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Flared Pipe Connections?

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Nigel Graham 230/06/2022 23:55:35
2287 forum posts
33 photos

On live-steam applications...

Has anyone any practical experience of using flared rather than nipple type unions, for live-steam model "plumbing" ?

Advantages? Disadvantages? Pitfalls to avoid?


I have a Parkside (Lidls if I recall aright) flaring tool and have started experimenting with it.

It consists of two bars that clamp together, with holes of various sizes along the joint, to grip the pipe. Fine ridges on the hole walls enhance the grip. The flaring tool itself resembles a bearing-puller that straddles the clamp; and an adjustable protractor shows 90º included angle on its cone.

Its immediate disadvantage is that the holes are all metric for tube with mm outside diameters; whereas we normally use Imperial tube and fittings.

Its deeper disadvantage is that it needs the male fitting to have a coned end, whereas of course all our bought-in fittings and those detailed on the published drawings have internal tapers for solder nipples or compression olives.


However, I have a length of copper tube that seems neither one thing nor the other, but fits the 5mm clamp well enough for me to have flared a sample piece with apparent success.

I did anneal the end and remove the internal lip left by the wheel-type tube-cutter, first.

For the immediate application, supplying the blower on my steam-lorry, I can make the two fittings with male tapers. They are the bulkhead union bringing the pipe through the smokebox wall, and the stub on the blower-ring itself.

So the specimen tube is intended for this.

I used 1/8" BSP (a shade over 3/8" crest dia.) on the bulkhead connector, countersinking the inner face of the nut to hopefully nearer the flare's outer profile.

I have yet to cut the tube to length and flare the blower end; and this hiatus led to me finding a union nut a size smaller - 5/16" X 32 ME - fits the flare very neatly, though its tube opening is slightly over-size.

It might be harder to obtain a full seal - that remains to be seen - though a very slight leak is probably insignificant on the blower feed. Annoying because I want it right, but not as critical as, say, on an injector water-feed where an air-leak would be an invisible source of nowt but trouble.


Pros & cons?

- Potentially neater fittings (proportionately smaller union nuts).

- No soldering needed (may be best to anneal the tube first).

- Fittings need male tapers, with a short clearance diameter for the nut's internal run-out.

- The nut needs be a running fit on the tube, not on a union nipple, and an internal countersink to snug against the flare. This might require making the nuts, though standard ones can be modified.

The particular tool I have also indents the tube surface, analogously to vice-jaw marks. Passable inside a smokebox, but not for somewhere visible. It is also metric, but it may be possible to make split mm/inch adaptor liners that fit its larger clamp-holes. Or indeed and probably easier, make an Imperial copy, machined from square steel bar stock.


Incidentally, as a prototypical point, I think I have seen some of the larger union-nuts for iron pipes fitted with screw-on nipples, made octagonal to reduce their overall bulk. It may also be to help access in cramped installations.

Howard Lewis01/07/2022 06:45:09
6314 forum posts
15 photos

Merely random comments.

The LIDL flaring tool is most likely intended for use on vehicle hydraulic brake pipework.

Being marketed by LIDL which is a German company,, it is most likely to sized for Metric pipes.

A British version, such as those by Sykes-Picavant would be more likely to be suitable for Imperial pipe sizes.

Since the pressures involved in hydraulic brakes are much higher, sealing against the lower pressure used on model steam locomotives should pose little problems

It may be, however, that the fittings associated with such pipework will not look "prototype". If so, it might mean that fittings unique to the particular application would need to be made, rather than bought, for the model to look "right"..

If the wish is to avoid hard or soft soldering nipples to pipes, small compression fittings are available, and would be more likely to look more like the "original".

Octagonal nuts seem to be popular among the providers of central heating fittings. Whilst being "fit for purpose" in providing a clamping load to effect a seal, Hexagonal fittings ere probably the norm in full scale locomotive work. (Apart form those requiring the use of a C spanner, as used on some glands. ).


Hopper01/07/2022 07:42:44
6695 forum posts
347 photos

I don't know about on steam models, but I do know from experience that flared copper tubing on vintage motorcycles can tend to fracture in a circle at the end where the flare meets the parallel tube, right about where it sticks out the hex fitting. Annealing the copper flare area after flaring can alleviate this, as does putting a "vibration" loop or two in the line to allow it to flex without stressing the end point at the fitting so much. Or at least curve the line a bit so it can expand and contract without putting pressure on the flare at the end.

Robert Atkinson 201/07/2022 08:14:44
1246 forum posts
20 photos

Two points:

Check the cone angle of any fitting. There are at least two standards 37degree (aerospace and industrial) and 45 degrees (automotive). Your Parkside tool is probably 45 degree

Most aerospace flared tube fittings use a support sleeve between nut and tube. This stops the breakage hopper described.

Robert G8RPI.

Mike Poole01/07/2022 08:15:31
3383 forum posts
77 photos

The grips are probably necessary for flaring Bundy pipe which is a steel based pipe and probably harder to flare than soft copper, Kunifer is another popular brake pipe that is a copper nickel alloy and may be harder to flare than pure copper. It may be worth making a smooth faced clamp for soft copper tube.


