William pony truck distance between hole centres vs. frame dimensions

[Arthur Jones 2Participant @arthurjones2]

Hi all,

I’m continuing to make slow progress on my William 3.5″ 2-6-2T (Martin Evans design) but am now at the stage of turning a pile of bits into a set of assembled frames.  One thing is now puzzling me before I move onto the next stages – the between-centrelines distance between the axle hole and the pivot pin on each of the two pony trucks, as there seems to be a discrepancy between the pony truck drawing and the frame drawing.

The pony truck centre distance is shown clearly: 4 3/16″ i.e. 4.1875″ (both pony trucks are meant to be identical).

But looking at the frame drawing, including the drawings of Stretcher C (front pony truck mounting) and Stretcher H (rear), something doesn’t quite seem right. Adding and subtracting the dimensions from the drawing seems to give the following:

Front: 2 7/8″ + 3 3/4″ – 2 3/16″ + 5/32″ – 3/8″

= 2.875″ + 3.75″ – 2.1875″ + 0.15625″ – 0.375″ = 4.21875″ = 4 7/32″

Rear: 5 1/4″ – 1″ + 5/32″ – 5/16″

= 5.25″ – 1″ + 0.15625″ – 0.3125″ = 4.09327″ = 4 3/32″

(Note: the 5/32″ measurements are implied rather than shown explicitly on the drawing but are half the difference between the stretcher width 1 1/2″ and the stretcher mounting hole centres 1 3/16″).

So there is 1/8″ discrepancy between the two centre distances on the frames, 3/32″ difference for one pony truck and 1/32″ for the other.

Am I missing something (or doing something daft) or has anyone else spotted this problem and know whether this is deliberate or an error? I guess this is just “rounding error” arising from Martin Evans laying this out on his drawing board and measuring, rather than calculating? Presumably I simply adjust the dimensions of the pony trucks to match (and mark which is which!) then stop worrying about small discrepancies? I’d have said that 1/32″ is probably too trivial to worry about in this context, but 3/32″ seems an awful lot of “rounding error” on a model of this size (equivalent to 1 1/2″ in full size!) and is making me ask questions so the wheel flange doesn’t foul the rear “arch” in the frame on curves. I have not yet machined the pony truck castings or wheels but they are among my next jobs, put off by me as I am trying to hone my rather meagre skills on stock metal bar before tackling expensive castings!

I am trying to make everything accurately but am also aware of the old adage that the loco is which is made to a high level of accuracy is the one that won’t run due to fits being too tight! (Whatever reasons mine may find not to run, I doubt it will excessive accuracy).

Any wisdom from people who’ve made a William in the past would be greatly appreciated – as was previous advice on things like piston material, piston rings etc (pistons, rings etc. are now all made and awaiting assembly – thanks).

Many thanks

Arthur

[BazParticipant @baz89810]

3 days have passed since this was posted and not a single reply so I strongly recommend to Arthur that he posts on the Model Engineering Clearing House Forum

[Arthur Jones 2Participant @arthurjones2]

Thanks Baz, wise advice.  I am not currently a registered user of MECH, so getting an account on that is clearly my next job once I’m on the laptop a bit later.   I was anyway reluctant to “cross-post,” but in the absence of the usual specific and helpful replies here, I think I can safely do so.

And all the best to everyone for 2026, despite the gradual loss of suppliers, etc.

Arthur

[Bruce VoelkerdingParticipant @brucevoelkerding91659]

Arthur,

I thought I had posted a Reply to your Question, but I don’t see it here. I came across the original Drawings in my Collection of ME’s. I drew the Frame, Stretchers and Trucks. There is at least one Error. If you wish to discuss further, please respond.

Bruce

[LukerParticipant @luker]

Hi Authur,

I made this loco a few years back, subassemblies should be spotted on the frame to check fitment, this is typically how I build so I never noticed any discrepancy. There are a few issues with this design, unfortunately Martin Evens never built this loco, and he never had access to the simulation tools modern designers have.

The steam regulator doesn’t work out.

The valve lead is too much, causing hunting. There was a problem with one of the links.

The grate dimensions don’t match the boiler and cylinders.

The ash tray is constricted.

The blast nozzle doesn’t match the petty coat or smokebox dimensions.

The axel pump is too big for the loco (but you can manage this).

The leading pony really should have stops to prevent derailing, alternatively the springs need to be stiffer (but this has other issues).

You should add an injector (easy to make and fit on a steam turret), then get rid of the hand pump; the mounting is not designed correctly and causes major issues.

