Metric Stuart engine

[JAParticipant @ja]

Before COVID there was a long topic on metric model threads on the Forum. I suggested someone should try to make a metric Stuart engine.

About a year ago I bought a set of new castings for a Stuart two cylinder marine compound engine from a deceased local model engineer’s estate. This is a Saturday project, secondary to my main project, and would be a true metric engine (as far as possible) made to mm and using metric fixings. One could ask why? – Because I want to!

Between working in the workshop (which should be my priority) I have been producing metric drawings. The re-dimensioning has been relatively simple. Most inch fractions are quite close to a whole number of mm. The one fraction that is not is 1/4″. This leads to the first problem, the 1/4″ diameter steam transfer pipe between the two cylinders – to my knowledge one cannot get 6mm or 7mm diameter copper tubing. Fixings are not difficult (7BA=M2.5 & 5BE=M3) if one is happy with stainless steel. However I like to black the studs and nuts which means mild steel. A German firm will sell mild steel model screws and nuts to the UK with a delivery time of about a week. However their longest screws are 20mm, too short to turn into the studs for the steam chests and covers. They do sell M2.5mm galvanized thread bar (interesting). Again 2.5mm diameter mild steel bar appears to be unavailable.

I do not intend to post a blog on the project (life is too short) but to appeal for help and advise. So:

1. Do anyone know of a source for 6mm OD copper tube?
2. Likewise for non-coated M2.5 mild steel threaded bar or 2.5mm diameter mild steel bar?
3. Does anyone have knowledge of removing zinc from mild steel (acetic acid perhaps)?

JA

[JasonBModerator @jasonb]

Soft copper brake pipe is easy to get hold of and bend so should suit your needs, comes 6mm OD

All my metric models including the Stuart based ones that have M2.5 fixings have the threads cut onto 3/32″ steel and I have not found it lacking strength when screwed into holes drilled 2.1mm tapping size, also passes through 2.5mm clearance holes OK, I’m not a fan of threaded rod so just thread each end

If you really want 2.5mm mild steel then get some welding rods and knock the flux off.

Or go to one of the other German suppliers like Knupfer who do M2.5 bright studding

[Robert Atkinson 2Participant @robertatkinson2]

Lots of 6mm OD copper pipe out there

https://www.bes.co.uk/copper-pipe-coil-6mm-x-10m-6939/

https://www.stevensonplumbing.co.uk/6mm-copper-pipe.html

Just two random examples no used either supplier.

[noel shelleyParticipant @noelshelley55608]

Contrary to common belief the world is not and may never be Metric ! Many things are still for various reasons made in imperial dimension, structural steel, aluminium, pipe fittings Etc Sitting on the bog at afriends house in Belgium the gas main had 1.25″ cast into the fitting. Ah well. Noel

[Charles LamontParticipant @charleslamont71117]

If you don’t like the idea of using 3/32″ rod for your studs you could run 1/8″ rod down to 2.5mm using a bush type travelling steady. It does not take long, but if you run at high speed, the length you can run in one go is limited by whirling of the unsupported end.

[noel shelley]

On noel shelley Said:

Contrary to common belief the world is not and may never be Metric ! Many things are still for various reasons made in imperial dimension, structural steel, aluminium, pipe fittings Etc Sitting on the bog at afriends house in Belgium the gas main had 1.25″ cast into the fitting. Ah well. Noel

Um… Noel; no argument there.

However, I, too, would built a Stuart engine with metric fasteners; cheaper than ordering in BA taps and dies from overseas. I’ve got a fantastic set of Metric, pretty good UNC/UNF, but minimal BA/ME, so why not use what you are comfortable with?

Currently finishing a Martin Evans “Ivatt” locomotive, bits made in NZ, NL, and Canada; all dimensions converted from fractions/decimals into mm.

My first locomotive was in BA and inch; after I lived NL for a while, I found the metric system for machining much easier.

Others prefer inch, but so what? It’s the end product that counts.

Respectfully, Another JohnS.

[DiogenesParticipant @diogenes]

I’m pretty sure that I’ve had / have small metric MS ‘self’ studding from EKP;

I need to get over to the shed and remind myself whether that’s true, and what I think of it – I’ll report back..

[DiogenesParticipant @diogenes]

It’s not true; ..if I did have some, I’ve used it all up now – apologies.

[JasonBModerator @jasonb]

EKP do list it but out of stock in M2.5

[John PurdyParticipant @johnpurdy78347]

Zinc coating can be removed easily by a short immersion in dilute  hydrochloric acid ( muriatic acid, concrete cleaner). Immerse till it stops bubbling, remove and wash with plenty of water, dry and oil or it will rust immediately. Do it outside as HCl fumes will rust everything in site! I have a small plastic tub outside that contains 20 degree baume (31% ) muriatic acid (sold as concrete cleaner) diluted 1:5 (acid to water) which I use for this. Also good for removing the black mill scale from hot roll steel.

John

[JAParticipant @ja]

Many thanks for the advice.

I had intended to make the studs. A tedious but not difficult job. Of course 3/32″ is all but 2.5mm. A “durrr” moment, must be old age.

I will trial 6mm microbore copper tube to see if it is usable. I distrust domestic plumbing from expience.

JA

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