Your guide to: Silk Sabre Mk.2

1977 Silk 700S Sabre Mk.2

While other manufacturers chased power at the expense of weight and handling, Silk wanted light weight, great handling and plenty of power

Words and photos by Phillip Tooth

The 1970s was the decade of the Superbike.

Motorcycles were faster and more sophisticated, but also bigger, heavier, and more complicated. As far as George Silk was concerned, the fun of riding one was being eroded as manufacturers chased power and paid for it with handling problems. He believed that discerning riders would prefer a small, lightweight motorcycle with bags of torque, a decent turn of speed and race-bred handling – a modern version of his vintage Scott.

When George left school, he started work as an apprentice at legendary Scott specialist Tom Ward of Derby. He later took a job at the Rolls-Royce jet engine factory, also based in Derby, but left in 1969 to set up Silk Engineering with his friend and fellow Scott fanatic Maurice Patey. They started making goodies for the ‘yowling two-stroke’, including a redesigned, stronger crankshaft and a variable delivery oil pump that was linked to the throttle opening.

George’s carefully tuned, methanol-burning 500cc Flying Squirrel was timed at over 100mph – when they were new, a Scott would struggle to hit 80mph. In the early 1970s, there was no way that anyone could make a decent living from manufacturing parts for obsolete motorcycles, but fortunately Silk Engineering won the contract to service narrow gauge diesel locomotives used by the National Coal Board. That was enough to guarantee a cash flow and pay for eight employees.

George indulged his passion for racing by building a special with a low and light frame, made by Spondon Engineering and based on one designed for a 250cc Yamaha. This was another Derbyshire company with links to Rolls-Royce; one of the company founders, Bob Stevens, was apprenticed there before he started making race frames. George chose a gearbox from a prewar Velocette, turned through 90 degrees so it lay on its side and with Venom gears inside. Cooling was by an LE Velocette radiator. Spondon telescopic forks, swingarm rear end, a Spondon-made twin leading shoe drum brake up front and a Spondon 7in disc with mechanical caliper at the rear completed the package. The Silk-Scott racer debuted in August 1970, and a short time later

George was asked by one of his regular customers to make him a roadster version. That first frame was modified from one designed to accommodate a 250cc Yamaha engine.

The first road bike was ready for the 1971 Racing and Sporting Motorcycle Show in London and featured an engine bored and stroked to give 635cc. Coil ignition replaced the original magneto and a new Amal Concentric replaced the old Amal Type 276 with its separate float chamber. Twin exhaust pipes were siamesed into an expansion chamber, and there was the usual road equipment of decent mudguards, a toolbox, lights and a dual seat. The fibreglass side panels and alloy petrol tank were finished in classic Scott purple, with cream panels on the tank and Scott logos.

When Matt Holder, who owned the Scott, Velocette and Royal Enfield brands, heard about it, he got his lawyers on the case and an apologetic George renamed his motorcycle the Silk. Another 21 Silk motorcycles with modified Scott engines were built; the crankshaft, pistons, barrels, heads and oil pump were all George’s designs. They were sold in the UK in kit form to avoid tax, although a couple of roadready bikes were shipped to the USA, but to move his project forward he really needed a new engine.

And then George got lucky. He was approached by a company that made grass mowers for trimming parks, football pitches and roadside verges. They wanted him to design a water-cooled two-stroke twin that delivered plenty of torque and also had impressive fuel economy. George and David Midgelow – yes, you’ve guessed it, another Rolls-Royce aero-engine man – sat down and designed an all-new engine.

At the time, large capacity Japanese two-stroke engines delivered searing performance but dismal fuel economy. Now seems like a good time to look at two-stroke scavenging. Basically, this is how the incoming charge helps to drive out the burnt gasses from the cylinder during the transfer stage. Since the transfer and exhaust ports are open at more or less the same time, different methods have been designed to prevent the fresh charge disappearing down the exhaust pipe with the burnt stuff.

