Any joining process that involves molten metal cooling to room temperature will cause residual stress in the area of the joint. This is because the metal solidifies at a high temperature then shrinks as it cools down to room temperature. This shrinkage is similar to the effects seen with bi-metalic strips and the forces can cause distortion in the assembly. If you machine something with large stresses after it has cooled the stresses can equalise out in a new distorted position, stainless steel has a large coefficient of expansion so suffers from this if too much heat is used when welding. Stress relieving can help in some cases. All you can do with your assembly is leave material to be removed after the joint is made and do the final work when it is cold. You may find some movement occurs in the material as some is removed. For instance if you were turning a diameter you may find that the amount removed does not result in the expected diameter.

Martin

Half inch machine reamers are cheap cheap these days. You can also buy a good British brand name used reamer on eBay even cheaper and usually they are still plenty good enough for home use. Will ream pretty much right up to the end of a blind hole.

Silver solder first, ream later is the only way I can think you will get the job done.

I've never made anything so small, but when I worked in engineering pretty much any fabrications that required accuracy would be welded first, heat treated then machined. On large pressure vessels the amount of movement post heat treatment, was sometimes staggering.

Could you not have soldered first and then bored out on the lathe or mill ? I realise it would be a bit awkward to hold the part, but that flat base should give a good start.

i guess you might be able to clean up the existing bore by boring out slightly larger and making a new piston, or sacrificing your existing reamer and chopping the tapered portion off.

Or a D bit – will cut right to the end. A fiver from Tracy Tools.

Hi Greg

This is a problem I too have encountered. Here is how I solved it. First plan for a thick wall, 1/8 minimum after reaming. Lay barrel flat and put in place the joining web (hold it any which way you can), apply heat evenly and solder in place. Be sure not to use higher temp silver solder or the whole thing might just melt! Now put in 3 jaw and drill/bore to the desired undersize for reaming. Next solder the base plate in place – the previously added web will help stop things distorting – apply heat to the outside of the base but dont clamp anything as if you do it will surely distort. You can then add the end section and hand ream as described by others. When I make mine I tap the end and screw on the ball valve section to do away with the risk of any distortion after fitting the base. If you go down this route you can solder it all together and then bore and ream in the lathe. I also always add several O rings and find that they are 100% reliable. Hope this helps.

You can't do this with a blind bore, but with a through bore you can regain some hardness in the brass by forcing a hardened steel ball through the bore, it also makes the bore round and parallel.

Ian S C

And you can also get a knurled effect where the jaws of the pliers gripped the job too.

Don't ask me how I know this…