Monday, 3 June 2013

Silk 700 - The Ultimate English Two-Stroke

Silk 700S Sabre Motorcycle
Image Source
Think of the icons of British motorcycling and odds are you will think of one thing exclusively – four-strokes. All the great flagship cycles of English industry – the Commandos, the Manxes, Bonnevilles, Tigers, Interceptors, Gold Stars, and anything else of note from the golden age of British bikes was going to be operating on the principles of suck-squish-bang-blow. British two-strokes were relegated to small, cheap, entry-level machines that were aspired to by no one. Dirty two-strokes were the domain of the Japanese as far as most of the British marques were concerned.

Silk 700S Motorbike
Image Source

There was, however, one important exception: in the earliest years of British motoring, the Scott Motor Cycle Company produced the fastest and most sophisticated two-stroke machines of the period. Seventy years later, in the waning years of the British cycle industry, George Silk would take that same engine and attempt to build one of the ultimate road-going British two-strokes that would offer a far more refined alternative to those cheap and fast Japanese 'smokers.
Scott Motorcycle Logo
Image Source

The story of the Silk motorcycle must inevitably begin with the Scott. Founded in 1908 by Alfred Angas Scott, the company produced a line of well-developed and advanced two-stroke parallel twins that would become famous for their speed and sophistication during the Edwardian period. Scott was a remarkably talented engineer who began designing two-stroke engines in the late 19th century while working in the shipbuilding industry. His initial designs were for marine engines operating on Joseph Day’s piston-port crankcase-compression two-stroke principles, but he also developed many prescient mechanisms that would later find their way into motorcycle design. You might be familiar with one of his devices: he designed a clever ratchet and pawl mechanism driven by a foot-operated lever for starting a motorcycle engine. That’s right, Scott helped design the kickstart, which would become the most elegant and masculine way to break your own foot outside of a bullfighting ring.
Scott Two-Stroke Parallel Twin Engine

 The first production Scott motorcycles featured a 450cc parallel twin, utilizing cross-flow scavenging and liquid cooling, placed in a patented triangulated steel duplex frame of Scott’s own design. The Scott would prove to be one of the most advanced designs of the Edwardian era: aside from kickstarting, a well-braced frame and liquid cooling, the Scott featured telescopic front forks, and his machines were the first to incorporate a multi-speed transmission – only two speeds, but still twice as many as the competition.
Scott Motorcycle Engine Crankcase
Image Courtesy Roger Moss

Alfred Scott designed from first principles and built his machines to be the best from the outset – his work was free of influence from “tradition”, however recent such conventions may have been in the in nineteen-oughts. But being the most advanced on the market is pointless if your product doesn’t perform, or dun-blows-up as soon as you wind out the throttle. Scott understood this, and designed his namesake machine to be durable, reliable, and well developed. He would prove his machine’s prowess in the traditional, time-honoured manner – by kicking the competition’s ass on the racetrack. In fact Scott motorcycles dominated British road racing and so thoroughly trounced their four-stroke opposition in the 1910s that organizers declared them “overly efficient” and slapped them with a 1.32-multiplier displacement penalty (thus a Scott of 450cc would be forced to compete against 600cc four-strokes). You know you are doing something right in racing when they rewrite the rulebooks to handicap you.
Scott Motorcycle Racing Wheelie
Image courtesy Roger Moss

Competition success begat a loyal following and a series of increasingly powerful and advanced models, culminating in the famous Squirrel series of 500 and 600cc machines introduced in the 1920s. Scott also developed a highly advanced manufacturing facility that would be considered state of the art, even after the Second World War – some 30 years after it was initially devised. Unfortunately Alfred Scott passed away in 1923 after contracting pneumonia while testing his Sociable three-wheeled vehicle. He had effectively left the motorcycle business in 1917 to focus on the Sociable project, an unsuccessful attempt to build an inexpensive and practical vehicle for the masses. Without his leadership, the company became technically stagnant and continued producing warmed-over Squirrels for decades with minimal development – what had once been 20 years ahead of the competition was now 20 years behind. Supposed improvements led to more weight and more complexity, which led to outcry from traditionalists. Rather than rework the designs to move forward, Scott continued producing old models alongside the “improved” versions to satisfy their nostalgic followers. In reality even the “improved” designs remained that of the interwar Squirrel series with modest improvements. Evolution was extremely slow and conservative, and as they years passed Scott went from being a highly advanced manufacturer to a curious producer of relics from a bygone era.
Scott Motorbike Racing
Image courtesy Roger Moss
Things fizzled out by 1950, at which point the Scott name and facilities were bought out by engineer/investor Matt Holder, who would go on to purchase the rights to Vincent, Velocette and Royal Enfield. Holder moved production from the traditional Scott home of Shipley to Birmingham and restarted production with some improved chassis powered by the same “long-stroke” twin that remained more or less unchanged since 1928. “Birmingham” Scotts were produced in limited quantities until the end of the 1960s, when the works was mothballed and put into storage at the old Triumph Number 2 factory in Meriden. A few bikes would be assembled to order until 1979 and Holder maintained his ownership of the copyrights and spares stock, but series production ended in 1968-69.

