Retrospective: Silk 700S: 1975-1979
Never heard of a Silk motorcycle? Not surprising. It was an English bike, very few were ever built, and probably less than half a dozen ever made it to the United States.
It was an attractive motorcycle, but had a rather unusual-looking 656cc parallel twin leaning forward about 40 degrees, with liquid cooling and no fins on the alloy cylinder barrels.
It was a two-stroke engine, which could loosely be compared to Suzuki’s GT750, a.k.a. Water Buffalo, the liquid-cooled transverse triple that was introduced in 1971 and sold quite well for half a dozen years. However, by 1975, motorcycle manufacturers understood that the two-stroke engine was on its way out due to its emissions in this environmentally conscious age.
More’s the question, was this Silk a technological step forward, or a step back? To understand the Silk motorcycle, one needs to understand George Silk—who was a racer, engineer, machinist and dreamer. He also had a passion for Scott motorcycles (Retrospective, September 2008) of all ages, and in the 1960s was working to keep the old ones alive and well.
Thus this story really begins with Alfred Angus Scott, who developed his first two-stroke twin in 1899, designed his own frame, liquid cooled the engine, and by 1910 was building his own 450cc Scott Motorcycles. In 1912, his new 486cc machine won the Isle of Man TT. The Scott Engineering Company prospered, particularly with the 1921 Squirrel model. Production was interrupted by World War II, then started up again in 1946, only to founder financially before Birmingham businessman Matt Holder bought it in 1950. He improved the chassis and left the engine pretty much alone, but by 1970 was only building one or two a year and soon quit.
In the late 1960s Silk, who loved both two-strokes and vintage racing, built a mildly successful race bike around a Scott, and soon there was a demand for road versions. Silk bought several dozen Scott engines and began building Silk-Scotts to order. He wanted to continue with the Scott engines, but unfortunately he and Holder could not come to an agreement and, in the end, he had to develop his own motor—which had a lot of similarities to the previous one.
The first true Silk, the 700S, went out the door of Boars Head Mill, Darley Abbey, Derbyshire, in 1975 for a rather lofty price, half again as much as one would pay for a Norton Commando 850 or a Triumph Trident, although it was still under the bike’s actual costs. Fortunately Silk’s main income came from his engineering firm.
The all-aluminum engine was a direct descendant of the Squirrel, a parallel twin, with two-stroke piston-port design using what Silk called a “velocity-contoured charge system.” That had to do with the design of the ports and the
aerodynamics of the inlets and the transfer passages. A single 32mm Amal Concentric carburetor supplied the mixture.
The engine was intended to last, as the crankshaft ran on four roller bearings that were well lubricated. The oil was held in a separate tank and, using a Silk-designed pump, was injected directly to the bearings, then combined with the gas mixture to lubricate piston rings and other essentials. The oil pump was linked to the throttle, and Silk claimed the mixture ratio was about 50:1, with the oil tank’s contents good for 1,000 miles. The size of the alloy gas tank depended on the customer’s wishes.
The liquid cooling was very simple, some called it old-fashioned, using a thermo-syphon system in which the water, after doing its job, would come to a boil and the steam would go back up into a “header” tank just behind the steering head, and then the radiator. No pump needed, nor thermostat. Simplicity.
Ignition was officially called “Lumenition;” it was transistorized and used two coils, with a seven amp-hour battery and 150-watt alternator supplying electricity. Joseph Lucas’s influence was still in evidence, as the horn and headlight were Lucas items. Silk had thought of putting an electric starter in, but felt that it would merely add weight and complication…and a two-stroke twin was not hard to kick into life. A fairly lively engine it was, putting out 45 horsepower at 6,000 rpm and 45 lb-ft of torque at 3,000.
The crankshaft had the flywheel in the middle for better balance, and the primary drive was just to the left, using a Reynolds duplex chain. This went back to a six-plate clutch designed by Silk on a 4-speed gearbox that looked much like one found on a Velocette. Final drive was by fully enclosed chain, with adjustment done by eccentrics.
Silk was using a frame design that had come from the famed chassis company Spondon—also located in Derbyshire—that Silk had used when building his race bikes. It was a traditional twin-cradle, made of tubular steel. A swingarm with preload adjustable Girling shocks was at the back, a 39mm Spondon-designed fork at the front. The 18-inch wheels were spoked Borranis with alloy rims, although cast Campagnolos were an option later. The rear used a drum brake, while the front had a disc—two if desired, also by Spondon. A protective cover was on top of the disc, a thoughtful addition. Wheelbase was a sporty 56 inches, with a seat height of 28 inches.
In 1977 some changes were made, adding fins to the cylinders to give it a more attractive appearance, a higher compression ratio and a bigger carburetor. This added another three horsepower…although there is debate as to just how many in all. The name was now 700S Sabre Mk 2.
Back to the Suzuki GT750. That triple had another 80cc and more ponies than the Silk. But here is the difference: the dry weight of the Silk was just 305 pounds, while the GT750 weighed well over 500 pounds. This was the joy of riding the Silk…its lightness. Everyone complimented the 700S on its handling, its ability to move effortlessly around curves, sharp or sweeping, with lots of cornering clearance. Power was very linear, non-peaky, and four gears were enough. Top speed was over 110 mph.
Over the years, the high price went higher still as these bikes were essentially hand-built. The final Silk came off the line late in 1979, the last of only 138 that rolled out the door at Boar’s Head Mill. George Silk’s vision was a success in that he built a very good motorcycle, but it could not withstand the crunch of costs nor the increasingly strict environmental laws.
(This Retrospective article was published in the March 2013 issue of Rider magazine.)