(that’s all I can remember)

This isn’t all doom, its an incredible loco if these minor issues are corrected. Loads of fun and decent pulling capacity! https://youtu.be/D8ZiUie3NlA

Cheers,

Luker

[Arthur Jones 2Participant @arthurjones2]

Hi Bruce and Luker,

Thanks for the responses – very much appreciated.  As I’ve implied on MECH, I’m just going to make the pony trucks fit the frames (and live with the fact that they won’t be interchangeable) as I’m in no mood for re-making parts.  My work rate is slow enough as it is without multiple attempts.

But I’m very interested in your comments.  Starting with the positive, that’s a very encouraging video, Luker, and your verdict on the engine matches the sort of positive comments found on the Station Road Steam archive.  I chose the design partly because, as the SRS website says, it’s “big enough to be interesting” and is a good runner (and pretty plausible as a loco design to look at), but small enough that a scrapped casting isn’t going to sink the family finances, and simple enough that I stand a chance of finishing it before I myself am finished!

But the catalogue of mistakes mentioned by Luker is a bit alarming.  Luker, please can you elaborate a little on:

– The steam regulator doesn’t work out. (Do you mean it doesn’t fit, or it doesn’t function correctly?  I guess it needs more than a 90 degree turn to open it fully, as it is just a screw valve)

– The leading pony really should have stops to prevent derailing, alternatively the springs need to be stiffer (but this has other issues).  (Stops where?  Please can you suggest a similar example?)

– The blast nozzle doesn’t match the petty coat or smokebox dimensions. (Do you mean it ends up in the wrong place, or the nozzle proportions are incorrect for it to function properly?)

I’ve taken the soft option on the axle pump and bought one from a reliable supplier at MMEX (not very much more than the cost of the William pump casting and materials!) and made a special (but simple) frame stretcher to hold it.  But in retrospect I should have listened to the supplier’s comment “isn’t it a bit big for a 3 1/2″ gauge loco?”.  It is the same bore as on the drawings (3/8″) and I assumed Martin Evans knew best! But in my view a pump is safer being too big than too small.  I may make an alternative eccentric with smaller throw to reduce the flowrate and put the two eccentrics on the same axle so I can swap between them (without disturbing the quartering!) once it’s working and once I can see how it performs in practice.  (At least the commercial pump can be adjusted in terms of position to compensate for different eccentric throws).

I am planning to fit an injector (and possibly an electric pump) anyway and the boiler has some extra bushes to allow that.  Thanks for mentioning that the handpump is trouble as designed.

I did run the valve gear through (I think) Charlie Dockstader’s simulator and I could not spot anything alarming,  I seem to remember using or finding a lead of 0.011 in, and events looked pretty symmetric between forward and reverse, but I’ll look out for the problem Luker identifies.  At the very least, I’m going to make a pair of mock-up valves from brass so I can check that the dimensions work out before wasting the proper materials on valves that aren’t quite right, and apply some “trial and error” on dimensions if required.  If I need to re-make a pair of links (presumably the combination levers?), so be it.

Bruce, sorry, I didn’t quite follow what you meant – are you saying you re-drew the William drawings to overcome errors you found, or that you were the draftsman who drew the fair copies for Model Engineer etc. in 1983?  I’d be interested to know whether you found the same mistakes as Luker, or different ones!

Thanks again and looking forward to being enlightened (and forewarned!)

Arthur

 

 

 

 

[duncan webster 1Participant @duncanwebster1]

Several chaps in our club have 2 injectors and hand pump, no axle pump, but I’d be loathe to do away with the hand pump. If you get low water and low pressure at the same time, your injectors won’t work. Unless you are one of the supermen who never run short of steam an axle pump will deliver as long as there is enough pressure to move the loco.

However, most axle pumps are too big, someone worked our that it should deliver 1/150 of the total cylinder volume per rev. Can you fit a sleeve to the bore, then easy to change the bore after test

[LukerParticipant @luker]

Hi Authur,

The steam regulator had dimensional issues, just check before making. You would very seldom need to open by more than 90Deg on this loco.

Just put any type of stops on the front stretcher that limits sideways movement of the leading pony. I made screw on cylinders.

I had to change the blast nozzle height position and dimensions and the grate open area to correct the overall thermodynamic performance. The loco does run if built to drawing, so that’s a good starting point. If you want to run circles around larger loco’s then the design needs some optimisation.

