Moving directly on from the last post at the end of January, I thought I’d share a little progress on the exhaust front.
Although I’ve been completely absorbed in doing engines for other people, I’ve been wanting to find some time to go back to basics and map the Gibsons made exhaust that I’ve been using on the Scott racer. Now this isn’t an easy thing to do completely accurately as the exhaust is shaped to wind its way through the frame, but I think I made a reasonable job of measuring it out.
I drew it up with CAD, but it’s actually easier to see on sketch up.
I know you can’t see all the detail but it’s interesting to see the lengths involved and tapers used. A couple of things are interesting, the stinger ID works out to be around 54% of the ID of the start of the header pipe. Now I don’t know much about expansion pipes except what I’ve read and been told, but the stinger ID is a critical dimension and very small dimensional changes can make noticeable differences in the effect of an exhaust. If too small a stinger, excess pressure can cause overheating in the engine. Now back at Cadwell in 2015 I was told that as a rule the percentage difference between the stinger and the header should be around 60%, which would make it ~29.8mm ID. That’s 2.8mm difference, which is not even that small. Also, there’s another thing: this is a 2 into 1 pipe, although I’ve only drawn a single pipe in the example. I’ve a Scott engine behind me on the bench with a similar exhaust duration as mine; mine is around 160° duration and the one behind me is around 157°. If this bike had a pipe for each cylinder, then it would have ~203° of crank rotation from the closure of the exhaust port to the re-opening on the next cycle. Since this is a 2 into 1 pipe, the exhaust has ~23° of crank rotation from the closure of one exhaust to the opening of the other. That’s around 11% of the time at any equivalent RPM or even if there’s a mathematical reason why that’s not completely true, it’s a lot less time to do the same job.
Given that the 60% guideline for the stinger is supposed to be based on a one pipe to one cylinder design, am I to extrapolate the time difference to apply to the cross sectional area of the stinger? Again, I’m sure the maths are well above my pay grade for that, but what’s obvious is that it’s about time and area, and most bikes rev faster than a Scott.
A quick googling leads to me think that an early 70’s TZ 250 would rev to 10,500 rpm, for example. Given that the tuning data from Bell would have been used on actual racing machines in that era, then it’s fair to assume that this was representative of the kind of engine speed that the 60% Stinger ID to Header ID rule of thumb, was worked out for. For sure the exhaust durations are different too, but I need to look at this to make a guess at what the stinger ID should be.
Also, having been able to establish the design and measurements of the Gibson pipe, I found that it’s actually made for an engine that revs quite a bit higher than mine.. like my dad’s! (although it’s even on the high side for that. Working backwards, the dimension between the piston face and the mid point in the deflector cone is 1244.5mm, which according to Bell makes it tuned for a whisker under 5500 rpm. Now Roger’s engine, with his substantially balanced four bearing crank will spin up past 5500rpm, but that’s not a place he likes to ride it anyway. Dobsy used to rev it hard, but even my dads engine is getting a little out of breath at that point.. just a limit to what the ports can flow, even with the differences on his engine. My calculations would suggest that I need a tuned length of around 1530mm for 4450rpm. 5000rpm is about right for max revs, but as I’ve said before, I just don’t think it can breath properly up there. Thats 285mm longer, not insubstantial in terms of getting it to fit on the bike. We might be looking at the muffler the other side… I know that’s been done.
So I bought Bell’s two stroke tuning book as well as John Robinson’s to go with my copy of Jennings and have been working through some possible re-designs. I purchased a TIG welder a few weeks ago and so that’s another step in the right direction. I think hydroforming would be the best way of testing some designs.. the most fun you can have with a pressure washer as far as I can see!
I’ve been very busy but two months is a long time since the last post and I shall endeavour to do better.
One of the things that has taken a lot of my spare time in the last months has been teaching myself to draw using a 2D CAD program. I didn’t do technical drawing as an apprentice and I’ve always looked upon drawing as being such a vital skill, especially if you need to be able to get work made out by others. Of course, you can do a lot of things with a simple sketch, but it’s undoubtedly quicker and simpler (once you know how) to make changes and amendments using a digital file.
