Hi again ,

A lot depends on whether the tubes just act as flues or whether there is substantial dependency on them as load carrying stays as well .

Full size tubes were fitted in lots of different ways early on but the favourite was 'clunking' (to use R Stephensons delightful term ) . Basically tubes were made with a bit sticking out each end and after expanding into tube hole the ends were rivetted over – giving a very sound joint .

There are two designs of simple expanded fitting :

(a) Expanded into the hole but no more – will seal against pressure but not reliably hold against end loading .

(b) Expanded so that two 'larger than the hole' circular bulges form – one inside and one outside . This is a very strong fitting and will seal against pressure and take end loads .

Other methods use rammed in ferrules and (just rarely ) screwed on ring nuts .

Notes :

To do expanded tube joints properly they have to be dry fitted . No sealant and no solder .

There is no certainty that either thickness of tube that you mention is better than the other for making expanded joints . My best guess would be 18 swg but you would have to do some test assemblies to be sure .

The tubeplate holes have to be properly designed to avoid the tube expanding process from simply shearing the tubes off .

Thats it for now .

Michael Williams .

Most of the tube expander suppliers give the spec for the hole for a given dia and thickness of tube.

This thread on HMEM has some chat about expanders that may be handy

Edited By JasonB on 10/07/2012 19:05:48