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Elmers no.7 square

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Jon Cameron28/01/2018 00:57:53
314 forum posts
90 photos


I'm about to build my first engine from plans

After a visit to the local metal suppliers, I came away with a few bits and bobs.

The Copper tube is not part of this project, and is destined for my friends Stuart boiler thats missing a chimney, once the ends are tidied up. This will repay my machining favour for him sending me a bench grinder.

We have

1/2" square brass
3/16" mild steel rod
3/8" Brass round
3/16" x 1" brass flat
1/8" x 1/4" Brass flat
1" square round mild steel
1.3/4" Brass round

The issue I've got is after measuring the internal of the square tube it's already 0.002" smaller without me machining anything, am I better to hog the brass down from a larger size? What's the tolerance on the piston fit, I'd have thought around 0.001" once it's been machine, but would welcome input.

Look forward to hearing any tips on making it.

Raw stock

Thor28/01/2018 06:54:26
1218 forum posts
37 photos

Hi Jon,

There is a few photos here, and a build log here. I hope this may give you some answers.


Edited By Thor on 28/01/2018 06:55:20

JasonB28/01/2018 07:47:38
17882 forum posts
1954 photos
1 articles

Don't worry too much about the internal size of the tube, as it says on the plans the piston should be made a "close free fit" in the tube and it only gives the tube wall thickness as "Approx" 1/32".

I would stop measuring your piston as you near the internal size of the tube and then take very light cuts using the tube as a gauge to get the size right. If you measure it afterwards it will probably be a thou or less difference between the two.

Jon Cameron28/01/2018 11:37:34
314 forum posts
90 photos

Hi Jason the square tube is 1/2" 16swg, the piston was to be made out of 3/8" piece I already had. I hadn't done my maths before and there' a 2thou gap already, I'm thinking about what I have that I could make the piston out of. 2thoj gap before machining is only going to get bigger, meaning a sloppy fit.

Thor thanks for the links they could provide very useful.

Martin Connelly29/01/2018 19:49:18
1267 forum posts
152 photos

Could you squeeze the brass to expand the diameter, do you have access to a press? Maybe a hammer will do the trick.

Martin C

Jon Cameron20/02/2018 15:18:12
314 forum posts
90 photos

Hi Martin that's an interesting idea though I'm sure I'll find something the right size to work with.

Well I made a start on the engine tonight. Starting with what I knew was going to be a pain. The long thin hole for the steam feed. What do you know a dead centre doubles up as a wobbler stick to dial the hole in. 
The long thin hole didn't go according to plan which means I'll have to take account of that when I build the crankshaft and bearing. Also it appears my chuck wasn't done.up tight when I drilled the long hole which accounts for the wobbly hole, seems I missed a jaw on final checks.
The cylinder base and frame all sized to length a few more holes to drill.

Edited By Jon Cameron on 20/02/2018 15:35:05

Jon Cameron20/02/2018 17:59:32
314 forum posts
90 photos

A little more done before I was called in for house duties.

Turned the bearing to hold the crank, checked fit on engine frame and also on the crank, it will need polishing slightly to be a good fit, the end been faced and a chamfer added to aid it's passage into bearing.




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