You could try Noggin end metals, they are listing 4" 10swg approx 3.25mm (i think) but limited stock. Also look at M-Metals both do C106

David

As far as I can tell from the specifications almost any copper tube will do for a model boiler provided the wall is thick enough.

Although copper tube comes in bewildering variety, for drinking water, drainage, brake-line, chemical, refrigeration, electrical and medical purposes, the differences in terms of heat resistance and physical strength as needed for a small low-pressure boiler are minimal.

Some problems with different coppers cancel out. Water pipes come hard or semi-hard whilst brake-line and medical tube are both annealed (soft). I don't believe it matters. After brazing, water pipe becomes at least partly annealed, but it's still as strong as before and pressurising the boiler will tend to re-harden it. Annealed copper, though soft at the outset is still strong, and it too work-hardens on the job.

I suggest the main thing to avoid is thin-walled tube and buying specials like medical or refrigerator tube new. Not because they're mechanically inferior but because they're expensive. Buying medical grade copper tube is a waste of money.

Sam mentions a concern about concentrated heat on the tube even with water inside. Shouldn't be a problem. Water cools metal faster than a flame can heat it, which is why the bottom of a sauce-pan on a gas ring only rises to 100°C, not flame temperature. It's even possible to boil water in a paper bag over an open fire. Always exceptions, but a properly designed and maintained boiler should keep itself cool. (About 150°C at 50psi) Fault conditions can fire damage a boiler, for example by turning the protective layer of water into steam, or by allowing scale to build up, or by running the boiler dry. Don't use oxy-acetylene to raise steam quickly!

I've never tried to silver-solder a boiler but experience suggests the main problem is getting enough heat on to the joint quickly enough. Too low a temperature and the solder won't flow properly, take too long getting up to temperature and the flux fails. As a thick-walled copper tube will absorb more heat than a thin one, I suggest 10g tube will be harder to solder than 3mm, which is more difficult than 2.5mm, which is distinctly trickier than 1.5mm. My advice, practice on smaller jobs first and move up step by step because the beginner is doomed to fail on a big job if his torch, hearth and technique can't deal with smaller diameter thin tube.

Trying to save money is asking for trouble. Skimping on a small torch and an ad-hoc hearth, then not practising with real copper and silver-solder because they're costly is likely to be a false economy. Owning a big torch, 50 quids worth of insulating firebricks, and a giant gas cylinder make it all much easier!

Dave

What is a PYRTE traction engine as it dosnt come up on search.

David

Yes I saw that price after posting but it does not say which thickness they are selling at that price?

Did not know if anything else had been changed on the boiler when the tube size was changed

The cheapest way for many is via the club inspector and the clubs insurance but you can go the independant route of a professional inspector and tailored insurance