Even in the 1960s Scott ‘cycles remained well respected with a loyal following of die-hard owners who extolled the virtues of their advanced design, light weight, and smooth, torquey engines. George Silk was one such die-hard, a man who loved Scotts and who was well known for his exploits in the Scott enthusiast’s community. Silk believed the iconic sloping-twin could be improved with an up-to-date chassis and the application of some modern tuning techniques – after all, the Scott twin hadn’t been updated in 40 years and was initially designed to run on 60 octane fuel. So George set about building a properly modern home for the well-respected parallel twin with its smooth, torquey powerband.
Scott Motorcycle Pistons Roger Moss
Scott deflector pistons with transfer port cutouts. Note that these are new Moss Engineering pistons - Silk pistons had a similar arrangement.
Image courtesy Roger Moss

Yes, torquey – unlike more recent two-stroke designs, Scotts utilized a cross-flow scavenging setup that produced a wide powerband at the expense of top-end performance. In two-strokes, with both the exhaust and intake ports open simultaneously on the power stroke, the flow of mixture across the cylinder (called scavenging) must be controlled in some fashion. Too much flow without enough control will result in over scavenging, where the unburned mixture will just flow straight through into the exhaust. Cross-flow (also referred to as piston-port) scavenging uses a tidal-wave shaped deflector cast into the crown of the piston to direct the flow of incoming mixture upward into the roof of the combustion chamber, swirling it up and over the crown and past the spark plug.
Two-Stroke Cross-Flow Scavenging
Cross-flow scavenging.
Image Source

Cross-flow designs have been considered obsolete for a quite some time. Modern high performance two strokes utilize Schnuerle porting, better known as loop scavenging, developed in 1926 by German diesel engine engineer Adolf Schnürle and first adopted in motorcycles by DKW in 1934. Loop scavenging uses carefully arranged and angled ports to accomplish the swirling effect needed for proper performance, without the need for a deflector. Thus the piston can be flat topped, reducing reciprocating mass and allowing for a higher compression ratio – the tall deflector on a cross flow piston acts as a heat sink which increases the likelihood of detonation and piston overheating, thereby limiting potential tuning. There are, however, two distinct advantages to cross-flow engines – one is a much wider powerband and a flatter torque curve than a loop scavenged engine, the other is excellent fuel economy.
Silk-Scott Special Motorcycle
Silk-Scott Special
Image Source

That’s enough two-stroke theory, back to story at hand: George Silk Jr. was the son of an ardent Scott fan who had George apprentice under Tom Ward, a technician who had worked for Scott prior to the First World War. Tom wasn’t able to pay George an adequate salary once his apprenticeship was over, so George entered into the commercial engine rebuilding business at age 21. He tinkered with Scott tuning on the side, with some attempts at building competitive vintage racers that he campaigned in sprint and road racing in the late 1960s. While working on these racers, he formulated an idea to place the classic Scott engine into a modern frame to improve handling, and idea that would become the genesis of the Silk motorcycle.
Silk-Scott Special Motorbike
Image Source

George approached Bob Stevenson at famed frame manufacturer Spondon to modify a duplex cradle frame to fit a Scott engine attached to a Velocette four-speed gearbox. The resulting machine had tidy handling, but was let down by the ancient motor design. Even the best Scotts were lucky to crack a genuine 20 hp without serious reworking, so placing such a motor into a modern road-racing frame was certainly overkill, but the results were encouraging.
Silk-Scott Special Motorcycle
Image Source

