SILK 700S ....................................................
Few people ever clapped eyes on a Silk 700 when these two-strokes were new, back in the late 1970s. Less than 150 Silks were built and maybe one-third still survive… so it’s faintly amazing that Stuart Francis found one – in New Zealand!
Few people ever clapped eyes on a Silk 700 when these two-strokes were new, back in the late 1970s. Less than 150 Silks were built and maybe one-third still survive… so it’s faintly amazing that Stuart Francis found one – in New Zealand!
The superb 1977 Silk 700S Mk2 Sabre featured in this article is owned by Andy in New Zealand. It is a low mileage (only 10,798 miles on the clock) original condition machine that looks almost factory fresh, finished in a striking red set off by the gold coloured Campagnolo cast wheels and the polished chrome work. Andy currently uses it for rallies and runs – it was recently seen on the Vintage Car Club of NZ National Rally at Dunedin. Once it is over 40 years old, which is when NZ registration costs fall dramatically, Andy intends to use it a lot more as it is a very practical and reliable machine.
The story of Silk motorcycles and how this unusual machine arrived in New Zealand from the UK is quite intriguing. Bob Cleare, of Auckland, NZ, was impressed by the press reports about the Silk and wanted to buy a new Silk 700S Mk2 Sabre. Bob contacted George Silk in 1980 but found out that they were no longer being produced. However, George suggested buying a low mileage, secondhand machine and having it overhauled by Silk themselves. George knew of two suitable machines that were for sale and Bob bought one of them, a 1977 Silk Mk2 Sabre. The red Silk Sabre was imported to New Zealand in 1981 with only 2000 miles on the clock. It went through three further owners before Andy acquired the machine in 2007.
George Silk and Maurice Patey started Silk Engineering at Darley Abbey in Derbyshire in the late 1960s. The company was set up to provide spares, repairs and restoration services for Scott motorcycles, and they also offered a range of modifications to improve the reliability and performance of Scotts. While previously working for Scott specialist Tom Ward, George Silk fitted a Scott engine into a Spondon racing frame, which he continued to race and develop after starting Silk Engineering. After some
success with his Silk Special, he began building a road-going prototype.
Silk Engineering exhibited the prototype at the 1971 Racing and Sporting Motorcycle Show in London. Orders far exceeded their capability to produce them but 21 Silk-Scott Specials were hand-built between 1971 and 1975. As the supply of Scott engines dried up customers were asked to find their own. The situation became even more difficult when Matt Holder, who owned all the manufacturing rights to the Scott motorcycle, disputed the use of the Scott trademark. Holder also refused Silk’s request to make Scott engines under licence, forcing them to develop their own engine.
The new two-stroke engine was designed and developed by George Silk and David Midgelow (moonlighting from Rolls-Royce). Firmly based on the original Scott engine, it incorporated a number of improvements. They were also assisted by two-stroke expert Gordon Blair of Queen’s University, Belfast, who optimised the porting with the aid of specialist computer programmes. As a precision engineering company, Silk were able to make the piston-port twin-cylinder engine at their Derbyshire workshops.
The new engine retained deflector pistons, which gives good mid-range torque and fuel economy, but with porting improvements and a capacity of 653cc it produced significantly more power than its Scott ancestors. The pressed-up, four roller-bearing crank was a great improvement over the fragile overhung crank of the Scott. The engine ran on a 50:1 petroil mix, with a separate oil tank reserved for main bearing lubrication fed by Silk’s own design of oil pump. Silk’s variable delivery pump was a great improvement over the Scott’s fixed delivery Pilgrim pump, ensuring correct oil delivery without too much smoke.
The original thermo-syphon water cooling system was retained but a new radiator and separate header tank much improved what was always a marginal system on a Scott. State of the art electronic ignition replaced the Scott’s magneto, and twin siamesed exhaust pipes fed an Ossa silencer. The 653cc engine’s claimed 48bhp was developed at 6000rpm, giving good touring performance, and peak torque was at 3000rpm; comparable to the Suzuki GT750.
