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Triumph Model 6/1 – The First British Parallel Twin

It’s common knowledge: Triumph’s first production vertical twin was the 1938 Speed Twin. Well, not quite.

Credit for the first British parallel twin four-stroke motorcycle engine is usually given to Edward Turner’s 1938 Speed Twin. And it’s certainly true that Turner championed the format such that it became all but ubiquitous in post-WWII Britain. But his twin wasn’t the first.

Valentine Page became chief designer at Ariel in the late 1920s after leaving J.A. Prestwich Industries of Tottenham, London—makers of JAP proprietary motorcycle engines. Page understood engines and engineering well, but styling less so, and the range of four-stroke singles he first designed for Ariel were solid and dependable but rather plain. Meanwhile, Edward Turner had joined Ariel, and while his designs like the Square Four were bold and imaginative, his engineering proficiency was arguably less sound.

So when Page left Ariel for Triumph in 1932, Turner applied his styling flair to Page’s somewhat pedestrian four-stroke singles. The result was the admired Red Hunter with its smart red-and-chrome gas tank. But it was when Page and Turner were working together on the Square Four that the idea of a parallel twin surfaced. Removing one crankshaft from the two-crank Square Four, they created a 250cc parallel twin that ran remarkably smooth, regardless of whether a 360-degree or 180-degree crankshaft layout was used. 

Not surprisingly then, He also produced another range of worthy but prosaic four-stroke singles, which would again get the Turner treatment when the latter joined Triumph in 1936.

Page’s twin was conceived as a sidecar tug, one that would offer superior efficiency to the ponderous and old-fashioned 1,000cc flathead V-Twins commonly used in that role. The engine housed a 360-degree crankshaft with the flywheel outside the crankcase, but inside the primary case. A single gear-driven camshaft located behind the cylinders operated the exposed overhead valve gear in two separate cylinder heads fed by a single one-inch Amal carburetor. Primary drive was by herringbone double-helical gears, meaning the engine turned backwards. Also contained in the main engine casting were a seven-pint oil tank and the four-speed gearbox. Twenty-five years would pass before a major British manufacturer offered another unit-construction parallel twin.

The drivetrain slotted into a sturdy steel tube frame with stout sidecar lugs and a rigid rear end. Nineteen-inch wheels were fitted at both ends, with the front controlled by a heavy-duty girder fork. Like most Ariels at the time, instruments were housed in a panel in the fuel tank. Shifting was initially by a hand lever, though a foot shift arrived for 1935. Power was claimed to be 25 hp at 4,500 rpm with a weight of 412 pounds.

Like most of Page’s designs, the Triumph Model 6/1 was durable and reliable while also performing well, and showed some early promise in competition. Triumph sales manager Harry Perry rode a 6/1 with a Triumph Gloria sidecar in the 1933 International Six Days Trial narrowly missing a gold medal (thanks to a puncture) but still earning silver. The same outfit then covered 500 miles in 498 minutes at Brooklands, (including fuel stops). In the light of these achievements, Triumph was awarded the prestigious Maudes Trophy in 1934.

In spite of this success, the 6/1 was never a big seller. Perhaps it was the novel engine format and a notoriously conservative British motorcycle buyer, but total production over 1934-35 certainly numbered less than 600 machines, with some estimates as low as 100. Fewer than 25 are thought to have survived—and Dale Baston’s 6/1 shown here may be the only one in North America.

The fate of the Triumph Model 6/1 was sealed by Turner’s arrival as managing director at Triumph in 1936. With Page having moved yet again to BSA, Turner revamped Page’s range of workmanlike singles, creating the shiny and sporty Tiger 70, 80 and 90. The 6/1 was ditched in favour of Turner’s own vertical twin, the 1938 500cc Speed Twin, which used two gear-driven camshafts with external pushrod tubes instead of a single cam with integral tubes. And though it lacked the engineering sophistication of Page’s engine, Turner’s design trumped it handily in performance with 27 hp from just 498cc. It was also simpler and less expensive to produce, yet rugged enough to be tuned for performance, as demonstrated by the 34-hp Tiger 100 version of 1939. 

It’s not quite the end of the story, though. Having moved to BSA, Page drew up the basic layout of that company’s first parallel twin. Not surprisingly, it echoed many features of the Triumph Model 6/1 engine including its single rear-mounted camshaft—though the gear primary was replaced by more conventional chain drive, and the integral oil tank was also nixed. 

Page returned to Ariel in 1939. BSA’s 500cc A7 twin, with his indelible fingerprints on it, was launched in 1946.

By Robert Smith Canadian Biker #301


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