Also, where clearances are tight, and dimensions are precise, because it shows (of course) a true representation of what you are wanting to produce, it enables you to see more easily any mistakes you may have made in measurement or design. Like pretty much everything I do, 2D CAD is so old hat that on one hand it almost seems ridiculous to talk about it but it’s new to me and I think it’s the most exciting and interesting development in my education for years.
The initial incentive to push myself to learn how to use CAD was that I found myself unable to find a particular component for a Scott engine that I’ve been working on. It’s a mid twenties engine and it had a bearing cup which was both corroded and cracked. Enquiries about a replacement cup with those associated with the rebuilding of the older engines have been fruitless. However, before I go any further I should explain a little about the main bearing assembly of a Scott and why repair or replacement presents more of a challenge than for many other engines.
The Scott engine has its two crank-chambers separated by an external area which houses the central flywheel. The crank-chambers themselves are very slim by design so as to maximise the crankcase pumping effect and so is therefore the main bearing assembly, crank assembly and big end / rod assembly. The main oil feeds are visible on the picture and are fed direct to the main bearings. These are (generally) fed by a metering unit called a Pilgrim pump (sic) which allows the user to adjust the quantity of oil delivered to each cylinder. The delivery rate in use is defined by the engine speed. The main bearings and big end bearings are roller bearing units, which involve outer and inner races that are made specifically for the Scott. Rollers themselves are fortunately still available (!) but if Scott bearings are damaged, the work to ensure the accuracy and fit of the replacement assembly requires a great deal of precise toolroom work.
A picture is worth a thousand words…
You can see here the drilling into the outer wall of the crankcase main bearing housing
and you can see here the corresponding hole in the outer diameter of a main bearing cup. The groove is to give some rotational tolerance when fitting the cup.
Here you can see a cross sectional drawing showing the drilling through the rear of the later cup design:
I will one day go through the operation of the metal to metal sealing arrangement of the crank-chamber, but suffice to say that it is a spring loaded seal that in Scott circles is called a ‘gland’.
but back to the bearings.
The bearing outer is retained in the crankcase with an interference fit. This fit is necessary to provide strength to the bearing ‘cup’ during operation and to mitigate against the differential in thermal expansion between the steel cup and the aluminium crankcase. It also has to ensure the constant seal for the oil feed between the crankcase and the cup.
Even if there is no problem with the bearings themselves this interference fit diminishes over time and the cup can become less well secured. This can become evident by oil leaks noticed in the central flywheel area. Sometimes this can be oil leaking past the gland but sometimes it can be leakage at the connection of the oil feed drillings. It’s also very noticeable when you remove an old cup… often they require very little force to knock out.
The cups when newly fitted have 0.005″ interference between the outer diameter and the bore in which they fit. This means that fitting is done with a cold bearing in a hot crankcase and can be a bit ‘heart in mouth’. It is a real pain if the cup isn’t exactly square going in.
And that’s the start. You then have to make a steel ring that fits in an annular groove in the aluminium around the main bearing. This steel ring (called a shrink ring) is made to have 0.009″ interference between it’s ID and the corresponding diameter of the groove. The shrink ring is then heated until it’s red hot and dropped into position.
After all this, the mouth of the cup which is not supported in the same way as the rear, is compressed so that the bearing track is now tapered. The whole lot must be ground in situ… which is another story.
Anyway, back to my CAD drawing. I had an-usable cup and needed to have one made. Although I have some copies of Scott drawings I don’t have one for this particular component and so I thought I must make a drawing myself. After considerable efforts to get on top of the process, I am really happy with progress.
Hopefully I’ll have it finished by the end of the week.
Roger went over to the continent a number of times during the 1970’s, racing his Scott. On this early foray his Scott was pretty much as it is now, having been rebuilt by me around 10 years ago. By the late 70’s he was thick into development and had commissioned a new frame, tank and radiator and this is where his iconic current racing machine came into existence. The pictures are from Montlery, which was a great experience. One of the French racing contingent was a man called Christian Olivaux, who invited us all (the whole family went) to his apartment in Paris to stay, which we duly did on our return.
Many years have passed since this time, but my dad was sent a picture recently by Christian from Montlery in 1975. He remembers coming third in the race. Others in the picture are ( I think): Geoff Pollard (second in the race on his 1939 Tiger 100) and his son Derek (or Bill.. one was a nickname), Fred Ellis, Tim Maton (won on a Vincent twin). If anyone remembers anyone else let me know…
The picture of the group of them together is from Christian, the others are from the same meeting that Roger had already.
