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. model of current Gibsons pipe
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 set this site up mainly to provide a platform to share some of my experiences and endeavours in developing the engine of my racing 1932 Scott Flying Squirrel.
The simple reason that I’ve not been updating is that I’ve not been working on my bike much or racing it either.
I did have a rather memorable meeting at Cadwell park in June, but not for the right reasons.
I have not given up on petrol or AVGAS (as I have have most recently been using) but my experiences with seizures in 2015 made me realise that I had more development work to do before I could confidently go to a racetrack again using that fuel. I have my suspicions that the diameter of either the ‘stinger’ or the muffler outlet pipe on the exhaust is inadequately sized and that excess pressure build up is creating heat at sustained higher revs. I’m not convinced that the exhaust is actually the optimum design anyway (Supposed to be a direct copy of Roger’s, but Gibsons didn’t make it with quite the same dimensions), but it certainly goes well with it on methanol and I simply didn’t have the time (or the TIG) to make test modifications at that (or this) point. However, as I’m sure I reflected on at the time, the problem may be something else entirely or indeed a combination of factors. Certainly, the seizures were not something I managed to replicate on a dyno during short output-measuring runs, although I since discovered that the dyno can indeed be set up to provide a constant load condition to replicate the conditions experienced when flat out for a period of time. It’s not really about measuring power but simply observing what is happening. To make the most of a morning on the dyno doing this, I would need to get some datalogging going to be able to really study the effect. I have thermocouple ports in the headers but I could also do with the temperature in the water jacket being monitored as well. I can switch exhausts to a standard straight-through pipe to make comparisons.The radiator is only small on my bike and may be a significant factor as well. It would likely be a couple of hundred quid for a morning on the dyno so I would have to make it count.
So, with the petrol incarnation of my engine needing time to work through the options, I thought I would return to good old methanol to give me some limited riding in 2016.
I was still hoping to try to squeeze more power out of my engine, and had the combustion chambers to my cylinder head welded up to enable a new profile to be tried. I didn’t have much time for the work on this and Roger kindly did some machining on it for me. He checked the head for flatness, which he thought was fine and I built it up.
The premise is simple. The Scott engine is limited to fairly low revs by several design constraints; you simply have to make the most out of the available range, which is effectively up to ~5000rpm (with peak torque somewhere between 3000 and 4000 rpm). I had done some development work with the cylinder head previously and seen on the dyno the effects of lowering the compression whilst improving the combustion chamber shape; I lost low rev torque whilst the engine revved to higher RPMs. If the engine was capable of breathing at higher revs, then an increase in power may have been the effect but as (according to my calculations and observations) the ports restrict the gas flow required for anything above about 3750rpm, then there’s just no point holding on.
When you also take into account the three speed box the task as I see it is to maximise low rev torque and discount the pursuit of revs entirely.
Now I would like to say at this point that a good strong engine can be made to run very reliably on petrol (of course!) and I raced it without mechanical problems for years like this (with a far less radical pipe, prior to fitting the expansion chamber I received as a wedding present from Roger!) but it is at this point where you are trying to experiment where you have to accept that you might hit trouble. I have built many engines for other people and I wouldn’t build an engine for anyone else with anything but tried and tested clearances and configurations, as we know that these work and work well but I have always wanted to keep experimenting with my own Scott engines and in doing so you commit to the highs and lows associated with that development process.
The Cadwell park meeting in June was a big event. An anniversary meeting for the British Historic Racing Club and also the Scott Owners Club, which had a stand in attendance.
The Friday had given people the opportunity to ride around the circuit on their road bikes, with a full weekends racing ahead.
I had returned to methanol for the meeting, thinking that was the safest option as I had never had problems running on methanol before. I was also quite excited at the prospect of the improvement I may find with the new cylinder head design. That excitement was short lived however as the head gasket was obviously not sealing and the water was being blown out the radiator expansion pipe.
I will not make a long story out of this weekend as it simply consisted of various attempts to seal the head (with associated peaks of hope and troughs of disappointment) aided by several kind and enthusiastic helpers. Kev Bayliss and Alan Noakes deserve a special mention.. many thanks.
Unfortunately, It was not to be. In that moment, having hired a caravan and being accompanied by my wonderful wife and two small children and being surrounded by so many Scott owners who, I’m sure, would have loved to have seen Roger and I trying our best in the vintage races, I was left with the feeling that I simply had to re-assess my approach to racing given the available time and finances available.
I never wanted to simply ride around, and when you feel like you could get more from your engine, it’s hard not to succumb to the urge to tinker. You must have the time to see it through though.
My main plan for 2017 is to finish the rebuild of my lathe and build a workshop to house my other machines; a Thiel 158 universal mill and most recently acquired, my Jones and Shipman 1014 grinder, which is a true universal toolroom machine with surface grinding, cylindrical internal and external as well as tool and cutter grinding capabilities. These are currently in storage and I need to stop playing with bikes for a while and make them a home. This has to be a priority above racing and bike development. I will rebuild the engine but I may do it as a good road engine to run with a normal pipe and finally register it for the road. I’m not certain about it, but I could possibly do some local events, hill climbs, sprints etc and enjoy it in a different way whilst building up the workshop. We’ll see but I quite like the idea. Racing is very expensive and there’s a lot of tooling I can get for the price of a two day meeting.
