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Beginner's engine build. Simplex 5"g.

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Simon Collier29/05/2020 11:04:52
352 forum posts
56 photos

The build series was in Model Engineer, from memory from 1969-1970. Clubs usually have libraries from which the magazines can be borrowed and photocopied. This is what I did.

Old Crock29/05/2020 11:49:48
29 forum posts
18 photos

Interesting solution to the oiling problem David. Clearly the Super Simplex is a different animal to the original Simplex but still omitted to say how to get oil to the boxes.

Don't get me wrong, the original Simplex can be made to work and is quite a nice looking loco, but there are many hurdles for the beginner to overcome.

The Walschaerts valve gear for example will not work without a lot of easing, something that may worry a beginner.

Iain I see that you are only working from the drawings from GLR. I assume they are the same as mine originally from Nexus. When drilling the frames don't forget the 3/16" reamed hole 4 3/16" ahead of the centre axle and in line with the weighshaft. Your drawings will give you no information what they are for! They are to line up the pivot points in the motion plate link brackets, something which Martin Evans failed to mention until "a large number of builders of Simplex have been after my blood the last few weeks".

In the original build in ME this appeared six months after he described the motion plate and link brackets, where he said to locate the motion plate, pull the crosshead right out, slip the motion plate into position and allow it to assume its natural position relative to the slide bars. Fix in this position. This is repeated in the book. No mention that I can find of those little 3/16" holes. In the later admission he says of the large number of builders "They complain that the slide bars are 1/8" too short! Unfortunately for me, I have been telling these good folk that the mistake is not mine, when all the while there has been an error, though not actually in the length of the slide bars themselves". He goes on to explain that error is in the distance of the rear edge of the motion plate from the centre of the driving axle.

This error was caused because he left the drawing (on cartridge paper with a 6H pencil) in the sun and it shrunk. One of the first things I was told in this hobby was never measure from a drawing!

Anyway we are getting ahead of ourselves. Before you get to the motion plate and valve gear you need to tackle the rear cylinder covers! This is the full size drawing

full size cylider rear cover.jpg

This is the same item in the book with two 2BA bolt fixing for the slide bar:

book cylinder rear cover.jpg

And this is the slide bar:

book slide bars.jpg

Why is there a .010 relief? No mention of threaded hols for the rear of the slide bar where it fixes to the motion plate.

Enough of my whinging. The Simplex will work but not "out of the box". A beginner will need help, possibly lots of it..

My conclusion is that the drawings and write up were a mixture of original work and copies from other designs and they lost, in video terms, a little in continuity.

Keep going though Iain and do let us know how you get on. Good luck.


John Alexander Stewart29/05/2020 16:15:42
768 forum posts
51 photos


My first build was an LBSC Tich; there were issues during the build.

No matter - if you want to do it, do it. I did - people kept saying "start with a stationary engine! Build something larger!! Purchase one and fix it up!!!" but I went down the path I wanted to go.

But you have been warned - know that you have to plan ahead and think about fits and interferences. Talk to others and go through the mentioned on-line build logs. Maybe it's part of the fun?

I'm back on my Ivatt 2-6-0 Martin Evans' design; took a break and built a Kozo Hiraoka Shay (first version) and, on coming back to the Ivatt, it hit me just how good a designer and draughtsman Kozo Hiraoka is.

I think that I'd rather build another Kozo engine than a Martin Evans' one; saying that, there's 2 Simplexes at our club, (one regular, one super) and both run and run and run with minor maintenance. (and, I think one had to have its' rear axle replaced after many, many miles on a ground track)

Iain Bachey30/05/2020 16:58:03
13 forum posts
11 photos

It's the fabrication of parts and machining that interests me and in this instance, the result will hopefully be a complete model steam engine. I have been building and flying model aircraft for 30 years and have always built my own aircraft from plans or kits. Don't get me wrong, it's nice to just go 'buy and fly' but I think that is for the throw away generation. I like to put some effort in to get the reward, like most of you on here no doubt.

I will stick with the Simplex and overcome the problems previous builders have come up against, just like they have. You chaps will all help I'm sure, even if there are a few "I told you so's" along the way emotion

Good news is after a fair bit of research I have decided to purchase a milling machine. Hopefully next week. I've been wanting one for many years now and I've got a big birthday this year so I will treat myself I wont go into details of which machine im getting as that will only stir up everyones opinion and I'll save that for another thread! This will aid the build and speed things up a little, although I've not done much milling since my college days. That's another topic all together though!


