The latest addition to the Moss family arrived on August 8th. A healthy little girl and we’re all very happy. She’s not been in the workshop yet but I intend to indoctrinate them both to make sure that their idea of a family holiday is to share a damp tent at a racing circuit. I’ve taken a couple of weeks off for paternity leave and as well as spending some enjoyable time with the family, I also managed to get the Norton model 18 gearbox shell down to a local machine shop to (hopefully) have it fixed.
I’m a little nervous about giving it to the guy, not because I think for a moment that he’s not capable of working a lathe or a mill, but that unless you are going to prepare and supply an entire op sheet that details every check, every measurement, the way you hold the component etc, you are relying on someone really working to engage with the context of a component to make sure that they actually address the problem.
It’s not that I’m a control freak, just that I’ve found that people who are sparky enough to really engage like that are the exception rather than the rule. Anyway, I’m very excited at the possibility that I’ll be overwhelmed with satisfaction at the job!
Tomorrow, the Norton will dismount the bench having used its quota of time unwisely by presenting me with damaged gears and casing, rather than allowing me to put it all together. The Scott Super Squirrel racer will go back on so that I can check it over prior to its appointment with the dyno next Thursday morning.
I don’t really have a clue what it’s going to tell me, but I’m thinking that it’s likely to be over 30 BHP. The most interesting thing though will be to see the actual torque curve as the one I got from the last dyno test I did, which was prior to all the flow and port work I did over the winter was really strong at 3500 rpm and then everything tailed off well before 5000 RPM. I think I’ve got more revs now, and have lost some at the bottom. It’s going to interesting to see.
Also, to recap, I know that my inlet port area is 25% bigger than the cross sectional area of the carb, so I’m going to try to get it all ready for twin carbs for the last meeting and get the extra gas in. I’ve got to have cables made, carb machining work, float chamber/ carb flow testing before that can be finished which is why I need to get it back on the bench! I also want to get it back on the dyno before I go.
Hopefully the rest of the bike will be ok. The rear tyre did suffer at the beezumph as it was quite hot and large chunks get ripped out if it starts to hop around. I’m pretty sure that the last meeting in September will not be so hot that this is exacerbated.
Anyway, loss of sleep is taking it’s toll so I’ll sign off.
The fact I’ve managed to do anything on the bikes in the last few weeks is a minor miracle. My wife is expecting our second child and with D-day approaching fast, I’ve had to focus on preparatory DIY.
I have managed to move forward with my projects though, if not at startling speed.
The Super Squirrel racer is sitting patiently, waiting for a trip to be organised to Alan Jeffries dynamometer in Plymouth. After that I’ll be trying to get the new fuel system completed for the final Cadwell park race meeting for the BHRC (British Historic Racing Club) at the end of September.
The Moss/Silk Scott racer is just about to enter a new stage, as I’ve found someone locally who will soda blast the frame. It’s about double the cost of normal media blast cleaning, but I wont have to plug every aperture in the vain attempt to stop the abrasive media ingress. I also don’t really want it in my workshop if I don’t have to. It gets everywhere. In terms of my own time, I think it’s worth the extra. I’m intending to take it up there this week along with the oil and water tanks.
Roger (my dad) has also given me the engine (in bits) to make a start on and I need to do a dummy build to look at timings. A couple of months ago contacted a guy who lives fairly locally to me who has done some very impressive expansion chambers for more modern machines and told him about this project. He’s interested in looking at what would be necessary (or possible) in terms of pipes and he needs provisional timing information to run through his calculation programme. I’ll write more about it all in a separate post.
The main subject of my attention at the moment is the Norton Model 18 which we’ve had in the family for over thirty years now. The last time the Norton was used was I think 2001, when I actually used it for daily transport through the winter. I’d come back from five years of travelling and working abroad and simply didn’t have anything else. It jumped out of third gear a lot and pulling the chair only managed 47mph top speed but I really liked riding it, once I’d got used to riding an outfit.
