Category Archives: Engineering

Silk Scott racer project – the return of the frame tubes

I’m not sure that would make a successful Hollywood title but it sure as hell excites me!
I don’t know whether everyone does it but I carry moments of regret through life somewhere buried in a dusty box in the corner of my brain. They spill out sometimes and I go through an internal process of closing my eyes and shaking my head side to side as I remember the feeling. It is largely pointless of course as the moments are long gone.
The bike(s) I shouldn’t have sold, the engines I shouldn’t have blown up, the Rolls Royce I shouldn’t have crashed (it wasn’t mine.. I was working as a mechanic in New Zealand and being towed to a garage) and definitely in the mix.. the Silk Scott frame I shouldn’t have cut the front tubes out of. Putting the fact that it wasn’t going to be easy to swap engines without doing something ‘detachable’ aside… the Silk Scott sat rather forlorn for at least 10 years until I started looking at possibilities last year. It’s not finished by any means but Alan (Noakes), who’s been working on the detachable cradle has got a loose assembly together now. he just sent me the pictures: This is how Christmas should feel.

My initial sketches
My initial sketches
Frame connectors
Frame connectors
loose frame assembly 001
loose frame assembly 001
Frame loose assembly 002
Frame loose assembly 002
loose frame assembly 003
loose frame assembly 003

The next step is to braze the lugs up.

So that’s what Alan has been doing!

What have I been doing?
I’ve stripped the weighted cranks out of the crankcase my dad machined for the Silk Scott as I need to do some final work on the gas flowing on the transfer and inlet passages (I love doing this.. stick a MotoGP DVD on and needle file for hours). I’ve also written to Rex Caunt, an ignition expert, to get advice about the process of setting up a charging system using the ring of magnets on the side of the flywheel.

It’s all too much. I’m off to bed.

January 2015 – Progress report

Well, we are moving in the right direction (I think) but I’ve a lot of ambitions this year so we’ll see how it works out…

Last weekend I decided to drive up to Leicestershire to see Roger and to help him improve security at his place, following the break in the previous weekend that resulted in the theft of a Yamaha RD 350 LC YPVS F2 (the one I had when I was 18 years old.. 25 years ago). He’d bought lights and I bought an alarm system and over the weekend we fitted it all and made the best of a bad lot.
I love being in the workshop and he’s still pushing forward with improvements to machinery and tooling, the latest being a DRO on the beautifully made Smart and Brown 1024 VSL lathe. You don’t need them, but they do make life easier and quicker.
It’s also good to be surrounded by his Scott engine work and I think we help motivate each other to do the best we can to make sure that our racing Scotts are in good competitive order for riding this year.
Sometime on Sunday morning we had a visit from Eddie Shermer, the editor of ‘Yowl’ (journal of the Scott Owners Club) and a skilled engineer in his own right. He and his wife also put great energy into organising the annual ‘gathering’ of the Scott Owners Club held at Abbotsholme School in Staffordshire.

Eddie Shermer, Roger and Thiel 158 universal jig mill. They were smiling until the photo!
Eddie Shermer, Roger and Thiel 158 universal jig mill. They were smiling until the photo!
Eddie does a lot of Scott transmission work but also takes on complete engine rebuilds too, and came to discuss possible solutions to a particular problem he’d encountered with a crankcase. It was good to see him and always interesting to discuss the engine issues.
In fact, I kept a Scott line going through the weekend because I’d arranged with Alan Noakes in Lincolnshire that I’d take the Silk Scott frame up to him so that he could make a start on the frame connectors. Lincolnshire is hardly on the way back to Devon, in fact around 2 1/2 hours in the opposite direction, but it meant that I could see Alan and chat through some of the details and also see some of the things that he is working on. He had some hardened and ground clutch hub centres as well as pressure plates that he’d made on the bench, and also a very compact roller starter that he’d even cast the rollers for. Very impressive.
Anyway, I couldn’t stay long as I had a 6 1/2 hour drive back home! Just back by midnight.

