Britannia 70013 in 5” gauge – rebuild story 2018 - 2020


by Norm Lorton








Go to Latest Update





This series of write-ups will describe the rebuilding of a Modelworks 5” gauge Britannia. It will hopefully be of interest to some, and of use to anyone undertaking a similar task. The model will retain its identity as 70013 Oliver Cromwell, partly because in full size it is one of only two in existence, and also because it is nearby on the Great Central Railway.


I like many of Doug Hewson's thoughts on subjects. He wrote a good series of 18 articles published in Model Engineer between August 2012 and July 2016 that featured the rebuilding of two or three  early Winson kit models.  Doug says that “the introduction of machined locomotive kits was the best thing that happened to model engineering” and I agree with him. I also have been motivated to turn one of these kit engines into something to be pleased with and I am grateful that the club sale of this engine has given me the opportunity to do that.


I purchased the locomotive in November 2018 with the idea that I would sort out its problems, tidy it up a bit and get it running by following an intense three months winter work. The idea was to have this big engine to enjoy in 2019. But I soon discovered there is twice the amount to fix that you first thought, then I get picky and perfectionist and want to make it a better job, and now everything goes much slower and there are other things to do.  So, will I finish it in November 2020?


This engine was one of the last of the Modelworks kits with a 2006 dated boiler.  It had steamed a few times and was looked after by several club members, but had never achieved its potential. Back in my workshop I stripped it down the last nut and bolt.









The frames were stripped to bare metal and I was happy with them. They were strong and well braced.  The large top stretcher was made from 6mm plate rather than 1/16” sheet. The Modelworks design came straight from the drawings of the failed Winson company, but Modelworks did change several features. The motion and weighshaft brackets were improved and brass castings replaced aluminium. The various dimensions are more than 99% identical to the Perrier design.


One problem with the frames was obvious in that the weighshaft bracket holes were slots! Longer on one side than the other. It seems that Winson made an error in the drilling of one of the brackets. Unfortunately, this error was copied through to the Modelworks kits.


Clearly I had to look into this. I had previously been interested in the concept of valve timing and Walschaerts motion.  I read Don Ashton's book and others about valve timing. I measured the frames by mounting them on my Bridgeport using the DROs and a laser pointer to obtain all the critical dimensions for the left and right hand sides. I put these numbers into a CAD drawing and also into Don's spreadsheet.  I moved the weighshaft brackets forward by 0.050” and now had reasonable figures from Don's spreadsheet.


With the weighshaft brackets clamped in the calculated positions I drilled each one in three places, reamed the holes and inserted dowel pins.


All of the above took up the remainder of 2018. Today the frames are fully fitted with motion and have been air tested. I show them in this next photo for interest.









Part 2: Pistons, Valves and Motion.


It is reported by others that the Winson and Modelworks valves and piston rings do not seal. I expected to have to replace these but what I found was a bit worse. The cylinder bores were corroded and it would need a rebore. When I tried to pull the valve bobbins out of the valve liners the first discovery was that they were very corroded. It was a mini-brute-force job to remove them. The main cylinder pistons came out more simply but a lot of corrosion was seen in the cast iron cylinder bores. I estimated the main bores had perhaps five-thou or more of corrosion pitting, mainly at the bottom of the bore where water would have lain.









The pistons and valve bobbins appeared to be cast iron and the rings look just like brass in colour, although they might be a type of bronze. They had a little bit of spring in them but their non-sealing abilities were not surprising. The liners, each side in two halves, were removed following a baking at 240 degC to break the sealant. The liners had to be twisted and pulled out as they have shoulders that butt against the block.


Clearly a rebore of the cylinders was needed first. The cylinder blocks were mounted on my Harrison lathe using a big 15” face plate and the block clamped to a small 90 deg angle plate. I took some time to align it all. I had up to 0.020” on the bore to remove so I tried a 0.002” cut to see where it was touching. By the time I had the bore to a nominal 1.740” all of it was being cut and just the patches of corrosion remained, most deep at the bottom. I used pre-made plug gauges to take the bore to 1.748” by which point all corrosion marks had gone.


The main pistons were simple to turn from GR17 cast iron. They were mounted on their piston rods, gripped in a collet, and finally turned to size so that they would be concentric with the rods. Two piston rings on each piston are made from carbon filled PTFE (25% carbon). These sit on Viton 75 o-rings. The rings were stagger-cut on a mandrel following the scheme first drawn by R. Etter of South Africa and published in EIM. I have used these successfully on another engine and I know that Bernie swears by them. To me, the design seems sound by employing the flexibility, sealing and wear properties of PTFE.


The Motion:


I looked at the coupling rods and other motion parts and decided to redo it all. The finish was dreadful with laser cut edges everywhere and misshapen parts. Big metric nuts were on the crank pins and 5mm clevis pins affixed with split pins held all the motion parts together.


All the motion parts were reshaped and refaced by hand file and then 180 and 320 grit paper until they looked like they should. I spent two weeks on all the reshaping and then more time making oil cups, new castellated nuts and neat motion pins as described by Doug Hewson. The photo shows the old clevis pin and the new item, with the tool used to tighten the front pinned head, while a 6BA spanner holds a flat on the rear nut.