I brought it down to Devon sometime last year with the plan of returning it to the road. Unfortunately as I wrote in one of my first posts on this site, their was a problem with the engine and I’ve temporarily replaced it with a later one. This requires a conversion to run using coil ignition and alternator.
Only last week I also removed the gearbox to have a look at the jumping third gear issue and unfortunately the news wasn’t great. Damage to dogs on Norton four speed gearboxFive gears had reasonable damage to the dogs and I’ve ordered some (also secondhand) replacements from the Norton Owners club. Also the gearbox casting is cracked at the lower gearbox mounting pivot on the drive side. Apparently this is a common problem and I will have to get it welded. It’s a design weakness here I think so I’ll have a look at some possible improvements to the arrangement. I need to order a bunch of other things for it, but it’s a really nice bike and is too good to sit around for another 10 years. My grand plan is to get it recommissioned and use it my everyday road bike. It’s much more of a sensible option than in many other counties too. The route between here and work is about 50% tiny lanes where you often can’t do much more than 25mph anyway. The roads through Dartmoor are more open than the ‘lanes’ in the South Hams but the presence of sheep, cattle and ponies create an element of surprise. The undulating nature of the land favours a engine rich in low down torque and flexibility. This terrain is the ideal stamping ground for a big single more than anywhere else I know!
Well, I tried to get the twin carb manifold completed before Beezumph 23, on the 12th July but it was not to be and I was glad that I’d made the decision to leave intact the entire single carb assembly, fuel lines and all, just in case I needed to put it back. It was a close run thing and I actually still hadn’t finished the bike when it went in the back of the van but in fact retaining the single carburettor gave me the opportunity to assess the changes I’d made on the engine with more certainty as to what had affected what.
Just to re-cap, the Beezumph is not a race meeting but a track day organised by the vibrant Trident and Rocket 3 Owners club, many of whose members bring their machines out for this spirited social occasion. I first went in 2001, I think, and Roger a couple of years before that. I think it was his first return with the Scott to the track after some years of working hard to build his workshop and business. I believe that first time he attended he was awarded the ‘man of the meeting’ award by Doug Hele after having caused great amusement having repeatedly passed very much more modern bikes invariably by diving up the inside of them into corners.
I got up early as I still needed to finish a few details. Rear chain tension and corresponding alignment of the rear wheel needed to done, followed by the wiring of the rear brake torque arms and wheel nuts. A good check over and then put the kettle on for the morning coffee.
The fog that had descended on Cadwell park the previous evening lingered for a while in the morning lending it a brigadoon-esque feeling of a world apart which I’ve always felt Cadwell somehow symbolised anyway. You can be a hero just for one day at a race meeting, away from normal sensible life, normally in battling to fix things in adverse conditions. Two years running I worked ’til two in the morning at the last vintage Cadwell meeting, stripping and rebuilding a jammed Scott clutch (having three gears is hard on a clutch). I’ve ridden hundreds of miles to fetch a replacement component to fit overnight. We fixed a hole punched through Rogers crankcase by a fallen transfer cover bolt with epoxy and underpants so that Paul Dobbs could continue to race the same day. It’s still the same today.
So the fog delayed a little the start of proceedings but when it did finally lift, it revealed a beautiful day which was at times almost too hot in racing leathers.
In short it was perfect.
Beezumph has become a family favourite, and until very recently (Babies have arrived) there were regularly several of us making an event of it. As it was, three Scotts and their owners turned up to support us and it made a very fine line up in the paddock. Richard Rawson and his fine Silk Scott and friend on his very nice Birmingham Scott, and then Alan Noakes on his beautifully detailed, girder forked, Flying Squirrel.
I hadn’t mixed my fuel and so set to work with the ingredients. The engine had not appeared to have enjoyed a surplus of oil in the bores from my inspections after the previous season and so I’d decided to reduce the acetone percentage in the mix in case this was simply stripping the lubrication out. Acetone is one of the things that came up in my research when I was looking at running on methanol and my understanding is that it’s used to help combat pre-ignition in leaner fuel mixtures and possibly aid starting. I had decided on a 5% Castrol M, 10% acetone and 85% Methanol mix previously but this weekend I reduced that to 5% Acetone to see whether that made a difference.