Yep, if you have the axel pump already, use it. Remember never to shut the bypass valve completely, and decrease the lift on the clack. The stress on the driving axel and eccentric will be too much, and the cold water inflow will drastically retard the steam generation. There is modern techniques to design these pumps related to the boiler heated area and swept volume, all the older pumps were overdesigned causing wear and poor steaming.

Those line diagram simulators are fine for concept phase design but not very practical for real world application. Proper simulations are the way to go, which is what I do with all my designs. The lead is incorrect for this loco.

You’ll get many different views on hand pumps and injectors. Experienced drivers will never use a hand pump, but you do need two methods of filling the boiler. I designed and made a wide range pressure pickup injector for my loco as it is only ever used when in the station waiting for someone with a hand pump 😉 In the end its up to you.

Cheers,

Luker

[Arthur Jones 2Participant @arthurjones2]

Hi Luker, Duncan and all,

Thanks for the clarification, and sorry for the slightly slow response (too much other stuff going on).  You have me slightly worried now as I have already made the regulator (I have not fitted the whole thing yet as I may re-make the spindle, which I made a bit of a mess of) but I couldn’t see anything wrong!  Time will tell.

In the meantime, I am plodding onwards…

Kind regards and thanks again

Arthur

[Bruce VoelkerdingParticipant @brucevoelkerding91659]

Arthur – I am building an LBSC Netta. I decided to look thru M.Evans’ William series to see if there were any Improvements in Boiler Design and Boiler Fittings as compared to LBSC. When I saw your initial Posting, I had the relevant Copies of Model Engineer at my Computer, so I quickly drew the relevant Parts in 2D AutoCAD. The Sums you listed in your first Post are CORRECT.

The LEAD TRUCK Wheel is 1/32″ out of Position compared to the Frame Cutout. The Pivot Pin needs to move 1/32″ towards the FRONT of the Locomotive.

The REAR TRUCK Wheel is 3/32″ out of Position compared to the Frame Cutout. The Pivot Pin needs to move 3/32″ towards the FRONT of the Locomotive.

The Error is not due to the two Stretchers possibly being mis-named (front-rear, rear-front). If the Stretchers are installed that way the Errors are worse.

The LEAD TRUCK could be considered CORRECT as is. If the Truck swings to the Extent that the one Wheel Flange is under the Main Frame, the Truck Wheel actually advances by .066″.

Personally, I would leave the FRONT TRUCK as it is. I would scrap the REAR Stretcher and make a NEW One to match the Design Dimensions of the LEAD TRUCK

It was a Senior Moment when I mentioned above that I thought I had posted a Reply but did not see it here. I posted a couple of Replies on the MECH Forum since you asked the same Question there. I look at the ME Forum only once per Month.

There are Photographs in the original Model Engineer Articles showing Parts made for William by Martin Evans. I have not read ever Article to see if he mentions a Correction, but one would think Martin would have updated the Master Drawings back then if he noticed the Error.

These Errors seem to occur because the Designers apparently were not conversant with industrial Practice used by multiple Designers working on Paper Drawings. You could never scale from a Drawing since Design Layouts were done on Tracing Paper, and it would grow and shrink with the Humidity. Everyplace I worked at here in the States used a Master Coordinate System on Layout Drawings. All the Designers agreed where 0,0,0 was and if you looked closely at a Design Layout you could see Coordinate Values on major Surfaces. The first thing the Chief Draftsman did was check that your Coordinates were correct on the Layout Drawings before he would check a Detail Drawing made from that Layout (the Coordinates were never shown on Detail Drawings since that would be confusing if a Part were to be used on a different Piece of Equipment).

Bruce

[Arthur Jones 2Participant @arthurjones2]

Hi Bruce,

Thanks for the thoughts and sorry for being a bit slow to respond.  Very informative and thanks for going to the trouble of drawing it out and checking.  I need to check which of the stretchers is one I was thinking of replacing anyway – I was hoping to get away with leaving it but on reflection it might be better to get it right if the dimensions are wrong anyway.  It’s a fairly trivial component to make anyway, though I am keen to avoid yet more delays to finishing the loco for a variety of reasons I won’t bother you with.

Or if I feel like cheating (and especially if the fault is on the otherwise “good” stretcher), I could probably swing the stretcher around by 180 degrees and drill a new hole for the pivot in the “new” correct place, and plug the bad hole so it doesn’t show when painted.

Anyway, good luck with making your Netta! I was glad to get the boiler for William through its hydraulic, and I am reluctant to make another in the future if I ever build another loco.  (I need to get this one finished ASAP before I even think much about that!).

Kind regards

Arthur

 

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