I hadn’t resolved to go to Stafford until the day before, but I knew that Roger was going and that it was also a good chance to catch up with people who I’ve known through my life around Scotts, but who I have seen rarely in the last few years, since I stopped building engines for a living with Roger. I also had a chance to talk to the technical guy on the Amal carburettor stand about the level the fuel should be within the emulsion tube of a type 76.
I’m constantly thinking of how to improve the power delivery right at the bottom end.
The Scott stand looked very good and had a back drop of screens with interesting photographs, including several of Scott’s in competition over the years. Sheelagh Neal had her father Ossie’s famous Scott racing outfit on the stand and there was a Reynolds special, Harry Langman’s TT racing outfit and a Sand racing Scott.
I also had the opportunity to have a look around the Bonham auction. They had a couple of quite iconic Scotts as well as a Silk. One of the Scotts was apparently a 1926 TT entry and was the first known use of the duplex frame. The other Scott is well known to me as it was the actual machine that I always thought to be the most beautiful Scott when I was very young. It is a Sprint Special and belonged to Dennis Howard, and then Glyn Chambers. It is pictured in the Jeff Clew book, ‘The Yowling Two Stroke’. A special bike I think.
My dad also had a small display of engine parts to enable people to see that there was new spares support for the marque. I think it also shows that the bikes are living and that people don’t have to fear using them.
I already posted a picture of an iron barrel my father, Roger, has been machining up from one of his castings but he’s sent me some more pictures as he’s almost finished.
In his email Roger said:
Pics depict a newly manufactured iron block showing inlet tract arrangement where it must be noted that as the “Spectacles” portion of the crankcase will be removed, then this forms the upper ceiling of an enhanced inlet tract.
Note the two 10mm counter bores which are used to enable accurate poisoning on holding fixtures during the metal cutting processes and by using ring dowels (Sleeves) to accurately position the cylinder head which also has such features.
Ports timings are more advanced than standard DPY blocks, but less than the aluminium competition blocks I make to special order which still give much improved torque at low to medium revs.
Dies and piston blanks to make 500cc pistons are currently in manufacture as are another batch of cranks comprising 15 sets of standard long stroke cranks. 5 sets of standard long stroke cranks to be fitted with Tungsten weighting slugs, and finally 5 sets of a special heavy duty crank variant for use with ball bearing main bearings and incorporating heavy metal weighting slugs. These can be used in standard cases after appropriate modification or in the Moss high duty large inlet competition crankcases.
My dad, Roger, has been working to make sure that as many new engine components are available as possible; pistons, heads, crankcases, cranks, transfer port covers are all available as new components. One of the items that he makes for the racing and sports engines that he produces for people is the cylinder block, normally in aluminium with hard chromed bores.
He’s got iron block castings but he’s not machined one up for years, but he was today and sent me a picture.
In the last days I’ve been contacted by the new owners of both the Silk Scott prototype and ‘FNT’.
FNT went through a period of development which resulted in the fitment of one of the early Silk engines, Georges own, Scott based design. In the intervening years it was forced into boxes for a little while, but is now in deepest Wales and being rebuilt for use on the road. The owner confided that he had some sense of guilt about that, but the various racing focused parts will be retained for possible future use.
The Silk Scott prototype
It appears that the Silk Scott prototype machine is in Spain now and seems to have remained largely unchanged. The owners have a collection of all manner of period literature and photographs which document its previous life and the Manx GP race in the hands of Stuart Hicken, who is now part of the management of Mallory park.
The machine is for sale on ‘Car and Classic’ for £25,000. I’m not running a classified section, but I thought it was very interesting to see all the photos and documentation that they sent me and I thought that others may enjoy it too.
As well as the images, the owner also sent a number of scans of period reports of the Silk Scott’s TT adventure, as well as other relevant information.
As my second daughter was born on August 8th, a certain amount of sleep deprivation was bound to be involved in these final weeks prior to the September 28th/ 29th British Historic Racing Club’s meeting at Cadwell Park. Since my entire year’s work on the Scott Super Squirrel has really been focused on this meeting, I was determined that I was going to do everything in my power to get the bike as good as possible before I arrived, sleep or no sleep.
