Hi Mk ,

Jason is certainly giving you good advice – make a new bush .

I've made one Tich boiler and helped with several others.

Here are a few notes on problems that can arise and how to get around them :

(1) When boilers are being silver soldered it is very easy for parts to become loose under heat . They then move randomly and boiler will need remedial work which is not usually very easy . Best to prevent problems by using simple mechanical fixings between parts . Many methods but a few same metal screws or rivets , peened over tabs or plate edges or sometimes just upstanding pop marks will all work depending on parts and location .

(2) The dome bush is not very well designed and a particularly sound silver soldered joint with a little fillet is called for . A way of doing this is to make dome bush sized so as to be an almost precision fit in the boiler shell hole – that is bush will go in easily but with not more than a couple of thou radial clearance . To hold in place while silver soldering either make a little through bolt clamp or use two tiny rivets fore and aft on bush ( located on top C/L of boiler ) .

Some people file tiny nicks in the hole in boiler shell to encourage solder flow to inside of joint .

(3) It is possible to have too good a fit between parts . Make for instance the backhead just an easy fit rather than any sort of press fit . A few pop marks around flange are again useful to maintain an even gap for silver soldering

(4) At several locations it is easier to preplace silver solder than feed in rod . Smokebox tubeplate and tube ends are good examples – sized rings just tight enough fit in/on work to stay put initially while flux melts work well .

Regards ,

MikeW

Edited By MICHAEL WILLIAMS on 04/08/2013 14:51:57

Hi Marinus,

Cadmium free alloys eg Ag40Sn are generally better for gap filling properties because of the wider melting range 650 – 710 degrees and require more heat. It won't cope with 0.8mm though! Max 0.2mm.

A cheaper option is to use 38% silver alloy Ag38Sn

The down side is they are not as free flowing and have higher melting temperatures. Ag40Cd is 595 – 630 degrees.

For good all round advice re silver soldering go to http://www.cupalloys.co.uk.

Go to "Best Brazing Practice" under Introduction to Brazing. There are also tips which include how to make your rings to size first time! Download it for a handy reference file.

Re your firebricks. Do not use conventional bricks or those taken from storage heaters. They ABSORB heat making brazing more difficult. Use lightweight refractory bricks

Incidentally, in this instance, it is cheaper to buy the silver solder ex stock in the UK and pay postage!

Regards

keith