Spurred on by the handling offered by his Spondon-framed prototype, George set about building a series of Silk-Scott Specials for discerning clients. He founded Silk Engineering with partner Maurice Patey in Derbyshire in 1971 and began to mate hotted-up Scott motors to Spondon frames with modern running gear. However, George hadn’t obtained the rights to the Scott name from copyright owner Matt Holder. Legend has it that some overly proud owners may have exhibited a bit too much arrogance regarding their new and improved Scotts, which led to Holder disavowing the endeavour and refusing to sell Scott spares to Silk. Another telling of the story says that Holder was annoyed that Silk was putting Scott logos onto the tanks of his wholly unauthorized machines, and swore him off. Whatever the circumstances, this meant that prospective Silk-Scott buyers were asked to supply their own crankcases to be able to construct the machines, no mean feat considering that Scotts were prized machines that were unlikely to be gutted to build a hybrid.
Silk 700S Sabre Motorcycle Motorbike
Image Source
Silk applied improvements to the engine as well as to the chassis - His experience under Tom Ward and his years of building racers had given him some ideas on how to improve the performance of Scott engines. Porting was modified, the crankshafts and bottom ends were improved, and cutaways were added to the solid-skirt pistons to improve flow from the crankcase into the transfer ports. The basic bore, stroke and timing of the donor engines were retained, but aside from that the motors were more or less new from the crankcases up. The delicate Scott radiator was replaced with a more robust and easily found unit taken from the Velocette LE. A four-speed Velocette gearbox was adapted to work with the Scott motor – it was flipped over and mated to the Scott engine via a chain primary and clutch designed by Silk. Being flipped upside down, the shifting was left hand, reverse pattern. The box had to be lubricated with the outmoded method of manually greasing the gears. At least one customer returned the machine to have the gearbox sealed and run in oil to make maintenance a bit less… intensive.
Silk 700S Brochure
Image Source

These Silk-Scott hybrids were produced according to customer specifications from 1971 to 1975 - total production during was a mere 21 machines. However original Scott engines and crankcases were extremely hard to obtain in good workable order, and Matt Holder’s blockade precluded the purchasing of new stock parts, so a solution was devised. In 1975 George announced a new, street-legal Silk machine would be built with a proprietary engine, designed and produced in-house to circumvent the Holder blockade. The Silk 700S would be heavily inspired by the design of the Scott parallel twin, but would feature many modern improvements and much higher performance to match the excellent chassis they had developed. Initial price tag was £1355, or around 150% the price of a contemporary Norton Commando, making it the most expensive production bike on the market at the time.
Silk 700S Mark 2 Engine
Silk 700S Mark 2 Sabre engine - note fins on the cylinders.

David Midgelow, a Rolls Royce engineer and a neighbour of George’s, was contracted to design the engine. Dr. Gordon Blair was hired to design the porting, exhaust, and optimize flow characteristics. The basic Scott design tenets were retained in the new engine – it was a 180-degree parallel twin sloped forward 40 degrees, with cross-flow scavenging, and liquid cooling via a pumpless thermo-syphon system that used convection to circulate coolant. The cases, heads and cylinders were cast in aluminum alloy, with cast iron bore liners. A 76mm bore and 72mm stroke gave 653cc. The deflector-crowned pistons were externally copied from a 1928 Scott sample, with a different internal shape and skirt cutouts. Because piston manufacturer Hepolite required a minimum order for custom pistons and Silk was such a tiny manufacturer, only 1000 of the 700S pistons were cast and all were in the same 76mm diameter. No oversized pistons were available for rebuilds – if you need a rebore, you need to get the iron liners replaced to maintain the stock piston diameter.
Silk 700S Sabre Engine

Fueling was via a single 32mm Amal Concentric running in the traditional Scott location, feeding an inlet tract cast into the crankcase between the two cylinders. A proprietary oil injection system was used that where oil was metered out as needed from an alloy tank under the seat. A device connected to the throttle dispensed oil according to how much throttle was applied, allowing for up to 1000 miles between oil tank fill-ups, with no need to premix gas and oil. Like the Silk-Scott Special the piston skirts had cutaways to allow better flow through the crankcase into the transfer ports, with the bonus of fresh charge cooling the underside of the piston crown and lubricating the small end of the con rod. At first glance most two-stroke savvy people assume the cutouts indicate a reed valve inlet but in fact they only affect the transfer ports, not the crankcase intake.
1978 Silk 700S Mark 2 Motorcycle

