Andrew,

I would go along with Duncan's suggestion as being the simplest solution and avoid the wider axlebox idea like the plague. It will interfere with axle centres and possibly umpteen other measurements.

I made a big error once but it was more easily solved. Not trusting the worn/damaged end of the rule, I marked out my frames starting from the 1" position. Ended up with large hole in frames for exhaust pipe, 1" adrift from correct position. Cure, drill out new hole in correct position and patch plate over original hole. If it was good enough for the GWR to bodge their frames with riveted on plates then it's good enough for me and realistic!

John

If it's the slot in the steel frame that is oversize for the casting, which I would guess is probably bronze:

First establish which side – front or rear of the axle – of the slot is the over-cut one. Or if both equally, which will make the next bit easier by making the error symmetrical about the axle centre.

Then sweat a brass shim to the horn-cheek side(s) as appropriate. Since it is only a spacer to fill an unintentional gap between two static parts, and takes no tensile or shear lead, soft-solder would be fine.

I know how you must be feeling, I have recently spent the best part of two nights machining a universally jointed drive shaft for a device that I am making. I know the device works because it was demonstrated to me by the designer who personally sent me the drawings before he left us a few years ago, but I had my doubts about the proportions given by the dimensions as I proceeded. It didn't seem to me to be robust enough for the amount of power it was going to transmit etc. but I worked on the principle that those who have had designs published must have forgotten more about engineering than I could possibly ever know so I plodded on regardless. And guess what? I was right and the drawing was wrong, the errors must have crept in when the design was published because when I eventually tracked down a copy of the magazine where it appeared the accompanying photographs confirmed the shaft on the prototype device was much more substantial. So I will have to do it my way guided by the photographs I now have, with another doubt and IMO pointless complexity being removed along the way. I don't mind the time and the lessons learnt so much as the waste of material and the expense of acquiring some more. Good job the MMEX is coming up soon to replenish my stock.

No – Do Not try to correct the mistake in a way that alters the wheelbase!

That is compounding the error and will lead you into all sorts of troubles.

Even a 1/16" is a serious error you'd need compensate for elsewhere: coupling and connecting-rod centres, piston-rod lengths, say. If the driving-axle is in the wrong place then you start putting the valve-gear in the wrong places.

However you correct the horn-plates, do keep the axle-centres to design.

A pair of asymmetric bearings is not quite Gold Medal winning but as long as the axles are where they should be, the locomotive should still work.

Do you know anyone with a TIG, or maybe a local fabrication shop that would build it back up for you for a few quid?

Martin.