Happy Memories indeed.
It’s been a quiet summer on the Scott racing front. All kinds of responsibilities and activities fill a Summer when you’ve got two very small children and I must admit to at least attempting to lead a balanced life, especially when the sun is out. Also, doing a bit of Scott work for other people does mean that I’m spending time on their engines rather than my own! It’s all good though and I was looking forward to getting to the September meeting at Cadwell park as this is my favourite meeting of all.
The Flying Squirrel (remember, it’s a Flying Squirrel and not a Super Squirrel as I discovered this year..) was still on the bench looking a little forlorn at the beginning of September. I had removed the top end after the seizures at Anglesey because I wanted to check the pistons and bores. The seizures had occurred on both pistons at the rear corners and the left hand piston was a little distorted because of it. I spent quite a bit of time filing the damage out carefully. The bores were fine, although I flex-honed them to freshen them up a bit.
The last time I had assembled the engine, I used a gasket compound called ‘three bond’ which has good gap filling properties and remains flexible. I used a tiny smear on various surfaces that I had scraped flat, just to ensure a good seal. I had a hell of a job getting things apart and in fact the bond was so good that it pulled the devcon epoxy out of some corrosion damage on one of the carburettor flange faces. There’s such a thing as too good! I think I’ll use it where a face isn’t good, but back to a silicone sealant where it’s needed.
So, with the engine back together and the repaired radiator back in (thank you again, Graham Moag), the bike just needed a few bits sorting out to get it ready. The front mudguard needed to be replaced and the L/H footrest needed straightening. A few other little bits as well.
Inevitably, some of these things ended up being left to the last minute, which was OK since I’d arranged to have the day free before I went to Cadwell just to make sure everything was finished. Unfortunately, my wife had a work deadline which meant that I had to look after our children on this day. This was an error in my calculations! Instead of an unhurried day and early afternoon departure to Cadwell, I frenetically worked on the bike from around 5pm till 9.00pm on the Friday night, then packed the van and went to bed at 10.30pm. At 1.00am I arose, drank coffee and set off to Cadwell… over 6 hours away.
The drive wasn’t so bad and the morning was crisp, with a low mist that hugged the warm ground. It was obvious that it would clear and clear it did. It was indeed a beautiful morning welcome from Cadwell Park.
I arrived pretty much as people were starting to get up and got the bike down to Scrutineering early. No problems here and I signed on and started to check the bike over myself.
Practice for solos was called and the bike started immediately. The twin carbs breathing without bellmouths because I knew I needed to make some that actually flowed properly and I hadn’t done testing to base a length on. I figured it would do at this point.
The Scott seemed willing at low revs although I was concentrating on the clutch, given the seizures it had experienced at Anglesey. Sure enough, at the bottom of the park straight it came. I was quick to get the clutch in and coasted to the marshall’s post to wait for the end of the session and the recovery truck.
Back in the paddock I started to go through the fuel system. A gentleman called Peter, who was interested in the Scott, was kind enough to help as I went through the fuel system checking the flows through the taps, lines and banjos.
It’s dangerous to assume but sometimes you forget when you have assumed. I had the bike on the dyno at the beginning of the year and had thought that this would show up any problems with fuel supply. It wasn’t that I was expecting it to give problems.. and it didn’t. I’d made sure that the fuel lines were all of a descent size and had put the lot together being as careful as possible to avoid flow related issues. The dyno runs were completed without the suspicion of a seizure, so I hadn’t thought there was likely to be a problem.
I did flow tests all the way to the banjos that fit to the bottom of the carbs and was surprised at the results. They were barely able to flow the potential of the main jet. With my new found assistant’s help, I drilled every fitting out and found that I’d increased flow by a couple of hundred cc’s per minute to each carb. Part of the flow improvement was that I removed a connector pipe between the feeds to the two float chambers from my twin taps. I’d thought that this was was a good idea in case one tap became blocked but in effect the T- piece connectors were just another cause of a pressure drop. Amal main jets are rated by cc’s per minute flow (albeit under certain controlled conditions that I wasn’t trying to emulate) but at least now I had almost double the flow specified by each jet. I thought that should cover that as a possibility of seizure.