Claimed power for the new mill was 45 bhp at a leisurely 6000 rpm, with 45 lb/ft available at only 3000 rpm. While not earth shattering by modern standard, you must keep in mind that an 850 Commando of the period made a claimed 53hp and weighed over 100lbs more than the Silk. Additionally, fuel economy was in the 50 plus mile per gallon range, exceptional for a two-stroke of any description. A loop-scavenged machine might have the edge in power - the 653cc Silk made about the same horsepower as a contemporary Yamaha RD350 – but a Silk would outgun the Asian ‘smoker in the grunt department. The Silk also had a characteristically throaty growl that is unusual for a two-stroke, helped in part by its “siamesed” two-into-one exhaust – it almost sounds like a four-stroke machine, but not quite.

Like the Silk-Scott Special a Velocette-type four-speed gearbox with gearsets cut by Roger Moss was used, but in the 700S it was integrated into the newly designed crankcases. Final drive was by chain and sprocket, but fully enclosed against the elements. Silk, ever the salesman, claimed that the spread of power was sufficient to not need more than four gears, unlike those peaky Japanese machines that required five or six gears. Indeed, reading the sales brochure for the Silk you would think it was the finest, most carefully designed and magnificently constructed machine of all time, such was the care and attention that was put into the hyperbolic descriptions. You have to respect the company’s enthusiasm, and the fact that they were able to design and build their own motor despite being a tiny, underfunded operation.
Silk 700S Sabre Motorbike
Image Source

Chassis-wise the Silk 700S had a duplex Spondon frame similar to the Silk-Scott Special. Spondon also provided the front forks, which were patterned after Ceriani items. The rear swingarm was a traditional unbraced tube-section arm suspended by a pair of Girling shocks. Borrani wire wheels were standard. Brakes were Lockheed or Spondon hydraulic disc with a cast iron rotor at the front, and a drum at the rear. A double disc setup at the front was optional - you could also substitute a twin-leading shoe drum if you were leery of that newfangled disc brake witchcraft. The bodywork was similar to that of the Silk-Scott Special, with a slab-sided aluminum fuel tank (available in 14 and 18 litre displacement) and locking sidepanels. The resulting machine was cleanly styled and quite modern looking. It was also remarkably light weight – only 305 lbs dry, featherweight for a liquid-cooled 650cc machine. This, along with the well-developed chassis, contributed to excellent handling that was praised by reviewers and owners alike.
Silk 700S Motorbike Brochure
Image Source

Production was slow but steady, and in 1976 the operation was taken over by Furmanite International Group. 1977 saw the introduction of an updated Mark 2 700S dubbed the “Sabre”. The engine compression was raised slightly to bump power to a claimed 48 bhp, and the engine cases were given a cosmetic redesign with finned cylinder jackets. A new seat and instruments were also installed. Campagnolo cast wheels were an available option, and with these installed the Silk looked like a thoroughly modern machine that belied its antiquated roots. If they were expensive at the beginning of production in 1975, they were breathtakingly priced by the time production ended in 1979 – at which point the retail was nearly £2500. The operation ceased in the face of steady losses (at least £200 per machine, despite nearly doubling in price since 1975) and increasingly strict emissions laws on the horizon threatening the very existence of street-legal two-strokes.

Silk production ended after only 138 examples were produced, making the 700S a rare and highly coveted machine that is highly prized among British bike aficionados. It is exceptional when a Silk-Scott Special or a 700S changes hands, and owners are a dedicated lot who hold their bikes in high esteem. After all, what other British machine offers some of the best handling available in the 1970s in an exceptionally lightweight package with sophisticated two-stroke power? The Silk was the product of George Silk’s enthusiasm for the refinement and potential of the venerable Scott engine, dropped into a modern chassis that could challenge the traditional four-stroke hierarchy in Britain.
Silk 700S Brochure Specifications
Image Source
 Interesting Links
Silk-Scott Reference Page with excerpts from "Yowl", the Scott enthusiast's magazine
Roger Moss Engineering, UK-based Scott specialist
The Vintangent on a Silk-Scott Special sold by Bonhams
Motorcycle Classics on the Silk 700S
Motorcycle Classics on the Scott Flying Squirrel
Rider Magazine Retrospective Silk 700S Mark 2 Sabre
Review and history of the Silk 700S

Contribute to OddBike on Patreon
Contribute to OddBike on Patreon