Nicholas Wheeler 101/07/2022 08:25:26
959 forum posts
88 photos
Posted by Robert Atkinson 2 on 01/07/2022 08:14:44:

Two points:

Check the cone angle of any fitting. There are at least two standards 37degree (aerospace and industrial) and 45 degrees (automotive). Your Parkside tool is probably 45 degree

Most aerospace flared tube fittings use a support sleeve between nut and tube. This stops the breakage hopper described.

Most aerospace tubing will be aluminium and uses a single flare. Car brake pipes are steel until you need to replace them. Then you'll be using 3/16" kunifer - 4.75mm - and mostly double flares

martin haysom01/07/2022 09:04:59
107 forum posts

its a cheepo brake pipe flaring tool the grips will damage the pipe better ones don't have this and are available with imperial and metric jaws

not done it yet01/07/2022 09:34:46
6888 forum posts
20 photos


I’ve only used automotive type flaring tools. I can say that there is a world of difference between these cheap ‘bar-type sets’ (like the lidl one?) and the proper workshop flaring tools such as some SIP and typically Sykes Pickavant.

THIS is the modern version of my older set(s). The cheap ones work, but I would not have been without these better tools.


Edited By not done it yet on 01/07/2022 09:35:11

Mark Rand01/07/2022 09:45:42
1314 forum posts
38 photos

I've got a Sykes Pickavant Flaremaster that I don't need anymore. I was going to put it on John's Lad's Homeworkshop site in the hope that someoune can use it. It worked very well doing Monel brake lines for a Mini and an Escort.

noel shelley01/07/2022 09:54:13
1445 forum posts
23 photos

Over 50 years ago I bought the best I could get, a Sykes Pickavant brake flaring set at £19, imperial, I later added the metric dies. ALL the dies had the tiny grooves or grips in the holding dies as otherwised the pipe would be pushed back by the swaging die. 37* is the American JIC form used in hydraulics, this type has a male thread and convex cone that can easily be damaged. Even a scratch can cause a leak, BSP has a concave cone on the male part that cannot be damaged (easily ). Flared pipes are used in many applications, but in standard sizes, so off the shelf components can be bought - but in the sizes we use I doubt you will find them at a sensible price if at all. They can and DO crack at the base of the flare ! I would stick with what we know works ! Noel.

Hopper01/07/2022 09:55:45
6695 forum posts
347 photos

For copper on steam, I don't think you want to be using double-flare brake tools though. Usually copper is done single-flare. You can even make your own tool to do it with a hole drilled through a split block and a centre punch ground to the right angle.

Nigel Graham 201/07/2022 10:40:26
2287 forum posts
33 photos

Thank you for these points.

Hopper, Robert -

I suspect the fracturing there is encouraged or caused by long-term, fairly heavy vibration much more likely on a motorcycle or car than on a model steam-engine; and obviously especially a critical point in aero-engineering. Even so, annealing the flared tube may still be a wise precaution.


On sizes, I looked at what ToolStation and Machine Mart have to offer, and these seem pretty well identical to the Parkside-branded kits save for being Imperial and including the double-flare dies the Parkside tool omitted. Enlarging the photos, the clamps on these seem to be serrated. They might not mark steel or Kunifer tube as much as they do copper, but no-one would notice it on a car brake-pipe anyway.


Regarding appearance, a common criticism of model fittings is that they, and particularly the union nuts, tend to be over-scale and often the wrong type anyway. My thought was that a flared pipe allows for a slightly smaller nut, so a neater appearance, assuming the lack of a visible tail to the union does not matter.

In full-size, unions were used but generally on smaller pipes and service installations, and sometimes with slotted rather than hexagon nuts. Otherwise it was more usual to use 2- or 4- bolt flanges; though some fittings were screwed into boiler bushes.

The official BTC Handbook issued to BR steam-locomotive crew-men reveals quite a mixture, but one significant detail is that the injectors and gauge-frames normally had bolted flanges (with a union for the latter's blow-down pipe only); but the brake and lubrication pipework was more generally "unionised". Both flat-face and cone unions were used, with hexagonal and circular nuts; and it's hard to see any pattern beyond established drawing-office practices among the railways and the individual components' manufacturers!

The diagrams show the larger unions on e.g. the ejectors, having slotted-circular nuts. The disadvantage of slotted nuts of course is that you need make them and their spanners. I have not seen definitive proportions but for strength, the floors of the slots cannot be closer than the across-flats width of the equivalent hexagon nut, so the overall diameter is not much below the A/C diameter.


My road-steam books are even less forthcoming, while my old generic engineering-design literature concentrates on the joints for large service and hydraulic installations, and malleable-iron fittings for threaded tube.

The one photograph I have for my steam-wagon's ancestor's footplate details shows the auxiliaries supplied through iron pipes with screw-on fittings from unions on valves screwed directly into the boiler bushes. The regulator is a very visible globe-valve with 4-bolt flanges on a big wrought-iron swept bend rising from the boiler top. No shelter from the elements: the non-superheated steam was somehow expected to survive this exposed, un-lagged plumbing well enough to drive the compound engine, then have enough strength left to drag the combustion gases up the tall chimney several feet away from the engine. All the valves and pipe-joints were probably commercial stock items.


I have not seen octagonal nuts on domestic plumbing parts. Those I have seen were large-diameter, galvanised-steel parts for heftier pipework such as might be used in commercial premises.

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