I was therefore hopeful when I got out in my first race, though aware that I still may be missing something.
It seized at the end of park straight on about the second lap. Bugger.
I started to think about the history of this problem. Roger had presented my future wife and myself with the exhaust as a wedding present in 2011. She was overjoyed of course :). It had been made by Gibsons in the south east and they’d had the bike to fit it to properly. They hadn’t run any calculations but had copied my dad’s pipe which he’d supplied them with for that purpose.
I went to the Prescott hill climb early in 2012 as the first outing with the bike and .. guess what.. it had seized at the first corner. It sound so hard edged and got so hot that I’d ended up fitting another head gasket at the next outing (BHR meeting at Lydden) simply to get it to stop overheating. It didn’t overheat (though it still got very hot) but it was slow. I thought perhaps that all this was a sign that if I wanted the power, it was a case that I’d need to make the engine dissipate heat better. Maybe my radiator, which is very small, was incapable or that my block (cast iron) was too slow in getting that heat away. It started to look very expensive.
That was why I switched to methanol in the first place…that I thought it might provide me with an easy alternative.
Methanol had worked, but was there another problem that it was hiding?
I had been wondering somewhere of course whether the pipe was in fact unsuitable for some reason. Since none of the problems existed before the pipe, it seems like a clear possibility. I talked to Rex Caunt (BSA Bantam tuner) on the Sunday and told him that I wondered whether the stinger outlet was too small and that it was the exhausts inability to get rid of the pressure quick enough that was causing the heat build up. He gave me a ‘rule of thumb’ to work out whether this was the case.
Apparently, the inside diameter of the ‘stinger’ pipe outlet is normally around 60% to 62% of the inside diameter of the beginning of the header pipe (around 50mm).That would make it around 30 to 31mm ID. I couldn’t get to the stinger, but I could measure the silencer outlet and it was around 26.8mm. That’s under 54% of the ID.
When I start to think about it more, I think there’s another reason why it might need to be bigger. It’s a 2 into 1 pipe and although the operation of the phases are separated, there’s not the time to dissipate pressure that you get with a normal 1 pipe for 1 cylinder operation. In fact it makes sense to me that the outlet should be bigger than a standard pipe for this.
So in terms of racing it was a terrible weekend but in terms of development, I feel happy that I’ve got some new direction for the off season. I’m going back to basics with the fuel system and the exhaust and maybe even make some experimental pipes simply for the dyno. I also have started to realise what the dyno may or may not be useful for. I’ll still use the dyno to show what relative power I’ve achieved but I wont assume that it’s all I need for testing.
I’ve got lots of reasons to be excited about this winter. Roger finally took his bike to Motoliner in Maidstone to have the frame and forks checked and straightened after Steve Plater’s crash at the beginning of the year. It will be good to see that make progress.
The Moss Silk Scott needs to start moving to the next level… a dry build to see what kind of tank and seat unit it’s going to need and where the exhausts can go. I really look forward to moving forward with this.
I’ve got my lathe to finish scraping/ re-building, my milling machine to re-commission and tool up for and various other engines and gearboxes to work on in the meantime.
I still have a dream of making my Flying Squirrel the machine to beat in the vintage class and will be yet again edging toward that over the winter. I can but try!
I’ve not written much recently but there’s plenty been going on.
My racer is still on the stand, although I now have a new front aluminium mudguard to replace the one damaged in the lowside crash at Anglesey earlier in the year. I also set about the paintwork on the headstock to try and find trace of a frame number, which would help me to register it for the road if I so desired. I found it in the end (but not before I’d removed a decent amount of perfectly decent paint) and amusingly it confirmed that the frame was never a Super Squirrel frame, but a 1932 Flying Squirrel. I have actually called it a Flying Squirrel in the past (and a Sprint Special when I was very young and simply wanted it to be one…) but the idea that it was a Super Squirrel stuck for some reason and I can’t even remember why. It’s not even that the engine type was the same.
So from this moment on I shall call my noble steed by its rightful title: My Flying Squirrel racer.
