thinking and sporting performance. Suddenly, singles and V-twins looked stodgy and dated.

Compared with a single-cylinder 4-stroke of the same capacity, power delivery was smoother, thanks to twice the power strokes for the same revs. That also allowed for lighter flywheels, so pickup was faster. Compared with a V-twin, parallel twins were more compact and typically lighter, too.

Stylishly late

Associated Motor Cycles (AMC), AJS and Matchless' parent company, had been focused on producing 350cc and 500cc singles, and arrived a few months late to the twin-cylinder party. No doubt AMC designer Phil Walker was aware of what the competition was doing, but his design for what would become the Matchless G9 and AJS Model 20 incorporated many unique features into the 2-cylinder format.



In addition to roller bearings at each end of the crankshaft, Walker added a center main shell bearing. All the other parallel twin makers followed Turner's lead, using only two mains. The third bearing gave extra support to the crankshaft and helped to prevent flexing. It also allowed feeding oil through the crankshaft to the two big-end bearings, providing an even supply for each. All other contemporary parallel twins fed oil from one end of the crank, creating the potential for the farther big-end bearing to be starved of oil — with the inevitable result. Another bonus: the center main bearing located the crank laterally, allowing it to "float" on the two outer roller main bearings during expansion and contraction.

Like Royal Enfield's designer Tony Wilson-Jones, Walker chose a massive one-piece iron crankshaft with integral counter weights rather than the bolt-up arrangement BSA, Triumph and Norton used. And like Enfield, Walker also opted for separate (interchangeable) iron cylinder barrels topped with light alloy cylinder heads. Under the alloy rocker covers were four eccentric rocker shafts: Adjusting the valve clearance required only a screwdriver once the pinch bolt was slackened. Walker also chose two separate oil pumps driven from the ends of the two camshafts — the exhaust operating the oil feed pump and the intake cam the return. Walker's engine was, as a British worker would say, a "proper job."

Bilgemaster
9/5/2018 9:09:59 PM

You might want to fix that year typo in the article's title: "Plumstead’s Posh Parallel: 1920 AJS Model 20". 1920? Yeah...NO. Pretty bike though. Thanks for featuring it. AJS seldom gets its fair due.




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