The finished motion was now looking smarter.









New return cranks were made and these replicate the early Britannia two bolt design, although this would have been onto a square crank end.









Piston Valves:


The piston valve liners are made from bronze. I knew from others that their fit in the blocks was a problem and stories of liner to block leakage and constant exhaust blow were reported.  I tried fitting them back into the blocks with plenty of Loctite but that did not seal when later tested. I had to take the liners out again, cut two small grooves on each liner half for 1.5mm o-rings, and ease them back in.


I designed new stainless bobbins with screw caps to hold a pair of cast iron piston rings at each end of the bobbin.  It was the first time I had made CI rings and all seemed to go well. I used the simple heat treatment technique of holding the rings open on a blade while keeping at red heat. But these rings did not seal because the sides of the ring nearest to the ring gap were not touching the bore of the cylinder. Perhaps a one or two thou clearance which allowed a big blow past.  I suspect the heat treatment part of the process created the deformation.









The photo shows a valve bobbin with the four cast iron rings and the sleeve that separated them below. Fortunately, I was able to design a simple replacement cast iron sleeve which the end nut tightens to, and in place use thin virgin PTFE tyres. These are shown fitted to the bobbin above.


A similar design has been used by Howard at NSME but I have lost my reference to that and cannot recall the sizes of the tyres.  My design uses a thin tyre working out at 0.064” wall thickness and underneath is a gap of 0.0035”. This under gap hopefully means that thermal expansion is accommodated. Virgin PTFE expands a lot when warm and these tyres will grow by around 0.007” in diameter. The tyre width is 0.001” more than the fitting gap to minimise steam blow by underneath.


It would have been better to put a very thin o-ring under the tyre with sealing benefits against blow-by under and by keeping the tyre out against the cylinder. It will be interesting to see if my rings still seal cold on an air test after they have been exposed to steam a few times. If not I can pull the bobbins out from the front and add those o-rings.


The Air Test:


After reassembly the air test was pleasingly free from exhaust blow by. The engine ran nicely at well under 10 psi, in full forward and reverse, and with the die block taken back to near mid gear and the pressure upped to15 psi. The exhaust beats became a nice even 1-2-3-4 rather than EeeehPhufff-3-4 once I trimmed the valve pushrod lengths by 0.020” and then 0.010” adjustments, a job made easier by the hexagons I had machined onto the ends of the valve rods.


The air test has been posted on YouTube and can be seen from this link or just search on the words “brit 70013 5 inch air test”


Part 3: Springs, Front Bogie and Pony Truck


The Modelworks main springs were made with mostly tufnol leaves and looked to be either flat or slightly concave. If I pushed down on the frames there was barely any movement; the springs were 'dead' and some of the horns were binding. Removing the bogie and pony truck and testing those gave the same, dead and un-springy effect.


First I had to ease three of the six horns with careful filing, scraping, and emery to get all three axles rising up and down freely, and then they needed rechecking when the coupling rods were attached. I would guess that previously the horns had not been finish machined in the frames after being fitted.


My design principles were to say that if the all-up locomotive weight was 99kg then you might have 23kg on each of the three driving axles, 15kg on the front bogie and 15kg on the pony truck. These are a distribution of weights that will be sufficient on the bogie and truck to keep them on the track during loading bay ramp movements and when crossing ground level points.


I had built a small jig that would measure a spring's compliance by deflecting it a known distance and measuring the load applied; a bit like a vice pressing on some electronic weighing scales. The Modelworks tufnol springs were measured to have compliance of 6.3kg/mm, a bit stiff because they were collapsed and simply not bending properly.


I made a set of six new springs from spring steel, each blade hardened and tempered, and held those in a buckle, mounted on new pillars, and with threaded hangers that would allow the spring to flex. When tested each oiled spring was measured to be 3.3kg/mm, near enough to my target to be fine.









This meant that the new bogie and truck springs needed a spring rate of  less than 3.3kg/mm per side. The Modelworks bogie springs are a dummy leaf and a pair of small coil springs hidden each side. The whole bogie when tested was far too stiff. I rebuilt it moving the dummy leaf to give more room and fitting new, thinner gauge and longer coil springs to give a total bogie compliance of 4.0kg/mm. This equates to 2.0kg/mm per side and this figure sits nicely lower than the main springs.









The pony truck springs are inverted on the prototype and I built two new springs to replicate this. I knew they would need tufnol inclusions to reduce the spring rate and by testing settled on two top steel leaves and seven tufnol. Most of the spring is hidden anyway, and you can only see the top edges in the photo. The truck spring rate is 4.0kg/mm total, the same as the bogie so exactly where it was wanted.









The bogie central swivel block was modified to carry an extension plate and hole. This hole locates in a pin inserted in the frame cross member support. The pin now limits the swivel of the bogie and will stop it wiping out the drain cocks when the inevitable derailments occur – obviously caused by defective track and not any locomotive spring problem! The photo shows how close those poor little drain cocks are.









The loco was inverted in its bench frame and the main spring hangers all adjusted to get each wheel tread level, using a long steel rule. The pony truck had a new swivel pin made, and the bogie height adjusted with washers, so that when loaded each axle would be carrying the intended load.


24/02/20 To be continued....