I have also been rebuilding a couple of engines for other people and it’s obvious that I need to have better facilities in place if I am to continue doing this within any reasonable time-frame as I’ve had to rely too heavily on Roger fitting my work into his already packed work schedule. It’s been a lesson which has contributed to my resolve that this year must be the year of the workshop.
So it may be that in 2017 my posts are less racing oriented. I’m still working on Scott engines anyway so I’ll post about that as well as my machines and any related progress.
We’ll see what happens…
Happy New Year to all who visit and good Scotting!
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. Inside late twenties Scott crankcase.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…
Scott main bearing bearing ‘cup’ in engine.Scott crankcase with bearing’cup’ removedTwo variations of Scott main bearing cup
You can see here the drilling into the outer wall of the crankcase main bearing housing Crankcase main bearing oil feed drilling
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.
Late Scott main bearing cup (big flange)
Here you can see a cross sectional drawing showing the drilling through the rear of the later cup design:
Drilling detail through Scott cup
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.
work in progress. 1925 Scott crankcase cup drawing.
I thought I’d share this. There are lots of casting videos on you tube but I really like the work that ‘Myfordboy’ does, as he shows the entire process through pattern making, core layout, casting, machining and the final object (normally a model engine of some kind) working. Absolutely brilliant.
I’ve seen a few possibilities for purchasing potentially very useful tooling recently but for reasons of economics and priorities I’ve not been able to take them all up. One thing I did do though was to buy a number of collets specific to the Smart and Brown lathes made by Crawford collets. They are numbered 2804 and not easy to find… my lathe was probably made somewhere in the 1950’s.
Anyway, a little advance but since I am spending so much time scraping the slideways (almost 0.010″ off the second top slideway now but a lot of alignment work to go), it’s encouraging to be able to improve the possibilities of it’s usefulness when it’s finally rebuilt.
I went to a 3D printing show with a Scott owning friend a couple of months ago to look at the possibilities of using 3D printed models as part of the casting process for development work with Scott barrels and pistons. It was his idea, not mine but it was fascinating. There are various different possibilities: you can make a 3D CAD drawing and then print a pattern from which you then set up to produce castings in a conventional manner, you could possibly print an exact replica of the component you want out of wax or something similar and then investment cast the item such as they do with jewellery. You can also print the actual sand cores, rather than printing the patterns from which to make them. You can use a 3D scanner, either handheld or one that rotates around the component, which will interpret the image into a 3D image from which a CAD file can be automatically produced. It was very interesting, but it got me thinking about how little I really know about casting.
Roger used to have patterns made regularly, not only for his Scott but also on a far larger scale in the context of the family-run, Leicester-based special purpose machine tool company, ‘Moss Machine Tools’, that he used to be works manager/ managing director for.
That company, details of which you can find if you click the link through to ‘Moss Engineering’ at the top of this page was involved in making complex machines for automotive, MOD and other sectors from 1947 to around 1991 when economic conditions forced it to cease trading. I don’t know whether he’s ever cast anything himself but he has significant understanding through having had to work with foundrys and pattern makers and seeing the issues that have to be overcome within those areas.
I realise that I’ve been looking at all this interesting technology to possibly assist within a casting process but I still haven’t any actual experience of casting of any type. I’ve worked with castings of course, and understand a little of the some of the areas that have to be considered; shrinkage, porosity, premature cooling, incomplete filling of cores but have found myself thinking about it more and more and have gone where any self respecting modern would-be self-improver goes to nowadays… You tube.
I’ve seen several casting videos recently but here’s one that I really liked by a guy called Keith Rucker, because of the thoroughness of the explanation of the process. he’s not setting himself up as an expert but he’s doing it and seeing where it could be better. He’s also using 3D printed patterns to make vintage components! I’m not likely to be attempting to cast any time soon, but it’s all part of learning the language and gaining a better understanding at how a pattern has to be made in order to be able to make up a successful set of core boxes and ultimately a successful casting.
I’ve been trying to sort a problem out with the website which means that I can’t upload photos. Hopefully I’ll get it sorted out soon but in the meantime I’ll just have to do without.
I was brought up with Smart and Brown lathes; my dad has had a Model A for around 50 years and a 1024 VSL for around 20 years. It wasn’t therefore a huge decision what I would look for when I started to put my workshop together a few years ago. The lathe I bought (unseen off ebay) was a Model M Mk2 which was a very nice 4″ swing tool-room lathe from the late 1940’s to the end of the 1950’s. It looks better blurred…
It was in a pretty poor state and I think it was a long time since it was a cossetted tool-room machine. Undoubtedly shifted to some unswept corner of the maintenance shop to turn spacers on. I made several other posts about the assessment and strip-down but the most significant matter was the slide-way wear. There was 0.010″ over the length which is quite a lot!
