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Member postings for Paul Horth

Here is a list of all the postings Paul Horth has made in our forums. Click on a thread name to jump to the thread.

Thread: Thompstone Engine
15/04/2021 13:43:36


Thank you for your helpful explanations. And thanks also for the link to Preston Services. I didn't know that such small engines were produced commercially.Nor did I realise the high prices they can get. You would need after all a decent sized boiler to run one of these (with its certificate and insurance), and a pump, feed tank, water treatment etc and the space to keep all this kit. That's why I'll stick with models for my steam kicks.


14/04/2021 16:53:08

Hello Jason,

I have a few questions about this model design by Thompstone, and the full size engine which it might have been based on. Do you have an idea of the scale that Thompstone intended? It looks like quite a small engine, maybe a bore of 1 foot, or smaller than that, having a screwed gland?

I can't make out where the exhaust connection is. I would be interested in your opinion of some of the features which Thompstone has included.

  • Would the full size engine cylinder have been mounted on the two feet (difficult to access) or on saddles or flanges as in the Stuart engine?
  • Would the full size cylinder have had those decorative rings cast in?
  • Is the cylinder oil cup a practical proposition in full size?
  • Is there a reason why the full size engine would have had the bearing caps mounted at an angle?

Please understand that these comments are not meant as criticisms, I am just curious about the full size design features which I know little about.


Thread: Minimum equipment for TE build
19/03/2021 16:02:43

I'll just add that ploughing engines are indeed large, and therefore heavy, which could be a problem for a model.. The weight helped to anchor them. I have driven alongside a 2 inch ploughing engine and it dwarfed the Durham. They are also rather slow as models - , they were low geared because they were not intended to go anywhere quickly. I would like to see one use all its power to pull a model plough.

Good luck William,


18/03/2021 23:49:22

I made my 2 inch scale Durham traction engine with just a Myford ML7, with vertical slide. I do not possess a mill or a dividing head. I have a cheap drill but it is not to be relied on to get holes square and accurate. I bought the main drive gears, but made the gears for steering and water pump drive. The rear wheels at 12 inch diameter were a struggle, I lashed up a wooden faceplate mounted on a pulley countershaft securd to the lathe bed, with the tool height also raised from the toolrest. Everything else came off the Myford. So it is possible.


Thread: Could I try an IC engine?
16/02/2021 12:38:36

Hello again,

The forum has helped me greatly with my questions about making the camshaft. I would now like to ask about the crankshaft for the two cylinder Seagull.

Is there any objection to making a built up crankshaft instead of turning from the solid? It seems to me easier to make the two sets of webs and crankpins separately, held offset in the 4-jaw chuck, then put the shaft through both of them and silver soldering, with a spacer between the two throws. Align the crankpins by drilling through and putting a rod through both. Cut away the surplus shaft between the webs. Alignment is guaranteed. Less swarf, and easier to turn the pins when close to the chuck.

I made the crankshaft for my traction engine by silver soldering an oversize shaft through a block and turning the pin with throw pieces, then turning the shaft down to size. It was fine. Shaft is ten inches x 1/2 inch, throw 1 inch.


15/02/2021 12:02:53

Thanks to all of you for the encouraging and helpful messages with plenty of useful information. It's still very much a thought experiment for me.

I think I would be able to produce a camshaft using the turning jig method, without needing a rotary table, milling machine or a grinder. It is reassuring to see that a decent sized paper disc marked out in degrees is good enough to be practical, though I am surprised that the one inch disc with marker pen lines, shown in the video clip from Andy G. would be precise enough. Some kind of pointer has to be registered to the shaft, to operate this system. The complicated method used by Graham Meek in the link from Diogenes, or that used by Charles Lamont, would be too much too much trouble for me.

I understand Jason's point on making the cams in pairs, on the turning jig, which I could case harden, and polish, then mount on the unhardened shaft, though I would need to devise a method to set the angular positions of the pairs relative to each other and to the shaft keyway. With all four cams made together this would be taken care of. The pairs would have to be drilled through after making, then the excess shaft length cut off, before hardening.

I could use milling in the vertical slide as shown by Roger B, though this would need a reducing sleeve with two keyways to register the change wheel to the shaft, and 60 teeth would not enable the 130 degree setting for the exhaust cam flanks. But, in principle I could do it.

So each method has its pros and cons, with balances of time, effort and precision to be decided.

I understand the point about the fit of piston to bore, it has to be much closer than in a steam engine. This is one reason why a diesel engine would not be my choice because this fit has to be even closer than for a four stroke. And not only do I need a close fit, but it has to be dimensionally accurate, to use piston rings as provided in the kit.

