Why not thread the hole and screw it on to a carrier bar of suitable size. Once the flange is machined you can loctite the casting to a bar with the correct diameter to machine the rest of it. Add jubilee clips for extra holding if needed.

Martin

Here is an idea: Turn a piece of wood to the diameter of your boiler (where the chimney is to fit). Wrap it with a spiral of emery held with a drawing pin at both ends, leaving a section of a few inches between the pins. File the inside of the chimney face to remove rough casting flash etc, and attach it to a handle where the chimney proper will fit. The thread idea above seems sound to me for this. Then offer it carefully to the rotating emery, and with care this should produce a radius to match your boiler.

You might find it helpful as an alternative to slot the wood lengthwise, and use a full sheet of emery held in the slot. And wet & dry works as well as emery, as will garnet paper if your chimney base is brass.

This should give a good enough fit for soft or silver solder – but do be sure to remove every speck of emery or the solder might not run so well.

Regards, Tim

I have always done as you suggested, finish turn the bore and make an expanding mandrel long enough so you can hold it in the vice on a mill and use a fly cutter or boring head to finish the smoke box radius. You can then transfer to the lathe to finish turn the outside. I have given up using castings for chimneys as there never seems to be material left on where you need it and it is cheaper to buy a piece of continuous cast iron bar and use that.

Clive chimneys are usually attached with 6 or so small hex head screws.

Michael