This last week, I’ve removed the carburettors from the Super Squirrel as I need to make some modifications for the return to petrol. I never had the twin carb set-up developed for methanol anyway (switching back to petrol this year) so it’s not something that I wouldn’t have to do anyway. The two carbs are Amal 276, but are of different bore sizes at the moment. I need to get them bored out to 1 1/16″ and also have some short inlet ‘trumpets’ made. They won’t be very trumpet like, but will just have a better radius for the air flow.
I’ve also got some ‘K-type’ thermocouples which I want to fit to the exhaust, fairly close to the exhaust manifold, to give extra information when I take it to so a dyno setup when the inital carburettor modifications have been done. Petrol will make everything run hotter, and I’ll need to be careful to try to make sure that I do what I can to make sure that I’m not overheating the piston. I may also fit a temperature sensor to a spark plug to read that, as well as coolant temperature to see whether the small radiator is able to shed heat. It’s possible that I could pressurise the radiator to help the situation, but it might not be necessary. I also have a slightly rough, larger radiator that I bought many years ago from the late John Hartshorne, a prominent Scott enthusiast who had some wonderful stories, not least from his work with Wilf Green in bringing East German MZ’s into the UK in the years before the ‘Iron curtain’ came down.
Also on the agenda is crank counter-balancing. A Scott engine, being a 180° twin has reasonable primary balance, and normally they are known as being fairly smooth engines. They have almost no actual balancing on each crank assembly however, as the cranks are too small. The engine relies mainly on the large central flywheel to help dampen out the residual vibrations, which it does reasonably well. Some people have drilled the side on the flywheel on opposite sides to try to counteract the implied rocking couple effect but roger has never been convinced of the efficacy of this process. His own idea was to view each crank assembly in isolation and to work to try to balance at least as much of the big end weight as possible. In his ‘four bearing cranks’ he has two crank ‘discs’ each side, rather than the usual one. His enables him, using tungsten heavy metal (and Titanium rods) to balance the big end if nothing else. Rogers engine is very smooth, much more so than my own, and seems to want to rev more easily. We have ‘slugged’ single sided ‘Moss’ cranks in the past for a couple of people, and one of them cannot praise its smoothness enough. My cranks are not weighted, but I have some heavy metal slugs and I want to fit them. Normally, the holes are put into the cranks prior to heat treatment, as the final specification of the material conforms to around 50 to 53 HRC which is pretty tough stuff to machine. I’ve been looking into using surface coated solid carbide cutters rated up to 60 HRC to machine out for the slugs. I’ll post further information on the calculated % balance. I think it should be interesting!

Anyway, enough for now. things are happening! Even the Triumph head is back although a little more work needed here too.
I’ll try to not leave it so long as we go through the winter.

Triumph head news

I got the invoice from SRM today to say that the work to re-commission the Triumph Tiger 100 bronze head has been finished. It’s all money of course but if they’ve done a decent job I’ll be so happy. It’s twenty five years since I took that head off to get fixed and there’s a lot of water under the bridge since then. I like a bit of continuity.
I spent most of my School lunchtimes at Owen Greenwood’s motorcycle shop in Loughborough in 1987/88. I had my first road bikes from him. First a Simpson 50cc and then an MZ ETZ 125. I have no idea how many times I fell off them but he did call me ‘crasher’ Moss at one point. When he had the Triumph bronze head valves seats built up for me and my Dad, after my first season of racing with the VMCC, I doubt he considered that it would take this long to get the rest of the work done. He’d have called me a ‘messer’ without a doubt, but I know he’d be pleased that I was eventually getting back to it. RIP Owen Greenwood.

I’ve got to get barrels bored but I’m sure I’ve got a reasonable pair of pistons somewhere. It won’t be phenomenal but it should be somewhere in the mix in the 500 class. I don’t think the short stroke Manx Nortons need worry though…

The other main thing the Triumph needs is some attention on the breathing, I’ve looked into non return valves in the past but never got it completely sorted. It always did cover the rear tyre with a fine mist of castrol R. Nothing to the rear of the engine will ever rust on this bike. I’m sure a concerted effort will bring success, although I have seen that from 2016 all road competition bikes (two strokes too?) will have to have a complete catch tank under the engine. I think a scraper on the sidewall of the rear tyre into a dangling bucket might work better. It’s no fun hitting oil though, so I’ll do everything I can to get it under control.

