Tag Archives: Bob Stevenson

November 2014-Moss/Silk Scott racer progress

As with all good projects, you need a certain amount of momentum to get over the tricky bits. The first tricky bit for the Moss/Silk Scott racer is to rebuild the frame so that its strong and aligned.
I cut the front downtubes out about ten years ago, as we had intended to make the bike work as a test bed for engines we were rebuilding. The problem was that you couldn’t get the blinking engine out without having to partially strip it, or that’s what I remember anyway. Maybe others (Yuri Gellar?) would have had better luck. Paul Dobbs, who raced Roger’s bike at the time, agreed with my suggestion that we could have a detachable cradle… so I cut the front tubes out. I’ve often regretted it, mostly because of the extra effort required to get the thing back together. With some intelligent work though, it could be a really useful modification and it’s time that I pushed to get it sorted out.
As Roger is snowed under with engine work (and welding and brazing were never his thing anyway) I’ve been talking to Alan Noakes, an engineer and a Scott enthusiast, about the best way to approach this. Alan has considerable experience with welding and brazing and also has a frame jig for the duplex Scott frame which may just work with the Spondon frame.

Another reason that it’s great to be working on this with Alan is the fact that he has his own history with the Silk Scott. He sent me a wonderfully atmospheric photograph of him with a Silk Scott set up for racing when I first contacted him and after some encouragement he gave me some of the background.

The Spondon Silk (see below) you have pictured on your website could be Georges prototype but it does not have the double sided Fahron front brake which I would have expected as the first one was raced in the Manx GP by Stuart Hicken 2 years running either 71/72 or 72/73, by the way I believe Stuart Hicken is MD of Mallory Park now, I did meet him at a vintage meeting at Crystal Palace after his ride in the MGP and he said he was aquiring Scott parts to build a vintage racer with Georges (George Silk) help but I don`t recall ever seeing him racing after that.
The story of the Silk Racer in the photo is this, during the 1960s I met a local Scott owner by the name of John Farrar, we were both intent on tuning our Scotts to make them faster we also used to bother George Silk at race meetings hanging round his camp asking stupid questions etc. at some stage John had his crankcase fitted with Silk cranks and we carried on experimenting with different mods and sharing information with George, I did the engineering John paid for it. When George and Bob Stephenson shared a stand at a race bike show in London could have been 71/72 george had his racer and the first road bike on display, John had decided to order a racing chassis complete with gearbox but minus engine as he would use his existing engine and placed his order at the show this is the bike you see in the photo, on the day the photo was taken you can see that the bike was not finished no exhausts etc. we just wanted to make sure that the gearbox and clutch worked I had modified a Jawa speedway clutch to fit. The letters FNT on the bike stood for Farrar Noakes Tingley, Barry Tingley was a Local rider quite good had been given rides by Stan Shenton from Boyers of Bromley who later ran team Kawasaki. Our first race with this bike was at a Big international formula 750 meeting at Brands with the likes of Sheene Grant etc. we finished well down the field the following week saw us drawing up a completely uprated engine with reed valves flat top pistons alloy cylinder and heads etc. I did actually start making some bits for this engine but a change of job put a stop to progress and by that time John had decided that Georges new Silk engine would be a better option for the bike. John eventually sold the bike to Colin Heath.

Alan Noakes with FNT Silk Scott (1971/2)
Alan Noakes with FNT Silk Scott (1971/2)
FNT Silk Scott racer with Alan Noakes. (1971/2)
FNT Silk Scott racer with Alan Noakes. (1971/2)
Silk Scott prototype
Silk Scott prototype

The weekend’s spoils

The main point of this weekend was to go up to Worcester to pick up the lathe that I bought from a fuzzy picture on ebay. It’s a Smart and Brown Model M Mk2 toolmakers lathe from the 1950s and is soon to be manhandled (400kg?) into my little workshop. I’m going to need to get a phase converter as it’s three phase but I am really pleased. I think you can get much more for your money if you buy three phase.

It looks better blurred...
It looks better blurred…
This cost me less than ¬£200 (although a phase converter will probably cost as much). It’s a proper little tool-room lathe and it’s got collets and a three jaw chuck. I’ll just have to start picking bits of tooling up here and there and hope the thing works when it’s all in.

Also, I went up to see my dad to pick up my Scott racer’s cylinder block which he’s had to inspect. I thought I’d bring a few more Triumph bits down too but what I didn’t figure on bringing down was another bike; A Silk Scott.

Ten years ago or more, he bought this Silk Scott from Roy Lambert (not the late John Underhill as I had originally thought. Apparently John had owned it and sold it on previously).

The Silk Scott had been George Silk’s first incarnation of a Scott based motorcycle and had applied 1970’s two stroke tuning theory to the ports and the pipe of an otherwise pretty standard Scott engine. The cranks had been improved and the crank chamber sealing was done with a conventional rubber seal instead of the spring loaded metal to metal gland seal that the original Scott design used. He also created a better oil pump using, I believe, a modified best and lloyd pump design from the vintage period. He had a frame made for it out of Reynolds 531 by Bob Stevenson at Spondon which was basically a copy of the frames Spondon made for the small Yamaha racing bikes.

Yamaha AS-1
Yamaha AS-1

Here’s a picture of the Silk Scott prototype.

Silk Scott prototype

The road bike’s rolling chassis was finished with Spondon 38mm forks, a single sided twin leading shoe front drum and a mechanical disk on the rear. Aluminium rims and a light alloy tank certainly kept the weight down here at least.

Basically it’s a complete racing chassis, built to house an engine which had changed very little from 1928.

The soon-to-be Moss Silk Scott racer!
The soon-to-be Moss Silk Scott racer!

George undoubtedly released more power from the engine, but at a cost. Scotts are not a ‘Schnuerle loop scavenge‘ engine, they are a crossflow engine with the exhaust port and transfer ports opposite each other in the cylinder and using a deflector on top of the piston to send the transferred inlet gas into the top of the combustion chamber, thus scavenging the cylinder.

Deflector piston
Deflector piston

The Scott is notable as a two stroke for having a great amount of torque at low revs, probably because the design does not depend so much on gas velocity to achieve a decent scavenge. Loop scavenge engines, with the transfer ports adjacent to the exhaust port can be susceptible to losing charge directly through the exhaust if the revs aren’t high enough. There are other factors at play, but the torque of the deflector piston Scott really surprises people used to later loop scavenge designs.
If however, as is often done when tuning a loop scavenge engine for greater power, you raise the exhaust port and the transfer and extend the inlet duration, you tend to lose the bottom end. Maybe that’s ok when you’re able to get a engine producing a significant amount of power a bit higher up the rev range, but Scotts aren’t so keen to rev with that massive hump on top of the piston and also have completely unbalanced cranks, relying only on flywheel damping.
Plus the fact that the standard (long stroke) Scott only ever had a three speed box, and the Silk Scott only a four speed box doesn’t enable you to cover a narrower powerband and you start to see where modifications to the original design require an overall consideration of how these things link with each other.

Our plan is to build the Silk up with one of my dads racing engines, which only slight differences to the original port timings but has much better gas flow, and work to make a decent set of exhaust pipes to exploit the low rev range available.

We originally planned to make the Silk Scott a test bed for the engines we built for people so we modified the frame with the intention of doing a detachable front cradle to aid engine swapping.

It’s going to be tight to get it done this year, but I’m going to get on with it and see how it goes.