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Air tools: Heat treatment, running clearances etc?

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  • Air tools: Heat treatment, running clearances etc?

    I do occasional bits of (full size) hot rivetting, I've been short of a rivetting hammer piston and have been sharing one piston between two different hammers. Now that piston has gone missing (don't ask!) along with my best flush rivetting snap. I've been quoted about 60 quid for a new piston, it's really just a 'slug' of hardened EN36 (I think that's 3310 in US money) or similar. The snaps I've made successfully before, the only place I've found that I can get the equivalent ready made is from the US for a frightening price. These I have hardened myself using Kasenit etc. which has been good enough for two or three days' rivetting.
    Two questions: If I make a couple of new pistons, what is a reasonable running clearance to aim for (on 1 3/16" bore), and if I send them away for professional heat treatment will they come back the same diameter as they left here?

    A couple of years ago I asked the nearest heat treatment people about doing a similar job, they would only treat to their standard depth of case unless I was prepared to pay for their furnace time to be used exclusively for my job, hundreds of pounds. The recommended depth for this sort of thing is 100 thou, several times their standard depth. Does anyone know of a UK heat treatment firm which maybe does this sort of thing (deep case hardening) regularly and so can do my job for a reasonable fee? Alternatively, what would be the effect of putting the parts through the standard treatment more than once?