I had wondered about grinding and looked into the costs, but it didn’t seem to be a perfect solution. True it would be a lot quicker than scraping but I could see the possibility of knock-on problems. I wanted to keep the original position of the head, which provided the correct clearance arrangements with the feed drive gears at the rear. One grinder I spoke to suggested that they had ‘shaved’ gears to get over this problem. It just didn’t seem like the right answer. Whatever I did I was going to have to re-establish the original height of the saddle once the work had been completed to ensure that the half nuts still centered on the lead-screw, the feed shaft within the apron, and the main traverse gears had the correct clearance with the rack.
So, to cut a long story short, I decided to scrape the slide-ways using a hardened rail sitting on part of the original head location face and the end of the tail-stock slide as a guide. With a clock stand moving along that whilst clocking the front way, I was not only able to see the initial wear but it gave me a basis from which I could start the work, so that I could start to scrape the worst of it out and establish a reasonable state of parallelism before moving to the next stage.
Months of intermittent scraping have followed. A bit here, a bit there. A few weeks ago, I had arrived at the point of being within 0.001″ over the slide-way according to the clock and so I went to blueing the rail and using it directly on the slide-way to establish a better flatness. Getting there…Still further hours but at last the front slide is now ‘good enough’ to sit the rail on to start to do the opposite way. There’s a lot of work to come and this is why I haven’t tried to finish the front slide perfectly. When the rear slide is done, then I’ll use a precision level on my datum faces (under the head) to set the bed horizontal and then use some ground V blocks which span the two slide-ways to establish not only the angular alignment to the head face but to ensure, using blue under the v blocks, that the alignment is achieved with faces on the same plane, not just parallel. It’s almost inevitable that after I’ve scraped the rear way, that the two faces will be parallel but not on the same plane. The process to align will mean that at least one of the ways will have to be completely re-scraped again.
Anyway, months of intermittent work ahead but the first stage is complete.
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.
The Vintage Motorcycle Racing Section’s Montlery invasion 1975
Roger at ‘full yowl’ with open pipe, Montlery 1975. One of my favourite photos ever…
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.1″ type 76s without bellmouths… yet!
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).My Flying Squirrel’s Gibsons exhaust header pipesThat 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. Silencer outlet
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 thought I’d better post an August update since it’s September tomorrow!
I realise that I’ve hardly been diligent in keeping up the website over the last couple of months. I’ve been so busy! I shall be better over the Autumn…
In terms of Scott activity, I’ve had more involvement with Scotts in the last few months than I’ve had for several years, since I worked with Roger rebuilding engines. I have recently started to offer my services to people who either want engine work or gearbox work, or simply want some help to try to resolve why their machine doesn’t work as they think it should. I’ve a few engines in process now and it’s a happy thing to be involved again with people who are really enthusiastic about having a good engine.
Anyway, I don’t want to say that I won’t write about work that I’m doing but I don’t want this website to change to a vehicle for my own work. If something interesting comes up, I’ll write a bit about it.
One significant Scott based event this August was the Abbotsholme weekend. I haven’t been since the first ever event, around 7 years ago I believe. Eddie Shermer and his wife Margaret have laboured to create what Roger speculated as being the ‘high water mark’ of Scott rallies. The energy and organisation to create such a successful event cannot be underestimated and I was very glad that I was able to bring my family to such a warm and welcoming event.
We were there from Friday afternoon until Sunday afternoon and stayed in our own little dormitory which was entirely comfortable and gave us the peace of mind that we had somewhere to retreat to with the children and also that any late night crying (not me) was likely to have minimal impact. My youngest is only one year old so is a bit young to require grand schemes to keep her interested. The older one, who is three, thrives on interesting adventure and activity and especially loved dancing to the excellent Ukelele band after the dinner on the Saturday night. Wonderful.
Saturday had been forecast with a downpour of such an intensity not seen since some guy was advised to fill his boat with animals. Amazingly the day was largely rain free and in fact sunny and bright for the most part. This was appreciated as it was the day for the Scott run which took in some beautiful places in Derbyshire, not least Chatsworth house which was the location for our lunch stop.
I didn’t have a bike to ride and so I gladly accepted Rogers invitation to ride ‘ballast’ in his little 500cc JZR three wheeler. Such excellent fun and we were so lucky with the weather… until about ten miles from home when the heavens opened first with a hailstorm with hailstones the size of peas and then just plain cold rain. A lot of it.
Some pictures, pre deluge, at Chatsworth.
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So we got soaked in the end. It was all part of the days entertainment and what a great entertainment it was.
Sunday was the opportunity for people to put their Scott’s out on display outside the School, and there were a few people with stands offering spare parts. Lovely to catch up with lots of people that I hadn’t seen for years and also to introduce them to my family. I think everyone seemed to be enjoying themselves and although it’s a bittersweet feeling as it’s the last one of it’s kind, I was very glad that my family participated.
The virtual oily corkboard of a vintage motorcycle racing family