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

Super Squirrel racer – dyno test results – August 2014

I really enjoy taking the bike to the dyno. It’s so useful to see the results of changes prior to driving 300 miles to a race circuit and it gives you a deadline to get things done by.
The rolling road dyno I use is at Alan Jeffry’s engine tuning workshop on the Valley road in Plymouth. Alan’s a really nice guy but the main part of his work is cars so the motorcycle dyno is run by GT motorcycles (01752 485000). A single run (at time of writing) costs under £40 including VAT and Steve, who operates it, is a two stroke fanatic and a very experienced re-builder and tuner. He had an NSR 500 (GP bike) complete with carbon chassis on his stand when I saw him on Thursday. People with NSR 500s aren’t going to let just anybody work on them.
It was all the more enjoyable since I was joined by Roger who, having made the journey down to Devon to meet his new grand-daughters the day before, was interested to see the improvements I had made.
I was there at 9.30am as planned, and then again at 9.45am… this time with fuel!
As I said in the Beezumph report post, it didn’t feel any quicker to me in the way it was delivering power, but I thought that it was pulling through a higher rev range. This wasn’t actually the case since it is producing more power, in fact it’s almost exactly doing what I intended to do when I started planning the modifications at the beginning of this year.

The plan was to try and get the engine to breathe better with some carefully executed gas flowing and port modifications, and also to extend the inlet timing a little to see whether I could take advantage of any negative pressure pulled by the exhaust before the transfer closed. By using Jennings’ port time/area calculations I was trying to move the peak torque up the revs a little from the previous 3500RPM (ish) to nearer 4000rpm. I didn’t want to lose the bottom end and I knew that was too easily done.

See the graph below. The blue line (as indicated) is last years test after returning from the final Cadwell park BHR meeting.

August 28th 2014 Torque Curve
August 28th 2014 Torque Curve

How that translates into horsepower:

August 28th 2014 - hp graph
August 28th 2014 – hp graph

So, it shows that there’s a fair bit more power available and really well spread over the rev range. It may be that that’s why it didn’t actually feel more powerful, because it delivers it so smoothly over the range.
Whilst peak torque is up from 31.8 to 37.9 ft/lb (almost 20%), it’s interesting to see the change in revs that this occurs at. Previously it was around 3700 RPM and now it’s pretty much dead on 4000 RPM.
Also, driving out of corners should be much improved as low down torque is significantly better. I haven’t plotted the actual revs through the gears at corners for Cadwell, for example, but for slow corners like the Old Hairpin and the chicane after Mansfield this is where you really need that low down grunt, otherwise you get passed on the exit. At 3000 rpm, the torque is up from 23 to 31ft/lbs (35% increase). That’s pretty impressive to me.

So extrapolated from that, the peak power is up from 26 to 33hp, but still everything stops at 5000RPM. Steve said he could feel it wasn’t producing any more so he just shuts down. Whether it would actually rev any more anyway is another question. Unfortunately I have no idea what I am revving to because my Scitsu hasn’t worked since I converted to methanol. Apparently a common problem with inductive rev counters due to methanols highly conductive nature. Something I need to address sometime.
The main focus now is to fit the twin carbs to give me more intake mixture. The 30mm carb is definitely sized small and
I’ve got less than a month before the last Cadwell (27th and 28th September). It may well be that this will keep things going up toward the higher end of the rev range where the need for an easier ‘gulp’ comes into play. It may be that it loses some immediate pick up at low revs, but we will just have to see.
The other thing that I need to look at is the cylinder head. I’ve never worked on the head, it’s a standard ‘MOSS Engineering’ high compression head which is designed to raise the compression to a level acceptable on a fast road machine on petrol. Since I’m on methanol, I can deal with some extra squeeze and with a bit of time and effort I think I could get the compression a bit higher which would increase the burn speed. I’m then looking at 40 ft/lb as possible and maybe even a fraction more. I may even extend the rev range a little.
Ideally I need to get the work done in time to get to the dyno again before I go. Just over three weeks.. I’d better get a move on!

Paternity leave

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.

Holiday thoughts..

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.
Roger working on generator flywheel.

We're hoping it's all going to work out...
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:

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
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!

This weekends engine rebuild

After what must have been the longest Devon to Leicestershire trip I can remember doing, I arrived on Friday night with a view to getting the bottom end of the Scott Super Squirrel racer rebuilt by Sunday afternoon.

