(An 0-6-2 stretched Simplex)

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When this came up on Ebay I couldn't resist it as it was such a bargain! I got the whole lot for less than the cost of the cylinder castings. Simplex has a good reputation as a powerful and easy to drive passenger hauler and as the name suggests, is a quick and simple loco to build.



I've decided it's time to make a start on Simplex so that I have a bigger loco to take to the running days at Whitwich. Simplex is going to be a joint effort between my brother and I so hopefully it won't take too long. I'll be doing most of the machining work and Mick will be the platework and assembly man!

I had a look yesterday and made a start by removing the horns from the frames again as they need a bit of machining which had not been done. Fortunately they had not been rivetted properly yet and only had a couple of rivets in each one. These were just drilled out. The trailing horns require the front top corner chamfering off to clear the ash pan and the middle horns require the rear sides machining down to clear the frame stretcher which butts right up to them.


I've spent the last couple of days trying to make sense of Jim Ewins work on loco design and have been going through as many loco designs as I can to see how they fit in with his theories. The upshot of it all is that I'm not happy with the boiler design for Simplex and I'm going to make a few changes to fit his design criteria better. Hopefully this will make for a better steaming loco. It certainly won't do any harm in my opinion.

Basically, according to Jim Ewins, the grate area is too small for serious passenger hauling for long periods. It would appear that others have the same opinion and reckon on a couple of hours at the most before the loco needs attention such as cleaning the grate. Reducing the cylinder bore to 1.25" would suit the boiler much better but then you would reduce the tractive effort. In any case the cylinder castings have already been bored to 1.5" dia. and I don't fancy fitting sleeves.

What I intend to do is lengthen the grate by 1" which will increase the grate area from 16.2 sq. inches to 19 sq. inches which is about the maximum increase without extending the firebox too far into the cab. I will also be altering the tube arrangement to cater for the increase in grate area. I reckon on using fourteen ½" by 18swg firetubes and two ¾" by 18swg superheater tubes which will fit in on the tubeplates.

By moving the tall vertical frame stretcher from behind the driving axle the throatplate and firebox front tubeplate can be moved ½" further forwards and the backplate and firebox door plate can be easily extended another ½" into the cab.

The full height frame stretcher will be replaced with a shorter one over the top of the driving axle and another shorter one in front of the axle. In fact I'm thinking of replacing the rather complex double acting feed pump with two single acting ones side by side as on Helen and using the pump body as a stretcher instead. The loss of room in the cab will be made up for by moving the cab back ½" and increasing the overhang of the bunker.

There will be a few others mods along the way to incorporate a few 'improvements' (well, in my opinion anyway!) such as the axles will be silver steel running in needle roller bearings instead of plain bearings. The loco will be fitted with two injectors (so hopefully the axle pump won't get much use) feeding into the two clacks on the backhead and the clacks for the handpump and axlepump will be moved to the front of the boiler, either on the side of the barrel as normal or I might use the Super Simplex top feeds. No doubt other ideas will appear as work progresses.


I've had a change of mind (again!) about the boiler and following an email from a fellow model engineer who ran a Simplex some years ago with no problems at all from the boiler (Thanks Dr John!) I've decided to make life easy and stick with the standard grate size. I've also read a few more articles mentioning Simplexes as reliable passenger haulers so probably the extra work involved in my original plans to extend the grate would not be worth it. At least now I can still use all the boiler kit and not have to buy a load more 1/8" copper sheet and new fire tubes! I still intend to replace the single superheater flue with two 3/4" ones and fit in another two firetubes which will increase the tube area a bit.

What I'll have to do instead is design a new loco based on Simplex with Jim Ewins grate area etc. and compare the two! (well, maybe!).


Yet another change of mind! Some time ago in Model Engineer, someone described an 0-6-2T version of Simplex which gives a larger rear bunker and also increases the size of the cab and allows better access for driving. The modification is pretty simple and just means extending the rear of the frames and fitting a rear pony truck. I've decided to follow the same lines but also increase the length of the firebox as originally planned. The longer frames will make this a lot easier. I will also be altering the design of the boiler and making the barrel separate from the firebox outer wrapper. This means that I can make the wrapper from one sheet of copper and not have to extend it by adding strips at the bottom which is necessary if the wrapper is formed from the boiler barrel by splitting it and opening it out. I've never liked this idea as the internal reinforcing strips on the joints are bound to interfere with water circulation around the firebox. The strips can be dispensed with if you make a proper notched coppersmiths joint but they are very fiddly!