The earliest method, patented by Joseph Day in 1890 and developed by Alfred Angas Scott, used the cross-flow scavenging system. A deflector on the top of the piston directed the gas flow from the transfer port upwards in an inverted U-shaped loop around the combustion chamber roof, and then down and out through the exhaust port located on the opposite side. There were no ports in the piston. By the 1950s, only Scott and French moped manufacturer Velosolex was making engines with deflector pistons.

Those Japanese screamers used the Schnürle loopscavenge system patented by Adolf Schnürle in 1926 and first taken up by DKW. With the loop-scavenge system, the ports were located on the same side of the cylinder, but the transfer port was split into two and angled upwards, one on either side of the exhaust port. The gas flow was now a circular loop, and the piston could be flat-topped or slightly domed. Ports cut in the piston wall fine-tune the two-stroke cycle. This all combines to give a better combustion

“When Matt Holder, who owned the Scott, Velocette and Royal Enfield brands, heard about it, he got his lawyers on the case and an apologetic George renamed his motorcycle the Silk.”

Above right: This Sabre has a Lockheed master cylinder and a Spondon brake caliper. Disc is cast iron

chamber shape and flame path, delivering better combustion, particularly at high speeds. George believed that, even with reed valves, the loopscavenge system sent too much of the fresh gas straight out of the exhaust pipe and at the same time suffered from lack of torque at low speeds.

The Derbyshire engineers thought the answer lay in updating the Scott cross-flow scavenging system by incorporating transfer ports in the piston. They called in Dr Gordon Blair, of Queen’s University, Belfast, to advise on the torque characteristics and to optimise the port profiles of their new engine. Dr Blair was one of the foremost scientists in two-stroke technology, and the world’s leading two-stroke manufacturers – including Japanese and German ones – benefitted from the QUB computers.

The expansion chamber was designed by another professor, this time Dr Roe, of Manchester University. The mower engine and a marine version were ready by the end of 1974 and over the following winter George produced a small batch for clubman racers to iron out any bugs on the track, with the first all-Silk road bikes available in 1975.

The patented ‘Velocity Contoured’ charge-scavenge system produced 45bhp at 6000rpm with maximum torque available at a lowly 3000rpm, where the engine delivered a useful 30bhp. Top speed was about 110mph, and with such a broad spread of torque, the wide-spaced, four-speed gear cluster remained inside the new gearbox bolted to the back of the crankcase. Silk claimed his new bike averaged 60mpg – impressive for any large capacity motorcycle and almost unbelievable for a big two-stroke twin, with oil being burnt at a rate of one pint for every 300 miles.

The bore and stroke was 76mm x 72mm to give 653cc, with both the heads and barrels of the 180-degree twin water-cooled. Inside each separate crankcase of the new twin was a pair of small flywheels, with caged roller bearings for the big ends and four main bearings. Primary drive was by a duplex chain from a sprocket mounted between the two centre main bearings and a Weller-type tensioner. On the end of the left crankshaft was a 104W alternator, while on the right was the oil pump and Lumenition electronic ignition, which used an infrared beam and rotating disc with a piece cut out to trigger the spark.

While the gearbox internals might have been based on the Velocette original (the splines are different), the clutch was designed by Silk and used six Triumph plates. A single 32mm Amal Mk.2 Concentric was used. There was no water pump or thermostat. A header tank was mounted above the engine and a compact radiator in front, with simple thermosyphon cooling (where convection moves heated liquid upwards in the system as it is simultaneously replaced by cooler liquid returning by gravity).

Spondon made the frames in aircraft-grade tubing, and like the original Silk Scott had straight tubes between the steering head and swingarm pivot, which was eccentrically mounted for precise chain adjustment. Timken roller bearings were used in the steering head. Spondon also supplied the forks and rear drum brake, and buyers could choose either Spondon’s race-proven front disc brake, twin Lockheed discs, or a twin leading shoe drum. Other options included alloy or steel wheel rims, fork gaiters to keep the stanchions clean, long range (18l) or sprint (13.6l) petrol tank, solo or dual seat,

Oil tank holds enough for 1000 miles

and any colour you wanted. Experienced road riders appreciated the fully enclosed rear chain that used a fibreglass case linked to the engine by plastic bellows from an MZ.