My aim would be to produce an engine which could run on a table top setup, and, importantly for me, would drive a load which could be regulated. No ideas yet on what would be practical for that. Thinking is a lot easier and quicker than doing!


13/02/2021 21:40:49

Thanks, gents, for the replies, they were most helpful for me.

I was considering the Westbury Seagull two cylinder engine, available as a kit of castings from Hemingway. Then, doing a little more research, I came across the excellent diary of building this engine by Charles Lamont :


This description of his workings, all to tenths of a thou, I found pretty terrifying, with so many problems in making the cams, for a man who is evidently highly experienced. He ended up building his own camshaft grinder. Mr. Lamont is to be applauded for setting out this diary and for his sheer persistence, but the project is clearly beyond my horizon .

Even if I completed the engine I would then have to produce the auxiliaries, water pump, radiator with fan, coupling or pulley for power takeoff, and some kind of clutch, plus something for the engine to drive. Put it in a boat? God forbid!


11/02/2021 16:37:24


I am not a complete beginner, I have built and operated a 2 inch scale traction engine. I have been vaguely thinking about trying a 4 stroke IC engine, however it seems that the precision needed is at a higher level than for a steam engine. In particular, I am wondering about making the camshaft. I have read some posts on this forum, including the post from David K and the replies to him back in 2019..

My specific question now is about the precision needed for angular location of the cams relative to each other and to the register which locates the shaft in the timing gear. I do not have a milling machine or a rotary table. I reckon I could have a go at filing the cam flanks to a horizontal guide, using a micrometer, with the shaft in the lathe chuck, but setting the angular positions accurately would not be possible without some kind of dividing head.

I wonder how camshafts were produced in the 40s and 50s when milling machines and rotary tables would not have been possessed by many amateurs.

There are other precision tasks which would be difficult for me, such as making the valves tight, and getting the crankshaft bearings in alignment..

I would be interested in any comments, thanks.


Thread: Bronze fittings in steel boilers - corrosion?
04/02/2021 11:55:59

I ask this question just from curiosity - the boiler on my engine is copper.

If a bronze fitting is screwed into a bush on a steel boiler, below the water level, I would expect that there would be some electrolytic corrosion of the steel in contact with the bronze. There are many steel boilers in existence so I realise that this cannot be a practical problem, I would be interested to understand why, and what evidence exists for this kind of corrosion. Is it simply that the plate thicknesses typically used in construction are sufficient to accept some corrosion over a reasonable lifetime?

I would appreciate any comments.


Thread: 2 Durham and north yorkshire
30/01/2021 09:55:44


The email address failed, so did the PM, so I am sending this drawing through the forum, it seems the only route I can get it to you.

This drawing is the John Haining design for the firehole door, for some reason it appeared in only a later series.

Haining designed the door ring arrangement to avoid putting screws directly into the copper backhead. As your boiler is steel, you might choose to attach the ring directly, though with steel screws, you may never get them out, and with bronze screws there is a risk of electrolytic corrosion inside. I made mine as shown, except I used a thin copper liner, not a steel one, with tabs bent up inside to retain it. This is fine.

Make the latch big enough for stubby fingers, when hot, you will be flipping the door open and closed frequently. It has to close securely, if it swings open you will lose the fire.



firehole door 2 001.jpg

29/01/2021 15:33:51


Please can you send an email address, I am trying to send a drawing of the firehole door, but the PM service on this forum is not functional for me today.

Paul Horth

28/01/2021 12:32:57


I have sent you a message with some modification notes.

Best regards


26/01/2021 21:08:17

Hello Richard,

I made my selector ring by simplifying the Haining design. Hopefully the photos will make this clear. I made it with the two half rings bolted to a fabricated back plate with the pin integrally attached. When in high gear, the pin engages with a little brass slide attached inside the gear guard. This stops the tendency for the ring to swivel forward which can cause the gear to jump out of mesh.

I can offer a list of other minor modifications which I have made on my engine, to overcome various problems with the Haining design. Maybe you have not yet made some of these bits and you can pick up what looks useful.




Thread: 2" Durham and North Yorkshire
17/08/2020 15:35:45

Hello Jon,

I have picked up your post rather late, as I don't look on the forum very often, so it's likely that a lot of what I say below is already out of date, apologies if so.

I built the Durham engine a long time ago (1980 - 2000), it was my first model engineering project, and I do have quite a bit of hard earned experience as a result. I have been running it every season since then ( except this year!). However, after reading you post, and sympathising with what you are facing, I don't think I can give you any really helpful answers. I have sympathy for some of the bodges which you are having to do because I also did a lot of hole redrilling and so forth, cursing the idiot who built it.

