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It's Stuart Hammer time......

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Sanjay F:
I made a start on cleaning up the mouthpiece and have bored the hole to 0.600" ish and will now make a suitable plug and loctite it into position.

Question

The new centre hole needs to be concentric to the cylinder which shouldn't be a problem as I can dial in the boss on the cylinder side (red arrow). However just looking at the hole on the gland side it doesn't look concentric and it needs to be. Should I also plug gland side and drill/bore straight through from the cylinder side, then flip the piece over in the lathe and dial it in using the new centre hole, and rebore it to 7/8" (blue arrow) and thereby ensuring the alignment?

Jasonb:
Hardly worth plugging the large hole. Set it up in the 4-jaw and clock the smaller hole true then bore out the 3/4" until it is true. Then just make the gland to fit the new larger hole of say 0.900"

Sanjay F:
The hole is plugged and loctited and took me ages to dial in, but it's there ready for drilling.

Looking ahead at the various options for making the slot, I'm going for the insert method by John Bertinat outlined in the earlier editions of Model Engineer (see photo) - why, well it feels the most comfortable to me at my skill level and the equipment I have. In order to do this I have to start with a 1/2" hole to make the outer radii so I'll be drilling the new hole to that dimension and similarly on the new gland once I've made it

EDIT

The JB method calls for a brass/bronze bush to be inserted into the mouthpiece so looks like I'll be drilling out the plug I've just made and loctited! No great loss, it's dialled in and ready for action  :D

Chipswitheverything:
Hi Sanjay, many thanks for your flattering comment on my 'hammer, I blush as I eat my Weetabix here ...   It has been suggested to me that no actual steam hammer ever looked like that, it should be covered in dirt and crud ....     Will be interested of course in how your build progresses.

As Jason has been advising, it looks as if the castings can be sorted out without further assisting the shareholders of Messrs Stuart.
 I made up, as you may have seen, a silver steel dovetail cutter at a more shallow angle than the 60 degree cutters used for machine slides. If you have a small 60 degree one then it may be OK, but if you need to buy one specially, then it might be worth just doing what I did, making a little cutter,  and getting the preferential angle for little or no cost.
The method used by JB for the gland is ingenious, I might have done it that way had I not had the various bits of apparatus that I used on the Myford to slot out the shape.    Personally, I think that Stuart Turner and / or Stuart should, decades ago,  have had a broaching tool made up and done those two awkward holes for the builder, who they knew very well would be faced with something rather tricky ....  Dave

Jasonb:
I can't see any reason the piston rod needs to be shaped as it is. It would work just as well if it were a rectangular section of 1/2" x 7/16" with the corners rounded to say a 1mm radius.

That would make it very easy to mill out the two holes the rod passes through with a 2mm dia cutter.

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