I had arranged with my dad, Roger, that we would attend the classic track day being laid on by the circuits organiser’s (MSV) on the Friday of that weekend and so it was with some relief that the bike was finished late on the Friday night before, almost a full week before I was due to be leaving for my Dad’s.
A sensible man would probably have looked at the improvements on the Dyno test results and decided to have left it at that for the year. I was not that man however, having tried and failed to have fitted the much anticipated twin carb conversion before the Beezumph in August, I was determined to have it ready for the last (and my only) BHR meeting of the year. The weeks of September had involved late nights flow testing the twin float chambers feeds to the carbs and setting the whole system up.
A fair amount of guesswork was involved in the choice (or manufacture) of main jets and I knew that the needles/ needle jets would be on the rich side, but I’d given it my best shot.
I had a moment though on that final Friday night when somewhere near 11 o’clock and undoubtedly tired, I dropped a washer whilst working to mount the inlet manifold. This was an awful thing to happen as the inlet was open at the time and I wasn’t sure where it had gone. I used a torch and a magnet to look into the inlet and couldn’t see anything. I then found a washer that met the description on the crankcase bottom deck.
I made a decision… this must be the washer.
Of course, it wasn’t.
The Scott fired up pretty easily (thank you easy start)in the yard of my friends farm, deep in Devon’s South Hams, the following morning. I had worried enough about the potential for ingested grit from his track to fit gauze filter bellmouths to the twin 276 carburettors and I was heartened by the seemingly strong pull of the motor as I headed up the track.
I don’t suppose I’ll ever forget the sound of the washer being bitten in two, twice.. not that I absolutely knew what it was at the time. It did stop the engine though.
As I put the bike back on the bench, I think I really knew somewhere as the first thing I did was to check the compressions on both sides.
Left, perfect .. no problem. Now right…wait.. I’m sure that should have been compression.
The transfer port cover was removed to reveal the kind of damage you would expect from a piston that had just sheared a 0.030″ washer in two using the top ring land and the edge of the port as cutting faces. The exhaust then told a similar story.
It was Saturday night. Five days before I was due to go up to Cadwell park via Roger’s.
Sunday morning my wife and I took my daughters swimming. It was peaceful, the calm before the storm I guess. I knew what I had to do in order to make this right and what kind of effort it was going to require and I also knew that I couldn’t do it in my workshop. I needed a little help from Roger.
So Sunday afternoon I call him. I tell him what’s happened and I know what his first reaction will be: ‘It can’t be done, have a ride on mine’.
It is the only sensible answer, after all I’ve got work and I look after the girls so my wife can work and that’s that… except that I still have the thing that fits so well with racing Scotts; The knowledge that if it is merely work and time standing between me and a functioning bike on the start line, then I can do it.
At 73, Roger’s initial enthusiasm for this kind of effort is less forthcoming but it doesn’t take long for him to come around. It is, after all, part of the game. It’s part of the deal of vintage racing, not to wreck engines, but immersion in this little drama requires an absolute agreement with oneself to work tirelessly to fix a problem when the odds are stacked against. Otherwise, there are always so many reasons to give up.
So, I gave him bore diameters so that he could grind oversize pistons ready for me and put everything in the van. I took an extra day off work and made the four hour trip on Tuesday night, arriving at midnight.
Wednesday 24th September.
I put the bike on the ramp and gave Roger the cylinder block. The bores were pretty worn anyway and would definitely have appreciated a rebore, but not like this. There was fortunately no damage to the port edges so he started to set up on the big angle plate on his Thiel 162 horizontal jig mill.
He’d put the main ports in already before he’d ground the last of his old stock Silk pistons, 0.008″ undercut at the ring land and 0.004″ at the base of the skirt. I set them up to do the boost ports and cooling holes for the exhaust port bridge as well as the cutaways at the skirt for the extended inlet timing duration. This all sounds a lot quicker than it is.