The transmission was also upgraded but, as on the Scott, the primary drive was taken from the centre of the crankshaft. The new transmission followed Scott’s tradition of using a Velocette gearbox and clutch by using an inverted Velocette Venom four-speed gearbox and clutch. The final drive chain was fully enclosed for longer life, with the upper and lower runs being encased in telescopic rubber gaiters. Early Silks had spoked 18-inch Borrani alloy rim wheels, these were later superseded by six spoke Campagnolo cast wheels.
The Silk 700S was launched in 1975, with the new engine in a specially designed, steel tubular frame made by Spondon of Derbyshire, who also made the forks, yokes, disc/drum brakes and rotors. Priced at £1355 it was the most expensive production motorcycle of the time by a long way – the comparable Suzuki GT750 two-stroke cost £919, while the British-built Norton Commando also retailed for around £900. To be fair, both competitors produced much the same power as the Silk but they weighed significantly more.
From the outset the Silk 700S had an excellent power-to-weight ratio which, combined with superb handling, enabled it to compete with some of the best road bikes of the time. The claimed top speed was an impressive 110mph. However with no electric start, and the kick-starting technique taking some practice, it was always going to be an enthusiast’s machine.
The 700S continued to be developed along with the SPR Production Racing version. Deliveries were slow with just two motorcycles a week coming off the production line. Customers could select a range of options and choose from five basic colour schemes: British Racing Green, metallic blue or green, black with gold coach-lines or plain red. There was also a‘Silk Scott special edition’ in purple and cream, a very distinctive colour scheme previously used on Scotts.
The other striking feature was how relatively small and light the Silk was. With a 54-inch wheelbase, 18-inch wheels and weighing only 310lb, it was more like a 350 than a 700. It was even more noticeable when compared to some of its bloated 1970s contemporaries.
Silk Engineering was taken over by the Kendal-based Furmanite International Group in 1976 who continued production of the Silk 700S. In 1977 it was upgraded to the
700S Mk2 and called the Sabre. Improvements from the Mk1 included finned cylinder barrels, a redesigned seat, instruments and rear light nacelle and Lockheed brake calipers, and the option to specify those modern-looking Campagnolo cast wheels. The last batch of machines had some further porting and timing improvements, which with increased compression raised power to a claimed 54bhp.
In 1978 the 100th Silk motorcycle was produced, production continued until December 1979 when the company realised they were losing £200 on each machine sold – and that was with a retail price of £2482!
The last Silk motorcycle ever built was Clive Worrall’s 500cc model, based on a prototype that was never put into production. A two-stroke trials prototype, the Silk 350, was developed but it also never went into production.
These days, the Silk is much easier to live with than its occasionally fickle predecessor, the Scott. Once the kick-starting technique is mastered, starting with the electronic ignition is easy. It bursts into life with the very familiar, Scott-like, burbling exhaust note, and idles nicely. Pulling away, you need to slip the clutch until it is doing 10mph due to the rather tall first gear. Thereafter the broad spread of torque makes for brisk acceleration and is well matched to the four-speed gearbox. Accelerating through the gears produces a distinctive, Scott-like ‘yowling’ exhaust note.
As you would expect from a motorcycle which employs a bespoke, race-derived frame, the Sabre handles superbly ridden solo. The excellent brakes – twin discs up front – border on being too powerful for such a light machine. Back when the bikes were being built, purchasers could specify a 2ls drum if they preferred. The factory, and some road tests, claimed that the 700S would do 110mph but owner Andy has not exceeded 80mph and feels the top speed is probably more like 100mph. The inherent smoothness of a twostroke twin and broad torque makes for stressfree riding.
Ridden sensibly it returns about 55mpg thanks to the deflector pistons, and 200 miles can usually be achieved on the four-gallon tank. However, like most two-strokes petrol consumption increases significantly when really winding it on. It uses about a pint of GP50 oil every 300 miles.
The only real problem Andy has suffered in the last nine years is with the 12V Lucas electrics, but a new regulator box solved the issue. One other issue is the small dualseat which is set a fair way back; a hangover from the frame’s racing heritage which means it is not best suited to riding with a passenger.
Andy’s Sabre is an unusual and distinctive but very practical machine that any discerning collector would like to own. The Silk offers one man’s vision of what Scott motorcycles could have developed into… if the company had stayed true to its principles of performance, light weight and good handling.