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!

Real Classic - - News - Pho­tos by Stu­art Fran­cis, South­land Vin­tage Car Club, Mor­tons Archive, Chris Dickinson, Oliver Hulme

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.

Right: The seventh ‘pro­duc­tion’ Silk to be built

Right: The cool­ing sys­tem is just one of the many un­usual as­pects about the Silk en­gine. It doesn’t use a wa­ter pump but in­stead re­lies upon con­vec­tion to cir­cu­late the heated / cool­ing wa­ter through the ra­di­a­tor and around the en­gine – known as a thermo-syphon cool­ing sys­tem

Left: The Silk was in­tended to be a rid­ing ma­chine: here’s one now, be­ing rid­den

The pro­duc­tion form of the Silk 700S with disc brake and a new mar­que badge

The pre­cur­sor to the Silk pro­duc­tion run; a drum-braked Silk-Scott spe­cial, one of 21 which were hand-built in the early 1970s

‘Our new, patented Ve­loc­ity Con­toured charge / scav­enge sys­tem is a ma­jor fac­tor in achiev­ing abun­dant low speed torque and good fuel con­sump­tion,’ said the Silk brochure. ‘Max­i­mum torque oc­curs at 3000rpm, giv­ing real kick-in-the-back ac­cel­er­a­tion through­out the speed range and re­quir­ing only a sim­ple four-speed box’ Above: Silk pro­duc­tion was def­i­nitely small-scale. He­po­lite, for ex­am­ple, needed a min­i­mum or­der of 1000 units to sup­ply the 76mm di­am­e­ter pis­tons. So 500 sets of Silk pis­tons of that size were or­dered, which means there were no over­size pis­tons avail­able. If an en­gine needed a re­bore then the owner would have to re­place the iron cylin­der liner

Silk em­pha­sised how much ef­fort they’d put in to keep­ing the 700’s chas­sis light, to make the most of the com­pact en­gine. ‘Not only does this con­trib­ute to the fan­tas­tic per­for­mance, but it means that the bike is read­ily han­dleable – you don’t need a weight-lifter to move it round the garage!’

Silk Engi­neer­ing built 138 Mk1 and Mk2 Silks. To­day the silk-mo­tor­cy­ site lists around fifty 700S ma­chines which are known to have sur­vived

This hand­some ex­am­ple seen at a Stafford Show shows off the de­tail­ing pos­si­ble with small-batch man­u­fac­ture

Many items on the Silk (wheels, brakes, sad­dle, colour scheme, fuel tank, fork gaiters, han­dle­bars, etc) were spec­i­fied to suit the orig­i­nal pur­chaser, who cer­tainly got good value for the ex­tra­or­di­nary pur­chase price. All the nuts and bolts were stain­less steel as stan­dard

As you might ex­pect from a Spon­don chas­sis, the Silk’s ride is typ­i­cally on the firm side. Ge­orge Silk said ‘This was done de­lib­er­ately to give the op­ti­mum road hold­ing…’

Be­fore Silk in­tro­duced their own en­gine, they built sev­eral of th­ese Silk Scotts, plant­ing a Scott en­gine into a Spon­don rac­ing chas­sis. Thanks to Paul Miles for the pics.

Silks rarely come up for sale, even in the UK. Moto Corsa might still have this 1979 edi­tion in stock, priced at £17,000

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