After using an air grinder with a very small burr to oil groove the little end of the replacement right hand rod I handed it to Roger to hone to the pin size. All bores are done on fixtures to ensure accuracy as the alignments of the big and little ends on Scott rods are quite critical as as any out of squareness to the narrow big end tends to result in the 3/8″ rollers pitching themselves into the side of the rod with the tighter clearance and from then on (and with little chance for oil film) overheating themselves, the big end side plate, and the entire big end of the rod. Final assembly clearances are established with a hone.
After that, one side at a time, I used a tool Roger designed to establish the amount of back-facing required on the piston’s gudgeon pin bosses per side. Normally we aim to have a few thou clearance either side. The little end is therefore controlled but this controlled position is established using the free position of the big end assembly. It simply sits in the little end bush and, using a little knurled wheel, you wind out the centre which is on a fine thread until it touches the side of the bore. This gives a measurement from the side face of the little end bush to the bore face. Using a large set of vernier calipers you measure the gudgeon pin boss width and then you simply know that the difference is going to be about the right figure. Clever and relatively quick way to start the process.
Once you’ve got the figures you set the piston up on his vertical jig mill and very simply back face the bosses. It’s a time consuming job to get it right as you need to do it several times and continue to measure the clearances but it is a great job when done.
With me working on the engine, Roger sets about one last modification for the day; the slugging of my bars to reduce the vibration I’d had at the Beezumph.
He turns down 6″ lengths of tungsten heavy metal and bangs them into the ends of the bars. Job done.
Those are serious bar ends.
Thursday 25th September
The work to the pistons and clearances go on into the early afternoon, when Roger starts to gap the rings for the new block. Care has to be made to clear the existing ring stops and Roger wants to knock them in a little further. It’s something he’s done so many times…
Suddenly I realise that something has happened. I go over to see him and look down at the piston. The ring land is broken… a moments slip.
We’ve gone too far to quit though… it can still be done. The grinder is still set up. Coffee and then Roger sets to work on grinding a new piston.
It’s early afternoon on the Thursday. We were going to leave by 4 for the 2 hour journey North.
Of course at this point we can’t actually continue with the build together. Roger comes up with the freshly ground piston in quick time and I start the rest of the work.
Knowing that I have to also measure all the piston clearances to the head, I tell Roger to get up to Cadwell whilst he’s still got energy to do it. It’s about 7.00pm.
I knew that it was a bit ambitious to want to fit a new cylinder head but I also knew that I would have probably needed to have done some work to the piston crown or the combustion chamber on the existing head anyway. One of my ambitions for the year was to have fitted a high compression head that we’d over skimmed to allow a closer matching to the piston crown. There’s a reasonable amount of work though but this was the place to do it. High speed air grinders with big cutting tools make short work of aluminium and, using the extending rod that tells us the point that the piston is hitting the head, I started to match the two.
I had measured the original head volume at around 29cc, which I had thought to have potential to be reduced. I know Colin heath once ran a head with 19cc, and I once did one with pretty much the same, maybe a couple more. I knew that the expansion chamber was going to (hopefully!) also contribute to the amount of compression here but I thought a bit more squeeze would increase the speed of the combustion process and the efficiency of the engine. If you haven’t got an engine that can be made to usefully rev, then the alternative is to extract more power out of each and every one of the lower revs that you have. That’s what I’ve picked up anyway.
There’s a lot of ‘taking off the head and putting it back on’ involved but finally I got the clearance I needed and started the build. A shade under 25cc, roughly a 15% increase.
Around midnight things were going pretty slowly but were certainly getting there and by 1.30am I had the bike finished and on the van. It took me another slow moving half and hour to pack everything else up and I rolled out at 2am, drinking the fizzless remnants of an energy drink I’d had the night before on the long way up from Devon.
The roads are quiet early in the morning and the Lincoln bypass certainly helps. The long single carriageway section at the end of the A46 North was always a jam when we used to go up to Cadwell as kids. The old derelict (obviously haunted) mansion somewhere outside of Nottingham is now a health spar. So many points of reference have disappeared with the bypass, but it is quicker.
I got to Cadwell and crawled into the sofa bed in Roger’s van just after 4am.
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.
How that translates into horsepower:
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!
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.