Not having even run the engine since I started working on it at the end of last year, I was glad to have an offer to use someones starting rollers.
With the drippers set high feeding Castrol R through the non return valves direct to the main bearings she turned over for a few seconds before gradually starting to fire. A tell tale hanging of mist in front of the carb opening showed the effect of the extra inlet duration I had applied. I expected that that would only be present at lower revs, but we would see. When cold she always carburates poorly and there’s a significant lag on the throttle as if the cable has a length of elastic in it. After the engine warms up, she’s immediately responsive. Methanol simply runs so cold that when the engine is also cold the atomisation seems to be quite poor. That’s what I imagine anyway.
So up and down the pit road a couple of times and then out in our session.
The beezumph has different categories and they range from beginner (marshall led laps) follwed by classic, fast classic, open (any age of bike) and then expert classic. We go in expert or equivalent normally simply because you can get problems with people being unused to being passed around a corner in other classes. Last year in fact, Roger decided to go in the fast classic group thinking that class more appropriate. He was 72 and on a 1934 bike and so thought he’d give it a try. He found, as is often the case, that he was being passed down the straight by more modern, faster bikes which then proceeded to brake very early where a corner was approached. Thinking that they may be stopping to attend to natures call or maybe a sandwich, he would pass them. Some take seeing a pensioner riding a vintage girder forked bike up the inside of you at a ridiculous angle of lean with great humour and enthusiasm, but not all. Roger would invariably pass, undoubtedly at a significantly higher (if not warp) speed, until their desire to storm by on the straight was diminished by a growing sense of futility in the action. There-after some are merely crest fallen, whilst for the special few indignance steps in to protect a fragile self esteem..
One rider was so affected by this (whether through genuine fear or critically injured vanity it’s not known) that he complained and Roger was informed that if he wasn’t able to pass on the straights then he should not pass.
This year he returned to the self appointed experts class, where passing on corners is in fact expected.
The first session out, I was obviously quite sensitive to the engine’s character as much work had been done since last season. It seemed to me to have less torque low down and be therefore slightly less drivable out of the corners. However, the engine seemed to be be pulling longer through the revs and although the majority of bikes (750 Tridents and Rocket 3’s) were easily faster down the straight, it wasn’t the difference I would have thought. I thought I felt the engine tighten a couple of times (though I might have been over sensitive) so I took it relatively steady.
I raised the needle before going out the second time and was not to feel any hesitation again. I kept behind Roger for a couple of laps to get a comparison between his and mine and he thinks that I have about 2 or 3 mph on him on the straight. maybe 5. It’s doesn’t feel quicker than last year in the way it delivers power, but I think that actually I’ve made it breathe so much better that it’s simply getting more in at higher revs and therefore revving longer. Extra vibration has come with this, but we’re going to slug the bars with tungsten heavy metal to help here. Although my dyno tests last year are compromised by the fact that I’d blown a head gasket, I think the torque curve will be broadly representative and a comparison will show me what has actually happened. I look forward to getting it on there and will not change to the twin carbs before I’ve tested with the single. I’ll then swap the carbs before going back for another test. It’s going to be interesting.
So in the end, the bike was flying and little was able to get past and stay past. True, it’s a track day and not everyone is wanting to ‘ride it like they stole it’ though some are. It’s easy to walk away from a track day thinking that the bike is a rocket and that you are riding at the edge of human ability then go to a vintage racing meeting and get lost in the wake of serious riders on seriously developed machines. Saying that, mine is a seriously developed machine. By the time I get to the last vintage Cadwell it will have been my only race meeting this year. Family and work commitments coupled with a realistic budget have prevented me from attending more, but not in idle have we stayed away and I am hoping that when we do turn up to the last Cadwell at the end of September that we are able to move further toward the front of the field than we have before.