I've spent the last day or so playing around with drawings for the 'stretched' Simplex and 'improved' boiler. I've more or less come up with an outline which I'm happy with. I've obviously lengthened the frames to take the trailing wheels but also lengthened the cab and lowered the roof which I think improves the appearance no end but that's just my opinion! I got a bit of inspiration fron the LNER N4 tanks which are very similar looking but with inside cylinders. No doubt things will be 'tweaked' as I go along.

GA for the new 0-6-2T version

The boiler will have a much longer firebox - 7-3/4" as opposed to the original 5-5/8" - which increases the grate area from 16 sq. ins to 22sq. ins. This gives an Engine Factor of 0.157 which fits Jim's figures nicely. Unfortunately, increasing the grate area also requires a corresponding increase in tube area and that has been a bit of a struggle with the existing tube plates (which I want to try and use to save time and money). I've managed it by utilising four 5/8" diameter superheater flues and increasing the number of 7/16" tubes from 13 to 16. The smaller superheater flues will mean having to use smaller diameter superheater elements but I think I can still get sufficient cross sectional area in the bores to pass adequate steam to the cylinders. Most of the time the regulator only needs to be just cracked open so the amount of steam passed is not a lot. Anyway, I'll cross that bridge when I get there!

I've also shortened the tubes from 11-1/8" to only 10" so that they fit the Keiller ratio of length versus diameter.

Overall, the boiler now matches Jim's figures very well and it will be interesting to see how the final result will perform compared to the standard Simplex. There are a couple in the club so we can have an efficiency competition amongst ourselves!

For interest the figures are: Ee = 0.157, Eb = 81.4, Eo = 12.8, and Kt = 75

The trailing wheels have caused a bit of a problem in that if I were to use a normal pivotted pony truck, the pivot point would have to be under the firebox of the new boiler! I've decided instead to fit radial axle boxes to the trailing axle to get over this problem. A little more tricky to make perhaps but probably less work than making a pony truck. To get a bit more clearance between the frames and the trailing wheels, the rear sub frame will be attached inside the main frames with a 1/8" spacer between, reducing the width over the sub frames from 4-7/16" to 3-15/16". I'll probably have to make a new rear buffer beam as the rivet holes for attaching the fixing angles have already been drilled and they will now be in the wrong place. No big deal though. I'm also going to have to make new frame stretchers as whoever machined up the originals made them slightly too narrow! They are a bit rough looking anyway.


I've made a start on the frames at last. I bolted them together to do a bit of cleaning up on the edges and cut off the rear 3 inches, ready to accept the new rear frame extensions.

Cut down frames

I've had to re-arrange the rear frame stretchers to accept the new boiler so this will mean filling in some of the original fixing holes and drilling some new ones, including those for attaching the rear sub frame. As it turns out, the original frame stretchers are actually the right length after all (must have had a senior moment when I last measured them!) so they can be used. The front horizontal stretcher remains as per the original Simplex and the others can be reused by cutting them down. I've decided to go for the twin axle pumps rather than the more complex single double acting pump.

The horns have been already been machined to fit the slots in the frames and are a good fit. The only snag is that some of the fixing holes have been drilled for 1/8" rivets rather than the specified 3/32" and I don't have any! These are on order and should arrive shortly. Similarly, the buffer beams have been drilled 1/8" for the rivets to fix the angles on rather than 3/32". As mentioned before, I will have to make a new rear beam anyway to take the narrower sub frame.

I've had a quick look at the driving wheels and there is a slight problem with these in that on some of them, too much has been machined off the front of the tread bringing the spokes very close to the edge of the tread. Not drastic but annoying all the same. I've seen this problem on other machined wheels. It's caused by not checking the thickness of the casting before machining and determining how much needs to be taken off the back of the wheel before machining the front. If you don't take enough off the back, you finish up taking too much off the front when bringing the tread to the correct thickness.

I've still got to source a pair of castings for the rear pony wheels but there seems to be a few designs that use similar sized wheels so there shouldn't be a problem.

I've been pondering over the design for the rear pony axle and decided on a slightly more complicated design than originally intended ( don't I always!). As mentioned before, I've gone for a radial axle rather than a pony truck as the pivot point for a pony truck would have to be under the firebox to get the required radius. The simplest design for a radial axle has the axle boxes sliding in curved horns to give the required sideways movement and also slide vertically to give the springing for the axle. To ensure that the axle boxes don't jam in the horns when moving sideways and vertically, they have to be a very sloppy fit in the horns. Not really a problem but not an ideal situation.

To get around this, I've designed the axle boxes in two parts. An outer part will slide sideways in a curved horn the full width of the frames (acting as a frame stretcher as well) and an inner part carrying the axle will move vertically in the outer part and carry the springing for the axle. The vertical and sideways movement will therefore be entirely separate and the relative fits between the parts can be made much closer. More on this when I get to it!