Although Anglesey was a while ago now, one of the other things that happened at the time was that my dad’s bike was due to be tested the following Tuesday for Classic racer magazine by Steve Plater, a former motorcycle racer and TT winner. I made the journey to Cadwell the day after getting back from Anglesey and we were very interested to see how he got on and what he made of the bike. He’s used to modern machines and I don’t think he’d ever ridden anything like it before. My dad advised caution through Charlies because of it’s tendency to get out of shape on the exit, but apart from that let him work it out himself.
We changed the bars to give him a different position and he seemed to be gaining confidence quite quickly through hall bends, where we were watching. He was certainly moving. However, maybe the confidence was a little premature as he lost it out of Charlies as a result of a tank-slapper that he couldn’t control.
Noticing that he hadn’t come round for another lap, I feared the worst and ran up to the van just in time to intercept the recovery vehicle. I took a deep breath when they opened the back door as the steel Vincent straight handlebars were bent vertically both sides, like bulls horns. I could see the top fork links had bent significantly before I even got it on the stand, and by the time Steve told me that it had gone over a couple of times I already had a mental picture of what, in all honesty, was the worst racing incident it had ever endured in over thirty years.
Of course, we were all relieved that Steve was ok. It could have been very nasty for him. On reflection, I think we were naive to think that even a highly successful professional modern rider might just sit on something with as lowly relative performance as my dad’s Scott and be able to work it out easily. Riding a rigid bike, or more to the point, racing a rigid bike requires a whole skill-set of its own. The feedback to the rider from a rigid chassis with girder (almost rigid) forks has little comparable in the modern motorcycling world. Racing with modern tyre compounds winds up the chassis and causes some instability that you get used to and some you know you can’t. Even though you ride a different line to avoid the ripples or the sudden dip in track surface etc, arse off the saddle.. damping with your knees like a jockey… there are some corners where you don’t want to try to push the line, and the exit of Charlies always has been one.
At the end of last year, Bill Swallow had a ride on my Dad’s bike in one of my races and he got into a tank-slapper coming out of Charlies which certainly caused him to wind it back it little. He knew he couldn’t push any further.
Maybe Steve felt a little under pressure to perform? Possibly, although he is obviously a great rider with a lot of experience of real racing pressure so it’s difficult to believe. I think the truth is, that he just didn’t know what the bike would do and assumed that he’d be able to tame it. It was a sad end to a day which promised some exposure for the British Historic Racing Club, plus a wonderful chance to see what a well respected modern champion might achieve with the bike in the way Paul Dobbs did with such style, ten years ago. Rest in peace, Paul.
In the end, it was a mistake, just unfortunately one which will take a while to sort out. Now stripped, his racer needs to have to frame checked for straightness, the bearings, the wheels etc. The fork blades are bent and most likely other parts of the assembly too.
Of course it will be done, but at 74 Roger has a lot of engine work for customers and it’s just difficult to find the time for minor developments, let alone complete re-alignment and rebuilding work. It won’t be done until next year, for certain. We have to remind ourselves that beyond the feelings of sadness and regret over the incident there must remain one clear point:
This is what racing is.
Roger had a recent email exchange with a gentleman in New Zealand, John Stewart, who has had a long history around Scotts. His grandfather was a photographer and captured a wonderful photograph of his father in his workshop in Featherston, New Zealand. John’s son, Scott, repaired the plate glass image and apart from Yowl (the journal of the Scott Owners club), it’s not been published before. He has kindly allowed me to do so. The copyright belongs to John Stewart and I use the image here with his consent.
The attached photograph of my father’s workshop in Featherston may be of interest. This was almost certainly taken by my grandfather, GT Stewart on his glass plate camera (which we still have), sometime during WW1. The garage was established in 1906 as Stewart and Son, Later Stewarts Imperial Garage. The garage provided a wide range of services including maintenance of steam traction engines, motor cars and motor cycles and during the war, repair and servicing of army vehicles for the nearby Featherston army camp.. Two vehicles in the background are almost certainly army truck chassis.
The man at the back is Dick Rowe who was workshop foreman. The lass sitting on the chassis is Miss Freed secretary and the other figures apprentices and tradesmen a couple of whom appear to be working on Model T Ford engines.