I’ve never won at Cadwell, and whilst I do all this for so many reasons beyond achieving a position in a race; this is what drives my desire to develop of course. To win, one day on my Scott Super Squirrel against good men on good bikes at full circuit Cadwell park. Of course it’s a folly, but what a grand folly!
I’ve been away in France for a few days Holiday with my wife and our little girl and I thought I’d take a couple of motorbike magazines with me that I’ve never bought just to give me something to stick my nose in (as well as Jennings book on two stroke tuning, Tuning for Speed and a a great book about some of the lesser known stories from within the drama of the Tour de France). One of these was ‘Practical Sportsbikes’ which seems to be largely written by one man but has some really interesting bits and pieces in it. Mainly aimed at people who are interested in 70’s, 80’s and 90’s sportsbikes and still actually doing things to them to make them faster or better. One bit was concerned with the re-commissioning of a Suzuki RGV 250, which I’ve always had a soft spot for after having sat on one at the 1989 motorcycle show at the NEC. Anyway, they had decided to fit a programmable ignition unit which gave them the opportunity to pre-program advance curves and also to alter the timing using a plug in remote control. After realising that this wasn’t something that was that far beyond the realms of possibility, I wondered about using this on the Silk Scott racer, since no firm ignition set up has been defined as yet, beyond the use of the flywheel as part of the generator and ignition trigger.
Roger working on generator flywheel.
We’re hoping it’s all going to work out…I wrote to the manufacturer mentioned in the piece at the beginning of the week outlining my interest and telling him the current situation.
I’ve had an email exchange this week which has been interesting. He admits that the benefits of an advance curve are likely to be greater in a higher revving engine, but he reckons that all engines benefit from it. It also gives speedy possibilities to set up in a dyno session, where the timing can be changed very quickly.
I’ve never had an advance curve on the system I run on the Super Squirrel and I’ve always thought it was fine. Without actually putting it on a dyno, It’s pretty difficult to know though.
He also says that the spark output is really good at low revs which means good starting… much better than a PVL system he said, which didn’t really crank out the voltage until the revs were higher. That would be nice.
The programmable ignition system is ‘zeeltronic’ (apparently popular according to this magazine article) and the website is here:
http://www.zeeltronic.com/page/home.php
They do systems that also control exhaust valves at different revs but the one he specified just does a couple of ignition curve programmes.
It certainly means that there wont be the fiddle of trying to make sure that the pickup assembly is adjustable. Apparently you set the pickup to sense the trigger just before the range you are going to be using and then the actual ignition firing points are decided by you in two programmable maps.
Also, since we won’t have any ignition or oil pump related gubbins hanging off the doors, I think that we should also do a set of reed doors for it such as Roger made for his brother’s bike back in the late 1960’s. Recently he’s had more castings made as they were used on the ‘phased transfer’ engine that Bob Collet has designed and built using Scott components as a basis. Reed valve doors as designed, made and fitted to his brothers bike in the late 1960’s The more I think about it, the more I reckon that although reeds can be restrictive to flow in high revving two strokes, with the engine speeds that we are using there could well be a distinct advantage and with the high comp head and a resonant exhaust, it could be significant!
Whilst I work out the port timing calcs, here’s a very early picture of Roger’s Flying Squirrel racer frame to illustrate his description in the comments to the post ‘The evolution of the Super Squirrel racer’Top triangle was a design to remove the role of the standard frame’s lower rails to retain rigidity.
The crankcase in this shot is, I believe the last standard Scott case that he ever used but fitted with the four bearing crank he made to help stem the tide of standard longstroke overhung crank induced engine carnage. Note the odd shape of the doors with the bolts in the middle. These are just blanking bolts; once removed a slide hammer can be attached to the doors to extract them as they obviously have to be a good fit to support the crank assembly.