I knew that I was going to change the main bearings, as the ones I had were a bit notchy in the case. I also wanted to do some more gas flowing on the crankcase to allow me to use another inlet port that was blanked off by part of the crankcase as my calculations had shown that I was deficient in the inlet gas flow. I also wanted to check the static flywheel balance and the crank assembly end float and alignment.

The first thing I did was my porting as I knew I’d have to clean up the cases before replacing the main bearings.

Opening up the last inlet port.
Opening up the last inlet port.
It’s all so much easier with proper air tools! I’ve been spending hours with a riffler file to do stuff I could do with an air tool in less than half the time. Files are safer though! Easy to make a mistake with an air grinder.

Apart from a little de-burring here it is finished:

Just finished grinding the final inlet port access.
Just finished grinding the final inlet port access.
Scott cases ready and waiting for attention at Mossengineering
Scott cases ready and waiting for attention at Mossengineering
Roger working on a customers engine.
Roger working on a customers engine.

One of the first things we noticed when we looked carefully at the crankshaft assembly was there looked like there had been some movement on a crank taper. Wanting to err on the side of caution we set up a lap on his Thiel 158 jig borer to just make sure that the tapers were good and clean in the flywheel. A bit of gentle lapping and all was fine.

Lap for cleaning up minor surface damage in tapers.
Lap for cleaning up minor surface damage in tapers.

Next, we checked the static flywheel balance before ‘knocking up’ the crankshaft/flywheel assembly for checking the distance between inner control faces on cranks. This, we compare to the bearing face to bearing face measurement of the crankcase to determine the end float as you cannot feel and measure it by simply moving the crank side to side when installed as I used ball races and not rollers as standard.

Static flywheel balance
Static flywheel balance
Drilling flywheel for balancing
Drilling flywheel for balancing

See here Roger’s magnificent Thiel 162 horizontal jig mill. We dug out the floor with a mini digger and filled it with at least 1 meter deep of concrete to create a sturdy foundation for this. Table rotates 360° and flips up to 90°, whilst the whole machining column can move in and out. The spindle then can be moved forward/back and up/down.

Thiel 162 Jig mill
Thiel 162 Jig mill


The Smart and Brown 1024 VSL lathe is a good place to put up the crank and flywheel assembly between centres (he sells these if anyone’s interested) IMG_4182We measure skip and run-out just to make sure there are no problems.
Checking the crank assembly for run-out
Checking the crank assembly for run-out

I made a new key, using slip gauges to determine the width. You have to be careful to check the height of the key as well as the length in case these prevent the flywheel tapers from safely locating in the flywheel.

Flywheel/ crank timing key. This is to time only and is not for driving purposes.

After all this, and before the assembly, the old bearing were removed and the cases heated to accept the new ones. The 22 tooth drive sprocket was deemed to be too worn and a replacement was bored out to suit the spigot and fitted.

measuring for the new drive sprocket
measuring for the new drive sprocket

After that the new oil seals were fitted to the housing behind the main bearings and then the cranks finally installed and ‘knocked up’. The key doesn’t transmit load, it absolutely is not meant to… the taper has to do that. The crank tapers are driven in by tightening the centre bolt and then (with adequate provision to provide a dead stop on the other side) the centre of each crank is struck using an aluminium mallet, or large diameter drift alternately whilst continuing to tighten the centre bolt. There will come a point where the bolt cannot any longer be easily tightened and this is then considered done.

All in a very successful couple of days and a bottom end that should hopefully last for a while!

Super Squirrel tuning for 2014

Although I’m unlikely to be able to afford to do anything like a full season this year, I’m pushing to try and get the Super Squirrel engine rebuilt to be as competitive as possible. It’s quite heartening to know that it went as well as it did as there had been little in the way of time spent on the detail of gas flow and port timings within the engine. Sometimes you have to stand back and re-evaluate why you are doing what you are and whether the original reasons still exist. I’ve done this and have some thoughts for ways to extract more power.