You could buy a Triumph Trident for £1215 in 1975. Kawasaki was offering its awesome 135mph Z1 for £1249, while Ducati listed the 860GT for £50 more. If you were after the latest technological wonder, you could buy a Suzuki RE5 for £1195, but the tried and tested Norton Commando was only £928. A Honda CB750 was a steal at £979. The Silk was a hand-built motorcycle – and you certainly paid for it. There was no electric start and instrumentation was basic, with just a speedometer and ammeter. Yet the 700S was one of the most expensive motorcycles money could buy at £1388. You had to pay an extra £7.50 for the bigger petrol tank and an unbelievable £100 more for sidecar fixing points, but you could save £35 by opting for a single instead of twin disc set-up.

In 1977, the 700S was updated to the Mk.2 Sabre, with finned barrels instead of plain, a new seat, a rev counter, a larger carburettor and bigger bore exhaust. Porting and timing revisions plus a higher compression boosted power to a more respectable 48bhp, but by 1979 the price had risen to £2482. Even at that price, Silk Engineering was losing £200 on every bike sold. Light weight and great handling were not enough in the Superbike age and production ground to a halt that December, with 138 motorcycles sold.

If there is one thing that Joel Samick really appreciates, it is a motorcycle that handles like a racer. The owner of this 1977 Sabre is too modest to mention it, but he won the American Association of Motorcycle Road Racers premier Formula One class in 1980, riding a Yoshimura Suzuki GS1000. “The steering and roadholding of the Silk are remarkable,” says Joel. “Skinny tyres accentuate the airy feel of lightness, and corners can be attacked and strafed with wild abandon!”

There is no ignition light, but the ammeter needle kicks to the left when you turn the key. Scotts have always liked a rich mixture for starting, so lift the choke lever on the Concentric carb and pull back the clutch lever to swing the forward-slanted kick-start lever back to a position where you can give it a firm boot. A cold engine needs two or three kicks, but once warm, one kick is enough. If you get the chance to ride a Silk, you won’t fail to be impressed by the lowspeed torque. With the Veglia rev-counter hovering at about the 3500rpm mark, the Smiths speedo indicates an effortless smooth-as-silk 70mph (we just had to say it...). Contributing to the rock-steady feel are the Spondon forks, which don’t dive under hard braking or bounce around on rough roads. Ah, the brakes. The single Spondon disc is more than up to the job of stopping a motorcycle that weighs only 310lb (140kg), but it has as much feel as the disc brake on an early Norton Commando. Which is not a lot.

You won’t be making slick changes with the gearbox – it feels like it is filled with cold porridge. If you must push the engine to the 6300rpm redline, the Silk will take you to 70mph in second, 90mph in third, and max out at a shade over 110mph. But then vibration will make itself felt through the nose of the seat, and you’ll definitely notice that. Leave it in top, use the torque to drive you out of bends, and enjoy the race-bred handling.

Above: Thanks to the beautifully crafted frame, which features a duplex cradle, with straight tubes running from the top of the steering head to the rear fork pivot, the Silk Sabre steers like a racer

Joel Samick and his wife Lynn opened a motorcycle dealership in Delaware in 1985. He retired in 1998 to indulge his passion for restoring and riding classic bikes, and when his collection grew to more than 20 big twins from the 1970s, he decided to set up Retrotours from his home in Kennett Square, Pennsylvania. Groups of between four and eight riders get to swap classic bikes on trips that range from 400 to 2000 miles. The Silk is in his fleet of loan bikes. If you want to experience this sweet-handling stroker for yourself, check out www.retrotours.com