The strength of the crank assembly was proven in quite extreme circumstances when there he started it at a meeting and it fired on one before hydraulic locking on the other cylinder, in which there had been a water leak. The contest of strengths was lost by the crankcase, which split across the main bearings. So much for sorting the crank problem.
Another interesting thing to note is the blind head block, which I believe was aluminium. This didn’t have any kind of higher compression inducing form work in the top to match the pistons, as his detachable heads do, but it would have been lighter than standard and running Silk pistons as we still do now.
just found some photos of the whole assembly: Aluminium block and EN24T four bearing crank assembly. That’s a titanium rod as well, in about 1977!
Also here’s a picture of him working machining a crankcase for Ted Parkin’s Scott. I believe this has extra large doors to take a set of special extra long stroke cranks.
Thicker sections, bigger port tracts, better material and designed to take replaceable main bearings with modern oil seals.
I’m doing things backwards here. I realise that I need to give some more of the the original history of the Super squirrel and how Roger’s Flying Squirrel racer came about. As I said in the previous post, the Super Squirrel racer is really only just returning to having the potential of competitiveness that it did in the early 1970s. Super Squirrel racer, prior to frame modifications (around 1971/2). Note Roger’s Laverda SF750 production racer with race kit. An unusual racing stable.Roger on the Scott Super Squirrel racer (around 1971/2) racing at the New Brighton circuit on the Wirral.My dad, Roger, having found that the Scott was fast and competitive in racing was very much committed to finding the solutions to the bike’s shortcomings, namely in handling, gearbox and engine reliability.
The single down-tube frame of the Super Squirrel had broken once before at the seat post and had been re-inforced substantially. Tie bars had been created to give some tension to the lower engine and undertray (carries the gearbox on a Scott) mounts as the original lower frame ‘rails’ have to be removed to be able to race. Left in, they will dig into the track and have you off. Later shot of Super Squirrel racer around 1973/4. Note lower rails removed , replaced by tensioned ‘tie bars’ and no silencing!This did leave things a bit more flexible in this frame and he resolved that therein lay some of the problem. He was sure a stiffer frame would be a great improvement.
He addressed the handling issue by having a duplex frame made by Bob Stevenson of Spondon to a similar design and geometry to the Flying Squirrel, only using lighter tubing. To be honest, he’s always said that the duplex frame he had made didn’t actually improve the handling, but it did allow a bigger carb (because you didn’t have the single down-tube in the way) and it was quite a bit lighter as it was a welded construction and not lugged.
Years later it was Paul Dobbs, the talented Kiwi rider who suggested that he thought the handling could be improved by moving the riders weight forward. Paul Dobbs in inimitable action over the mountain on the Scott at Cadwell park, 2005My dad did this, moving the seat forward, and a big improvement was felt. In about 2010 he had the tank shortened to allow this to be more neatly contrived.
I also moved the saddle forward on the Super Squirrel when I re-built it, and swapped the ‘Brooklands’ style bars that my father favours with a set of wide straights that force your hands wider and make your body weight shift forward. The handling is far better for this, and actually I much prefer the extra leverage too.
The gearbox story is well explained in his story of the affair, here, and the pursuit of power and reliability were definitely linked, as the inevitable longstroke crank breakages inevitably took it’s toll on successive crankcases, prompting a decision to re-cast cases with better material and extra strength. Cases were redesigned to have larger transfer apertures and inlet port areas and cranks were re-designed to use the crankcase doors as an outer main bearing support to overcome the design flaw and material shortcomings of the original overhung crank.
The development of the Super Squirrel racer into the Flying Squirrel was not instantaneous though and it was largely about a substantial focus on re-engineering. In truth, that has consistently been the focus of his very successful Scott racing development work. In the process, he has developed his Scott to the point where some people even dispute that it is one. To me however, the Scott was Alfred Scott’s creation and he was a man of vision and ingenuity. He left the company that bore his name in 1915 and died in 1923. It’s impossible to look at the balance and finesse of those early shortstroke machines and imagine that he would allowed the bikes to have developed as they did, in both weight and fragility, had he stayed with the company.
