Here was a curious (not to say irritating) thing; I made a batch of five injector bodies. I happened to have some nickel brass in stock, so I used that. Some folk call it nickel silver, others German silver, but the "official" term seems to be nickel brass. Anyway, I used it because it's a very similar colour to silver solder, so it might not look so very obvious a fabrication.

All went well; I made them all with the same bar, used the same brazing fixture and the same stick of solder for all five. Four were absolutely fine, but one has cracked to a quite amazing degree:

What on earth could have happened? I can only imagine it's a material fault in the bar; but I hope it's not something that will suddenly appear in the others. I have heard about brass self-destructing due to high inbuilt stresses (stress-corrosion cracking) after extruding – particularly with thin sections, but not 3/8" round bar, surely?

Next batch will probably be in brass…

Try and enlarge them a bit…

Seems a bit clearer!

Ha! The mystery is solved! I've just picked up a "good" injector body and done a careful length check on it to be sure of its actual dimension before I started on the cones. It was miles out! Whaaat??? Check the others… They had all shrunk in length by about 0,2mm – and I'd made them all to within 0,01mm. What must have happened is that the body had expanded during heating, but my brazing fixture is made from a big block of hollow square cast iron. The body had nowhere to go and so it got squashed as it expanded and softened.

Thinking about it I've often seen dents in brass parts of a fabrication when I've tightened up a G clamp or a fixture screw a bit too tightly.

I've just taken a length of nickel brass bar now, turned it accurately to length and heated it up to dull red just by itself. No change in length once it had cooled down, and no cracking.

Then I put it in the fixture, heated it up to dull red again, and surprise, surprise, it's 0,2mm down on length and has cracked.

My scrap bin now has five too-short nickel brass injector bodies and I'm changing the way that the bodies are held to ensure they can expand unrestrained when heated. Back to the lathe now to make some more – and in nickel brass because I still have a fair length of the stuff.