Is the casting cast iron or bronze?

I would make threaded plugs big enough to accommodate the redrilled & tapped holes in a similar material to the casting and screw them in using Loctite or similar permanent adhesive. It used to be No 601 but I'm not sure of any new materials.

Then drill and tap in the correct places. I'm suggesting screwing them in because they will need to withstand the pressure of air or steam.

You won't be the first of us to have a mishap, the important thing is to be able to work around it.

Brian

Richard you can make your cast iron plugs by sawing down the length of a bit of round bar a few times to give you some small squarish sections that can then be held in the 4-jaw to be turned down to size and threaded.  Or if you don't have a 4-jaw then cut the round into six by making 3 cuts through the ctr of teh bar and the almost triangular segments can be held in the 3-jaw.

Another source of small bits of CI is to keep any chucking spigots that are cast into kit parts. If you keep both materials the same the drill is less likely to want to wander away from teh harder material into the softer one.

CI plug in the valve face of this cylinder

Edited By JasonB on 22/05/2017 16:55:06

Edited By JasonB on 22/05/2017 16:59:36

Richard, without knowing how much the holes are off, it could be easier to draw the holes in the covers to match the holes in the cylinder. If this would be blindingly obvious when done, Then don't do it. Incidentally when I undertake such a job, I generally make the covers and then use these to spot through and position the holes in the cylinders. It is generally easier to hold the covers in the chuck on a rotary table than hold the cylinder.

Chris Gunn