The interesting bit is of course the Scott on the right. This I believe is a 1914 model and was the machine that my father, H.H.Stewart raced on the grass track in Featherston with some success. The family left Featherston in the mid 1920’s and dad kept the engine and two speed gear out of the Scott along with the remains of an 1898 De Dion Tricycle and a 1900 Locomobile steam car. He carted these parts round the country during a number of moves until finally settling in Auckland in 1926. The two speed gear was used as a change speed gear on a turret lathe after WW2. The engine I gave to a friend many years ago who had unearthed the remains of his late brother’s 1914 Scott with no engine and the De Dion has been subjected to full restoration over the last 3 years which I finished in Dec. last. It runs superbly.
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 have a whiteboard in my workshop and it generally lists too many tasks that I am a reasonable amount of effort away from achieving. This last week however, I managed to get some significant ‘to do’s’ rubbed off the board.
The Silk Scott frame had been waiting at Alan Noakes’s workshop for me to come up with a plan for the final brazing solution. The lugs had been made to be brazed with a capillary fixing which really needed oxy-acetylene. I must admit that we hadn’t fixed the exact type of brazing material we were going to use and this was all part of the delay. I saw a clear weekend coming up and thought that this was my chance to push to get it finished. So the plan unfurled: first to get oxy/acetylene.
We’d had a plasma cutter at work that we’d bought from machine mart (please no comments). It was around £600 new and lasted a year and a day. The warranty department, were sympathetic but not quite to the point of being reasonable or useful, and I was told (after paying for the service) that it could be repaired for a sum of 500 and something pounds. I declined their kind offer. We’d only used it a handful of times to make register plates for woodburners and other bits and pieces. Normally around 3-4mm thick steel for a machine rated for 10mm. Anyway, since this incident I am resolved not to use them again and so was without any metal cutting equipment at work apart from grinders. So, I figured at £5 each per month for bottle hire, I could justify the oxy/acetylene. My Dad had an old portapak I could have and so I found myself at BOC sorting out an account last week.
I called Alan from the shop and enquired as to whether he’d got any prior arrangements for the weekend, and asked about rods. He didn’t and It didn’t seem that they had anything suitable, so I left with two bottles hoping that I could sort out the rest in the following few days.
I remembered a conversation with a man called Arthur Sosbe not long ago. Arthur, though now mostly retired, is a Leicester welder held in very high regard by my father. He is also a vintage motorcycle enthusiast and I believe used to race a velocette that used to lurk under a cloth in his workshop. I myself have known Arthur for many years and he has repaired many a Scott crankcase, as well as frost damaged barrels and many other fragile vintage parts.
Arthur had said that he’d naturally use silver solder, but when I quoted him a 0.010″ gap, he said that it was too wide and he suggested something else. I remembered this conversation and also that many lugged cycle frames used silver solder and I did a bit of research. A company called ‘Cupalloys’ came up as being suppliers of silver solder to model engineers, so I gave them a ring. I’d since confirmed the gap with Alan as being nearer 0.006″, which was within the capillary range for a 38% silver solder alloy which also apparently had the benefit of melting over a reasonably wide heat range, which gave scope to also create fillets.
I bought two packs of 5 rods, and at over £4 per rod, I hoped this was going to work.
So Friday came and later than I’d hoped, I packed the Moss crankcase destined for the Silk Scott into the van and I headed up country to Leicester to see Roger and to get the portapak and hoses/ regulators I needed to take up to Alan. I didn’t arrive until after 9pm but we gathered all the bits and pieces together, as well as a trophy I’d been awarded by the British Historic Racing club, the ‘Aotearoa Trophy’, gifted once to the club by a New Zealander. I believe it’s supposed to be the best performance of a 1930 and under bike. I won it last year also, and although it’s not that I actually won a race, it’s a beautiful shield with names going back many years.
We chatted, a lot, and eventually I got to bed around 1.30am.
So up and off to Alans, the offside front wheelbearing of my van starting to whine somewhat irritatingly.