To me the very spirit of the Scott is strongest in those machines where people have employed their skill and imagination to take the unique qualities of the Scott and develop them.
It is in the DNA of the marque and though I understand of course that there are those who have great enjoyment of their original machines, to me there is no Scott more a Scott than one that has been intelligently modified, and there is no Scott that can lay claim to having been been developed with more ingenuity, determination, focus and success according to its remit, than Roger Moss’s Flying Squirrel racer. Waiting for the call…
It’s not been the best couple of weeks for getting on with the Super Squirrel racer’s engine with both my wife and little girl both poorly and work being very busy indeed. A very good friend of mine also passed away, although this did rather contribute toward the quietness and patience required for gas flowing with riffler files.
So what is the plan?
A bit of history… In 2006 I finished re-building the old Super Squirrel racer and into it went a good Scott engine that I’d built with Moss crank. I sold this engine to fund another more radical engine build, and machined up my own head and heavily modified a Scott barrel to suit. I also welded up my own expansion chamber.
The crankcase I used had been a damaged case that we’d had welded, but had some evidence of cracks still remaining beyond the welding.
Anyway, I took it to it’s first BHR meeting at Mallory and it felt really strong for the first two laps, before it died. I didn’t really look at the engine until I’d pushed it back to the van, but when I did I realised that the damage was absolute to the crankcase. It was split in two and completely irreparable. oh dear.
Upon reflection, the case wasn’t up to the job. I might have had some tiny amount of piston/ head contact too.. I know they were close as I’d had it before during testing and had worked to increase the clearance. The main bearing assembly was experimental and I think that also may have been a weakness. You live and learn and competition sometimes just brings the answers a bit quicker.
I had been working with my dad building the engines for a few years and I think this just happened just as I was going to move on to do a contract working to changeover a cylinder head line to a new head in the casting plant at Nissan in Sunderland. As I was not in a position to build another engine, my dad resolved to machine up one of his crankcase castings to at least provide a sturdy basis for a race engine. Into this he built the internals of the previous engine, and the new engine was badged ‘Phoenix’ in reference to it’s resurrection from the remains of its predecessor. from the ashes…
It was a fantastic thing to do for me.. I think because he felt quite sorry as I’d put so much work into the previous engine and also, that he felt that the far stronger crankcase casting was a far better place to start.
So I ran the engine for a couple of years, on petrol, and it kept going, but it wasn’t really competitive. It wasn’t really ‘tuned’, just solid and although I really enjoyed my bike, it wasn’t anything like as good fun to ride as my dad’s Flying Squirrel, which just had a sense of thrilling urgency that mine lacked utterly.
The catalyst for the major improvements that came was when my wife and I received a wonderful wedding present in 2011, in the shape of a new expansion pipe that my dad had made to fit by Gibsons exhausts in the South East. My wife found this quite amusing. It was somewhat better looking than the one I’d welded up myself.. but did it work?
The first test came early in 2012 when we participated in the Prescott hill climb in aid of the blood bikes. With the same jetting as previously used with my old pipe I accelerated from the line hard and then pulled in the clutch quickly as it seized on the needle as I rolled it off.
We played around with it in the afternoon, changing plugs, altering the timing and jets but it just seemed to be running very hot. The next outing was at Lydden with the BHR club and we put in an extra head gasket to decrease the compression. It was still running a bit hot, but better… at least it finished a race. It really wasn’t quick enough though. I realised that I needed to make a decision.
It may be that the exhaust is not of the optimal shape and there may be a build up of heat because of this and not simply because it’s charging the cylinder so effectively… but we are not running a blank cheque development program (!) and so we needed to try and see if we could get it to work.
I figured I had three obvious choices. The first was to put my pipe back on. I did not want to do that .. It seemed such a retrograde step. The second was to work on getting the heat away. I’ve got a speedway radiator in the bike so a bigger one may well be much more effective. Also my dads bike has an aluminium cylinder barrel which also transfers the heat away from the exhasut port and cylinder head much more quickly than my iron block. Great. A new radiator would be about £1000 and a cylinder several hundred.