David Wasson30/05/2020 19:06:56
142 forum posts
42 photos


Congratulations on getting a mill! It will really make short work of things that you might otherwise be using a file. I assume you already have a lathe?


Old Crock31/05/2020 11:56:51
29 forum posts
18 photos

Iain, I hope you are making progress with your frames. As I mentioned earlier there will be highs and lows. For me today was a high as I got one of my Simplex running on air - after 8 years (not full time!).

From here on it should be plain sailing but I will believe that when it is finished. I just hope it doesn’t take another 8 years as I don’t think I have that time left in my tank😁.

So whatever happens persevere.


Jon Cameron08/06/2020 12:21:19
336 forum posts
90 photos

Hi Iain,

Hope all is going well with the frames, i was looking over the drawings and write up in ME over the weekend, i believe i may have found another to catch you out down the line. Appolagies if it was mentioned previously.

The axle pump eccentric has a 1/4" throw. The eccentric strap OD is 1.3/4" (excluding the bolts). So this is 7/8" radius + 1/4" throw of the eccentric, total is 1.1/8". The stretcher immediately behind the axle has a dimension of 1.3/16" to the centre line of a 3/8" plate. So the plate is 3/16" from centre line to its edge, 1.3/16" minus the 3/16" of the centreline of stretcher to the face of the strecher.....Ahhhhh 1" of free space to fit 1.1/8"......

A relatively easy solution to this would be to drill and file, or mill out a section of the strecher so that the eccentric does not foul the strecher at the rear, while it is in the flat and not mounted up on the frame. Say 2.1/16" high, and 1/2" wide, on the vertical centre line and 2.21/32" from the top edge. You will find this will remove the fouling of the eccentric, though it will mean that your frame strecher is effectively cut in half which is no good. You don't need to go full depth but by the time 3/16-1/4" has been removed the strecher is already weakened.

A better solution would be to move the frame strecher and holes to the rear by 3/8" giving ample clearance, but this will also possibly effect the length of the firebox between the frames. So the drawing and suitable amendments will need to be taken into account on the boiler and discussed with your boiler inspector.

Ill keep looking through and see what else i notice on the drawings other than those previously listed.

Wise words earlier in this thread, if it moves, see what else its close to that it could effect.



David Wasson08/06/2020 17:04:10
142 forum posts
42 photos

The eccentric is mounted on the front axle. There is plenty of room for the eccentric strap.

Jon Cameron09/06/2020 08:28:28
336 forum posts
90 photos
Posted by David Wasson on 08/06/2020 17:04:10:

The eccentric is mounted on the front axle. There is plenty of room for the eccentric strap.

My appolagies, must have been having a bad day yesterday. Don't know how i missed that??

David Wasson09/06/2020 10:36:24
142 forum posts
42 photos

No problem, happens to everyone.

David Wasson09/06/2020 10:36:39
142 forum posts
42 photos

No problem, happens to everyone.

Russell Eberhardt09/06/2020 11:26:21
2583 forum posts
85 photos

I may have missed it but I don't think anyone has mentioned yet the need to aneal the frame material before cutting. If you are using BMS it tends to distort after cutting out for the axle boxes.


David Wasson09/06/2020 14:37:50
142 forum posts
42 photos

Yes, sometimes BMS will distort when you stress relieve it, like cutting out the space for the axle boxes. I used 1/8" BMS, and it did not distort. Although, I did make sure that the material was as flat as possible before cutting. Also, to a certain degree, the frame spacers and front and rear beams will keep the frames straight. Make sure everything is perfectly square when you clamp and drill for the screws that hold everything together.

Phil H109/06/2020 15:10:52
273 forum posts
27 photos

Many people mention the BMS distortion but I have cut 6 sets of frames out using BMS and I have never had any distortion. Perhaps certain batches of BMS are particularly bad?

David Wasson09/06/2020 17:05:52
142 forum posts
42 photos

Perhaps distortion has happened to those that had frames that were particularly narrow. These frames are pretty wide. I certainly had no problem with distortion. Hopefully, it was not just luck!

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