I arrived at Alan’s somewhere after 10.30am and had thoughts that I might need to find a B & B for the night, as I didn’t know how fast the job would go with untested equipment and solder. I needn’t have worried because apart from Alan having to adapt some spacers to fit the crankcase, the whole process went very quickly. we did take a few minutes to try to establish that the frame was straight prior to fixing it, and to that end I took a sight from the rear engine mounting to a straight edge laid over the bottom face of the head stock. This looked spot on, although pretty much every other tube in the middle looked like it was in a slightly different position. I realised that it was simply built this way from new and that as long as the main datum points were correct, the bits in had some ‘tolerance’. That of course is one of the joys of a low production handbuilt machine. Every one is truly different!
So onto the hot work and (making sure the detachable lugs all faced in the correct directions) using a propane torch and the oxy-acetylene, Alan soldered the new cradle assembly into the frame within a few short hours.
I was very grateful for the sandwiches made by Margaret for lunch and delighted with the way the frame came together.
with a thanks and farewell to Alan, after he’d showed me some detailed bits he’d made for Scott TT replica forks, I shoveled my frame and the bits and pieces back into the van and headed back to Devon. A quick stop in at my dad’s to show him the results, but all in all a great result for the weekend.. and I was home on Saturday night.
I emailed Colin a couple of days ago to ask him whether he knew of anyone who had cut the bridges out of the transfer successfully and he sent me this. Thanks Colin.
“Just a little more about the pictures. These are the barrels used by Martin Heath in that one glorious season when he won 12 events, including his first win. This was at Cadwell when he was left on the grid as the compression was so high the rear wheel just slid instead of turning the engine over, on the old downhill start by the timing/ commentary box. He managed to start it eventually by vaulting on from the greatest possible height and it fired up. The field were disappearing into Charlies by now, but he caught and passed them all to win. Proper Boys Own Paper stuff.
However, the point is that the barrel, as we had no other, was reclaimed by boring out the broken skirt from a damaged set and pressing in a flanged liner from the top. ( It sat in a machined recess). The machining had broken through into the jacket so good old Loctite sealed it all up and provided what I think is called a ‘wet liner’. We could not bring ourselves to put dividing bars back in the ports so made them elliptical as you see and relieved them to give the rings a softer time. You will also see that we needed to use a ‘detachable’ steel ring for the lower seal – and this located on a small step machined on the liner o/d.
Another simultaneous experiment was to make the liner full length down to the very bottom of the crankwell to see whether preventing any piston ‘rock’ would help. You can see the remains of this in the pics after it was later cut off. The rods had to be scalloped to clear the base of the liner, but we still use them to this day with no trouble.
The heads are a type you are familiar with and the drilled/tapped holes onder the dome on the exhaust side were so that we could use a head steady onto a frame cross tube. ( This was our super lightweight T45 frame, it was more of a frame steady than an engine steady. It was so light it sang like a tuning fork even after the engine was cut).”
I also sent him the pictures of my piston for his interest and to get his thoughts.
“As for the fuel pattern on the piston crown, by using a close profile high compression arrangement I expected that any ‘clean’ area on the side was trapped fuel ‘end gasses’ that got rudely pushed/ sucked out of the way before they had a chance to detonate.”
and of the photo of Martin:
I think it was taken at Mallory but expect Martin will correct me if necessary. Incidentally the silencer shown was our first effort after introduction of silencing regulations. This one is designed on the principle of ‘silencing by controlled leakage’ whereby multiple small outlets are provided under the crankcase all carefully pointed in different directions. From memory this set up has five intentional outlets including the official one. The theory is that one noise meter will have difficulty covering all directions at once. It worked well enough, for we would have been excluded on open pipes, and to my surprise the machine seemed, subjectively, to go just a little better than before.
The way I’d finished 2014 was with a rough set up of twin carbs on methanol. I’d modified the needles and the jets, but it was all way too rich (post meeting dyno results here) and a rush job really. The truth is, sometimes you have to push through to get something done, knowing that it might need work later. I decided that I would move away from methanol anyway after that last meeting and so it was a case of starting again.
