Trust Roger to come up with that particular “cunning plan”, one that I’d almost forgotten about! Years ago I made a polystyrene mould for a ¾ ton lead keel by much the same method; there wasn’t a church roof in Essex that was safe!
Erik: Could you elaborate on why you want/need to replace the piston? It might help in stopping me offering daft ideas if nuffink else – like that’ll stop me!
Roger’s suggestion is a good way to go; surely it is possible to find a foundry within reach? Braintree College (in Essex) made and operated their own small blast furnace a while ago, probably the Health & Safety Mafia have put a stop to it by now but it goes to show that sometime it pays to cast your net wide.
My only reservation in this case would be that while burning the polystyrene out with the molten metal might be OK with a big mass like a machine casting, for something of small mass with thin sections I think I’d want to burn out the mould before pouring, it’s hard enough to get a really sound casting without adding a source of gasses!
What about coming at the problem from the opposite direction? How about sleeving the barrel and having it bored to a slightly smaller size? The existing piston can then be ground to fit or even the bore honed to suit the piston “as is”, the photograph may be flattering it but it looks servicable in the picture. My thinking here is that at least it saves the problem of having a new casting made and even if an off-the-shelf liner cannot be found good quality iron to make one from is readily available from metal stockists.
That would leave just the problem of what to do about new rings.
Taking one step back for a moment, if wear is not too serious a new piston and/or liner might not be needed anyway, you might just get away with new rings only. Now making rings is not a “garden shed” exercise if you think in terms of today’s hi-tec methods but then when your machine was made they didn’t have access to them either! They will have used a rather more lo-tec method and given a halfway decent lathe it is entirely possible to make good serviceable rings in the “shed”. It is only a matter of method and a bit of careful turning but the method is a bit convoluted and a description rather too long for this forum.