As it is, the port timings have been unchanged from standard, except for the inlet which has a slightly longer duration due to having relieved the skirt by about 1/8″. I didn’t change them because I didn’t want to lose the tractability and strong torque at low revs that the engine produces. I am still running a three speed box with ‘vintage close’ ratios and it’s important to have as much flexibility in the power band as possible. So, the idea was to increase the power and efficiency without narrowing the range too much.
The power of the engine seems to have been noticeably increased(though I lack proof of this) by using this new exhaust showing, i think, that it is effectively ramming unburnt gases back into the cylinder prior to the closure of the exhaust port. This has been a success which needs building on as I still think that there’s a significant amount of further power to be had by careful development.

I thought I’d start by looking at the obvious impediments to gas flow. I’ve been using Jennings book on two stroke tuning for guidance but essentially the idea is to have a gas flow which is not full of disruptive internal turbulance. The turbulences can be caused by changes in the surface of the ports, either things sticking up or the surface falling away (bumps or hollows). Also flow out of and into ports is facilitated by radii on edges. I’m applying this to all my ports and piston ports as a beginning, though minimally on the top edges of the ports where the timing will be affected. I’ve got a pair of Roger’s ‘high flow transfer ports’ which have no internal bridge and a non symmetrical shape, the idea being to send the transfer gas into the hump on the piston rather than over it. I always intended to spend time matching them exactly to the transfer ports on the crankcase and the block and two weeks ago I decided that the time was upon us.

I started with the crankcase and worked steadily on the left transfer aperture and found that when I’d absolutely matched the aperture to the cover, I’d increased the aperture from 904mm²(1.4″²) to 994mm² (1.54″²). That’s a 10% increase in area and the removal of edges over which eddies can form in the gas to restrict flow even further.

Transfer port work. 10% bigger on left before work commenced on right.
Transfer port work. 10% bigger on left before work commenced on right.

There was a bit of work to the covers themselves in matching to the transfer port openings on the block, but no work on the block on either the top or bottom edge , though a little at the sides to prevent the gas hitting the sides of the port. This gave me fractionally less area as it entered the ports. My understanding is that this is preferable as the gas speed is increased and the tendency to have internal turbulence affecting flow is less.
Once I’d dealt with these, I looked at the transfer ports themselves.
Whilst the engine was still together I’d noticed the height of the top of the skirt at bottom dead centre and found that it was around 1/16″ below the transfer ports both sides. To my mind this gave at least some opportunity to use some of this available space and at present I have elected to radius the bottom edge of the port to assist flow and also deepen the port in the middle adjacent to the bridge by that 1/16″ as well as radius the bridge on the transfer side.

before... (note blocked up inlets)
before… (note blocked up inlets)

I thought that there would be gas displacement when the flow hit the bridge and deepening the port there would give it somewhere to go.

The timings themselves are:

Transfer: 134°
Inlet: 129°
Exhaust: 159.5°

I have to be cautious about messing around with the port timings too much, as Scott barrels are no longer commonly (and cheaply) available should I completely mess it up. There is, however, a factor that has never really featured in Scott tuning before, that we now have got an expansion chamber exhaust which goes some way to (over) compensate for the effects of enforced silencing to 105 db. This means that the inlet gas, which was originally intended only to be subject to a pumped transfer is also possibly assisted by the expansion chamber extracting gases through the transfer, which would leave a negative pressure in the crank chamber. If this is taking place, then there is possibly also merit in extending the inlet duration further as there is possibility that more gas could be introduced without it spitting it back out. This also might be assisted by the use of the twin carb manifold and long inlet tracts contributing some inlet inertia to the situation.

previous engine with twin carb minifold fitted
previous engine with twin carb minifold fitted
Whether in fact the 289 carbs I have for this are too big to allow the gas speed and inertia required for this (and atomisation of the methanol) I don’t know. It’s be a suck it and see. I do have a rolling road dyno down the road and the smart money would be to run the single carb and then try the twin set-up and see the difference. Unfortunately my attempt to Dyno test at the end of last year to provide me with comparison figures didn’t go to plan as the head-gasket was blown from the beginning.

I need to do some port timing calcs (time/area) and continue the flow work to the crankcase and ports.

The evolution of the Super Squirrel racer

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.
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.
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'
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, 2005
Paul Dobbs in inimitable action over the mountain on the Scott at Cadwell park, 2005
My 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...
Waiting for the call…