I measured the port sizes last year and the inlet on each cylinder of the Super Squirrel is about 10.25cm². It’s actually less than standard, as we block the rear most inlet apertures up to allow us to put windows in our pistons to aid transfer flow. A Scott standard inlet gallery (cast into the skirt of the barrel) has a lot of small bridged ports which work fine in a standard road machine. In a racing bike, they are not able to flow enough gas quickly enough once the work has been done to enlarge the inlet tract in the crankcase which feeds them. This inlet tract is very restrictive in size and any serious attempts at performance usually involve some substantial work to open this up. The ‘floor’ of the tract rises up as it follows the radius of the central flywheel which is housed immediately beneath it. What we used to do was to reduce the diameter of the flywheel and weld up the bottom of the tract to enable it to be opened out. Of course such a measure means complete re-qualification of the bearing cups and barrel locations as the whole lot will be pulled in to the middle. Not a job for the faint hearted! You can of course improve performance by working on the tract without doing the floor, but it just depends how much performance you want to extract. My crankcase is one of Rogers improved castings with a better floor profile, thicker sections and far better material.
So the inlet was around 10.25cm², but I was using a single overbored Amal type 89, which had been cobbled onto a Scott flange many years ago. It was about as big as you could get stuck behind the single downtube and was/is a really good carb, giving great pickup out of corners with no hesitancy, possibly down to the good gas speed due to its small size. However I think it’s undersized really as its aperture is only 8.04cm². That’s over 20% smaller than one of the cylinder aperture areas.
Since the Super Squirrel frame will not fit a larger single carb due to the frame tube being in the way, a split manifold seemed to be a good idea. Eddie Shermer made the one I have.
I figured that I would try and get close to the inlet gallery area with the carbs, and so settled on the idea of 2 x 1″ Amal 276 (or 76) with remote float(s) as this pairing give an area of 10.13cm². Although I don’t know what the relative flow characteristics would be between one carb or two of half the area, I guess that the flow rate might be less with two due to a greater proportion of drag from the inside of the greater surface area of carb venturi wall. However, the atomisation could be better. We’ll see. I actually had a 7/8″ Amal 276 and and 1″ 76 and was toying with the idea of getting the 276 bored out. I decide instead to keep an eye on ebay to see if something came up and thanks to someone on the Scott Owners Club forum who spotted one, I bought a singularly unhygienic 1″ Type 76 a couple of weeks ago. It had exactly the same smell as I remembered when my dad’s shed was visited by rats many years ago. I remembered they were so big that the cats used to sleep on high surfaces to avoid having to meet them. It required a lot of scrubbing to get to a point where I could inspect it…
So just the body and the jet block, well corroded in. An evening of gentle heat, wd40 and physical persuasion wrought success and now I am on the way to getting the carb setup back together.
While I was doing this, I remembered that I’d been meaning to remove the head to see if I could detect any patterns which might give some information as to how the bike was running. Initially this came out of a conversation with Ignition and bantam tuning expert, Rex Caunt at Cadwell Park last year and even though I was changing fuels I thought I’d have a look.
I was very interesting…
It could be that the clear sides of the crown indicate that I’ve achieved a functional ‘squish’ clearance preventing detonation (excellent article here). I’d really like to think that but I don’t (next day edit: I’m coming round, it might be a bit of this). It looks more like a tell tale that you wouldn’t normally see, but that my significantly over rich mixture is allowing me to see; the loss of unburnt mixture from the transfers straight owner the top corners of the piston into the exhaust. I might be wrong, but that’s my feeling.
It’s easy to see why it would happen, the transfer port is a bridged rectangular port which operates onto a deflector which is not rectangular, but is rounded at the top corners. The crankcase has stuffers cast in, which is a legacy really from the times where we weren’t running extractor exhausts. With stuffers and an extractor exhaust maybe it’s all a bit high speed into the cylinder and too much is being lost. Although I would expect to have some fresh gas returned to the cylinder prior to the closure of the exhaust port, this initial transfer gas would be short-cutting the scavenge cycle and so is not only unlikely to be returned but also not actually scavenging the cylinder of the remaining exhaust gases. What can I do if it is this? Well, Maybe some better shaping of the transfer bridge would reduce the tendency of the gas to bifurcate and disappear up and over the deflector. Maybe some mild shaping of the deflector could help. I had thought about removing the bridge but it does look like it’s too wide for the ring to cope.
I think it might be a case of planning a better cylinder and possibly looking at this for the Moss/Silk Scott racer too.
It’s all quite late now … I did spend about two hours scraping flat the exceedingly distorted flange on the new carb I bought but it’s still not done. Still, It’s all moving in the right direction.
The virtual oily corkboard of a vintage motorcycle racing family