The first thing I would emphasise is to get the boiler hydraulically tested before doing any more work. I have the impression that your boiler was not all that well made, so there is a risk that it might not pass the hydro test, and then any work by you in fitting the hornplates, smokebox, axle bearings etc might be wasted. If it passes the test then you can breathe again and consider how much effort you are prepared to make.

I spent a lot of effort fitting the hornplates to the boiler so that they were parallel and the three shafts in their bearings would all line up and rotate freely. This required fitting the rear axle brackets to the hornplates first, then locating one plate on the boiler, then offering up the other one with shafts through the bearings, and very carefully clamping and drilling through to the hornplate stays. Even then there was a lot of fiddling about with shims before all the shafts would turn freely. It takes only the slightest movement to lock the second shaft. Then, i had to do this job all over again, when I had to replace the boiler after 9 years with a new professional one. The rear axle is easier, there is some play in the axleboxes (in mine at any rate) and a little stiffness will soon wear off. I would say you can live with the 1/64 oversize here.

Before you shorten the firebox lower edges, be happy about the attachment for the ashpan. These edges should be deep enough to comfortably put in a long pin to hold the sahpan in place.

The chimney base really does have to be properly aligned. More holes to be plugged and redrilled, and make sure the joint is airtight. And make sure the smokebox is properly square to the boiler in both planes.

The front axle journals with two flats? Familiar, mine are like that! But only a little, it doesn't show.

When you obtain the rear axle, give yourself a little extra length, to get all the clearances you will need so that the compensating gear doesn't foul the tender bolts.This will also allow the hornplates to be a bit further apart so you can fit shims, washers etc under the bolts. I cut mine too short and had the devil of a job getting everything in.

I have a whole list of other modifications to the Haining design which I can send you, if you decide you are going to move ahead with this engine. Good luck,

Paul Horth

Thread: Cylinder mounting.
28/05/2019 11:07:37

Hello Garry,

I have come to this thread a little late. Regarding the cylinder mounting studs, I have done this job twice now. I have a 2 inch Durham engine, now nine years into its second boiler (don't ask). The studs are fitted into through holes and sealed with Red Hermetite, and there has been no problem with leaks on either of the boilers. So this procedure will work OK.

I'd like to say a few words about fitting these studs. They are made of bronze and I found them tricky to produce and fit. You need to take care about the exact length, they need to show one nut thickness plus one thread or so, with the cylinder and its sealing gasket in place, when tightly screwed in. The cylinder flange thickness may vary, depending on how the saddle was machined, mine did. There is not much length to grip the studs with, so you need to make a good stud driver to screw them tightly home against the end of the inner thread.I reduced this problem by fitting nuts with silver solder to all the studs except one row, thus making them into bolts. One row of studs can be put in before the cylinder is placed, this should be done with the stud driver, the cylinder is then located and the other stud holes can be carefully spotted through taking care about the angle. I had to drill the holes with a hand drill, aiming by eye, because I couldn't get the whole boiler, hornplates, cylinder etc under my machine drill. So I ended up not getting the angle accurate and having to draw over some of the holes in the flange to get the studs through.

And before doing any of this, the cylinder needs to be accurately located square to the crankshaft at the exactly correct distance and in good contact with the saddle pad.. I made a fixture which connected the cylinder squarely to a dummy shaft which could sit in the crankshaft bearings. So the hornplates and bearings have to be already in place.

Good luck!


Edited By Paul Horth on 28/05/2019 11:14:59

Thread: Exploding gauge glass
27/05/2019 16:50:35

I was at a rally a week ago when the gauge glass on my 2 inch engine blew out explosively. Luckily I was attending to something else at that moment, or I would have received a faceful of glass fragments.On this size of engine I usually have to lean down closer to the glass to check the level.

I fitted a spare glass, re-filled, re-lit the fire and carried on with running, having a successful afternoon. However, when I got home I decided to heed the warning from Lord Murphy. I got out the gauge glass protector, which I had made in 2001 but never bothered to fit, and cut a piece of clear plastic from an old tape cassette box to fit in front of the glass. So the protector now protects me from the glass instead of the original opposite function. I hope it will withstand the temperature.

I have had glass tubes break before, but with a neat quiet crack, not an explosive blowout. I mention this for others to think about, though with larger scale engines there is perhaps less hazard due to more distance from the glass (but also a larger steam escape)..