I seem to be struggling to get into this build but have managed a bit over the last couple of days and the enthusiasm seems to be returning! I have been working on the drawings on and off though and spending plenty of 'thinking time' on various aspects of the loco. I saw a dome casting for sale on Ebay that looked very much like a Simplex one and took the chance and bought it very cheaply! It is indeed the correct one so that's another big saving money wise! I still need to go shopping for materials for the rear sub frame but there's quite a bit to do on the main frames in the meantime.

The horns are now rivetted in place and filed off flush with the outside of the frames. The rivetting was a bit awkward as the holes had been drilled too close to the inside web of the horns and I had to file one side of the rivet heads off to get them in! This also meant that I could not use a proper rivetting dolly to support the heads and had to use a flat ended piece of steel bar instead. The rivets are now flat headed rather than domed but you don't see them anyway so no problem. Not all the rivet holes had been drilled fortunately so the remainder were drilled the correct size (3/32") and in the right place!

Frames with horns rivetted in

Next job was to trim the ends of the horns flush with the bottom of the frames and this was done in the Micro mill by clamping the frames vertically against an angle plate bolted to the table. The mill was plenty man enough for this job using a sharp carbide cutter.

Trimming the ends of the horns

The next operation on the horns is machining the slots for the axle boxes but I don't think I'm going to be able to manage it with the equipment I've got. The Club at Whitwick has a vertical milling machine in the workshop so I may be nipping down there to machine them.

I've been thinking about the cylinders and the valves during the last few days. If I had not already got the castings, I would have gone for piston valves and altered the valve gear to suit but will be sticking with the slide valves. What I want to try and do though is to make the valves balanced to decrease the loading on the valve gear and reduce wear.

*The area of the valve face is 1.64 square inches which means that at a steam chest pressure of 100psi, the force on the valve is 164 pounds! The pressure in the steam chest probably won't ever get that high but there will still be a hell of a load on the valve. In a balanced valve, this load is reduced to around 40% of that of an unbalanced valve and the force needed to move the valve is considerably less. This is achieved by shielding part of the top surface of the valve from the steam pressure using a seal of sorts which bears on the valve chest cover. More of this later.*

** Further to the above, I did quite a bit of 'research' on balanced slide valves and it would appear that the area of the valve to be considered is actually the area of the exhaust cavity and not the area of the complete valve. There is only a pressure difference between the exhaust cavity, which is at atmospheric pressure, and the steam chest. There is no pressure difference between the steam chest and the outer surface of the valve surrounding the exhaust cavity which seals onto the portface. Therefore, for Simplex, the area of the exhaust cavity is 0.75 square inches (0.75 x 1) so with a steam chest pressure of 100psi the force pressing the valve onto the portface is 75psi.

It seems usual to 'balance' anything from 80 to 95% of this force by using a seal or balance piston on the back of the valve so the force acting on the valve is reduced to say only 10psi or less, just enough to make sure the valve still seals. The force required to move the valve is therefore considerably reduced.


Construction work has ground to a halt on this project due to rebuilding the 2½" Kingette but I've been working on the drawings on and off and bought some materials for the frame extensions and other bits.

I've abandoned the idea of trying to use the boiler barrel and flanged plates from the original boiler kit as it's too much of a squeeze to get the tubes in. Instead I've designed a new boiler based on a 5" diameter barrel and the tubeplates for the Super Simplex. I can now fit in three 3/4" diameter superheater flues which will make life easier. This will mean making the boiler from scratch but that's no problem. A chap at the club has recently picked up a part built Simplex with no boiler so I may be able to sell him my boiler kit to pay for the new materials!

Boiler design

I've redesigned the axle pump to use 'poppet' valves with O ring seals instead of plain balls. I saw this idea on 'The Home Machinist' forum and it looks a good idea. I've designed it so that the valve seats are just a flat surface on the pump body and no machining of seats is required unlike the ball valves. The whole pump assembly should be much easier to make than the rather complicated design by Martin Evans.


Not much to report again. I'm still sorting the drawings out bit by bit but have finalised the boiler design which has met the approval of our club boiler inspector (so long as the locos painted green!). Bob from the club is having the old boiler kit for his Simplex so I may be ordering the materials for my new boiler soon! I'm going to hack the former plates from 3/8" steel so I can make other sets of plates if the 'design' proves to be successful. The formers will be suitable for the standard Super Simplex boiler anyway if anyone requires a set of flanged plates.

I've had a look around for castings for the trailing wheels and decided to use Black Five bogie wheels which will finish at 3½" diameter on the treads and should look ok. The castings are available individually from Blackgates at a reasonable price so that's that problem solved hopefully.



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