The third way was not popular with my dad.’Dope’ I said, that’s what I’m going to do, ‘run it on dope’.
The positives of methanol are that it really cools an engine and allows a far higher compression ration to be run than with regular petrol. Methanol also burns more slowly and that can make for a smoother and more progressive power delivery. On the negative side, it’s comparatively harder to get hold of, more dangerous to deal with and you need a much larger amount to run on. I’ve also experienced lubrication issues since I’ve used it, but it may be that some careful development may improve that. It also doesn’t give much warning in terms of plug colour if you are running lean. It tends to let you know by melting a hole in a piston apparently.
So it was that I invested in a barrel of your finest methanol and talked to a few people who had experience using it. By far the most useful contact was Roger Cramp, who used to race the highly developed Velocette that his son Ian now campaigns with the BHR. He had been involved in the building and development of an ariel leader that he had run on methanol as well as a Greeves. He confirmed the research I’d done about the necessary changes to ignition timing but also said to be aware that methanol was singularly averse to atomisation (at least when bucketing it in) and that high intake gas speed really helped. This encouraged me to stick with my smallish single carb to at least try out and see if it worked.
I decided on a huge 980 main jet and then measured the needle jet with a taper pin. I then put the carb together and drew lines on the needle at 1/4, 3/4 openings at the top of the needle jet and then worked out the dimensions of the needle at those points against the aperture open for air inlet. I ended up with a pretty severe taper on the needle, but it seemed to make sense.
I advanced the ignition by about 7°, closed the plugs right up and pushed. It fired up and it ran, albeit a little roughly, before I put it in the Van to took it to the last BHR meeting of 2012.
It was brilliant. The jetting seemed to work fine and there were no holes in the power delivery according to throttle position. The bike pulled and was so much fun. It felt totally different to my dads bike, but the torque and flexibility of mine suddenly made it feel like a completely different bike. It absolutely hammered the clutch though (as it does tend to with three gears) and I was up to 1am on the Saturday night stripping, releasing, filing plates and and rebuilding it.
2013 came and one of the first jobs I did was to rebuild the clutch with new GFS plates, laser cut. They were perfect really because I had to dremel each one to fit with abut 0.010″ clearance. The less clearance, the less hammering… My dad also had some pressure plate he’d had made out of solid, which didn’t flex like the original ones. These didn’t have the adjustable clutch actuation pins I normally used, which are a pain to set up. I made pins up instead from some silver steel and got them within 0.001″ of each other using a cordless drill as a chuck, a file, a dremel and some emery cloth.
We took it to a couple of track days, and then the last Cadwell park in 2013 and (with new 21″ racing tyres robbed off my dad’s poorly Flying Squirrel) managed a couple of second places and even a fastest lap. It was flying, although not in the league of Mike Farrel on his Rudge, who was really out on his own and un-catchable for us at least. You can lose a lot on the start as it’s difficult to get off the line with three gears when you’ve geared top for a long straight. Except for a CS1 Norton belonging to the famous Lewis family who’ve been campaigning Triumphs and the Norton for many years. I think everyone else runs four gears. It does make a difference.
I ended the weekend having blown three composite head gaskets and with the feeling that there was a bit too much piston slap noise, but apart from that it was the best racing weekend I’ve ever had at Cadwell. I also knew I needed to strip the engine and that I’d do some gas flowing whilst it was apart.
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…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
Here’s a picture of the 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!
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
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.
My Father, Roger, had known Titch Allen for many years. Titch had been a founder of the Vintage Motorcycle Club and was a great fan of racing, modifying and using the bikes.
He lived a very long and interesting life and was a mine of great stories.
He used to visit us quite regularly when I worked with Roger, and one day I made arrangements to make a recording of a story.
This is his blue rollers story.
The virtual oily corkboard of a vintage motorcycle racing family