Thread: Designing Model Boilers (Thermal Design)
25/07/2018 22:09:54


Thanks for your comments. As well as being a great design aid, your spreadsheet provides a rational basis for setting the capacity of safety valves. Sizing the valve for the chosen capacity is another story, I published a method in the M.E some years ago, but I've since had my doubts about its accuracy.

If a Speedy boiler can be fired to produce 28 lb/h of steam, as your spreadsheet estimates, I wonder if the LBSC-style safety valves can pass that flow. On my engine, if I had the draught to get the grate loading, 28 lb/h would be well in excess of the feed pump capacity, and just about within the capacity of the two safety valves (estimated with uncertainty).

When I run the spreadsheet for my boiler with a reduced grate loading of 15 lb/hft2, it predicts a steam rate of about 13 lb/h, which matches what I actually see on tests. So your heat transfer calculations are pretty good. More details to come.

I agree with your comment on ash - there are always some still-burning cinders when I rake out the ashpan during a rally session, maybe not as much as 4% of the 3kg or so of coal that I use in a 5 hour rally. They do add some heat to the air intake.


24/07/2018 10:20:35


I have been reading your articles in the M.E and have just got around to looking on this forum. I really appreciate having access to your spreadsheet calculations and notes. I have spent much of my professional career doing calulations in heat transfer and fluid flow, and I admire a well-made spreadsheet as much as a well-made model.

I have a 2 inch scale traction engine, and I will enjoy using your spreadsheet to estimate the boiler performance, and comparing the results to my measurements of coal consumption, power, and efficiency. I will send the results back to you for your review and comments. I'll just make a couple of comments here first.

I think that the grate loading that a model boiler can be run at, depends on the draught. The draught in a traction engine is less effective than a loco, because the blast nozzle is right up in the chimney, well downstream of the tubes, and there is no petticoat pipe or choke to gather the flue gases. So in my engine, which is a similar size to Speedy, the grate loading is much lower than 40 lb/h ft2, it's typically 13 to 15 lb/h ft2. I have deliberately eased the draught with a larger blast nozzle, so that I can go for a drive without lifting the relief valves all the time. I also find no evidence for the large quantity of unburnt coal which you assume, the smokebox has a few spoonfuls of cinders in it after a long session. Probably due to the lower draught.

I will use your spreadsheet to see what difference these parameters make to the simulated performance.

Thanks again,

Paul Horth

Thread: Trevithicks Dredger Engine
10/12/2017 20:04:53

Error alert:

I was incorrect to say in the last post that the gas consumption will reduce the gas tank pressure. If the liquid in the tank is one component, (propane, or butane) and at a constant temperature, then the pressure will stay steady as the gas is used, until all the liquid has evaporated. For a mixture (LPG) the pressure will fall even at a constant temperature. I knew this professionally but was too hasty to check before sending my post. Apologies.


10/12/2017 19:07:43

Here is a further update on running the Trevithick engine, four months on...

Following a holiday, and then a bout of unexpected faults and troubleshooting, I have recently had a series of successful runs. The engine will now run continuously for the duration of the gas, about 30 minutes, at a speed around 160 - 140 rpm. This is still faster than I would like, but at least it doesn't stop. On compressed air it will run quite a bit slower. The pump works well, the flame stays alight from just the steam draught. The steam is restricted by an orifice 0.029 diameter, in the throttle valve, but even so the speed is very sensitive to the exact position of the throttle. I start at 20 psig and this pressure then drops to around 12 - 15 psi, where it is held steady.

All the operating conditions of this boiler and engine are just about in balance with nothing to spare:

  • The burner flame using a No. 5 jet is just about sustained by the engine draught through a 1/32 blast nozzle
  • The burner burns just enough gas to make enough steam to sustain the pressure at the lowest regulated speed throughout the run. Faster speed causes the steam pressure to fall. The gas pressure falls as the tank empties, for 3 reasons - the gas is consumed, the tank gets colder, and the propane comes off first leaving gas richer in butane which has lower pressure. The gas control valve has to be gradually opened to compensate for this. The average gas flowrate is 0.45 to 0.5 gm/minute.
  • The pump delivers just about enough feed water to match the steam rate (about 5 gm/minute) without needing a bypass. I did fit a bypass but I run with the valve closed.

I have not yet got the level gauge to function.. The level is visible before steam is raised but then it just bounces randomly. I will next make a deck to fit on the plinth so that all the feed piping is below the deck.

This thread doesn't really belong in the Beginner's Questions forum, but I don't know how to move it to a better place.


Edited By Paul Horth on 10/12/2017 19:11:53

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