The LNER Encyclopedia

How easy/difficult would it be to model an O4/4 using a Bachmann O4 donor?

To an extent it's as easy as you choose to make it. The starting point of the RTR model provides almost all the difficult elements ready to run*, the motor will fit inside a round top boiler, and the loco body is screw assembled and comes apart very easily; and then it is for the modeller to decide how this basis is going to be used. First step: obtain the drawings.

The most difficult conversion would be the O4/4, as it requires a frame extension, if you want it accurate.
The O4/5 lets you off frame extension, but now you have to substitute a drum smokebox and cast saddle.
The O4/7 is the easiest option as the replacement boiler 'slots in' between the original cab and smokebox.

I have not tried this, but if Gaugemaster have the parts, then the O2/3 boiler shell mouldings might suit, if you don't want to make the new boiler by DIY methods.
Common to all the conversions, the cab spectacle plate has to be patched, or replaced wholly or in part, as you choose, to remove all evidence of the Belpaire firebox.

I might suggest that it is worth looking for the GWR ROD, if you can get one at a reasonable price; as it has a more accurate footplate casting than the LNER versions. Whether it has some other features that make it less suitable, others might be able to advise.

*All the mechanisms I have seen need the cylinder angle corrected, so that the piston rod axis aligns with the crank axle centre. The only Bachmann model I have purchased with this ugly defect, it screams 'WRONG!', a very unfortunate error.

If there's any way that you can still get hold of one of Bill Bedford's 3D prints of cab and boiler for O4/5, you could, if you must have one of the two O4/4s, very carefully cut and extend extend the boiler barrel in order to get the O4/4 version. You'd still need to make the necessary extensions to the Bachmann frames, running plate and rear splashers of course...

Alternatively, the smokebox, extended boiler barrel and firebox, all suitably re-detailed (especially that smokebox saddle), from an old Bachmann B1 body, or better still from a "scrap" Hornby O1 if you can get one, plus a hand-cut new upper spectacle plate would perhaps do the trick. The angle irons of the cab roof would require flattening, and a mock blister added to the driver's side of the cab to give clearance for the re-located screw reverse mechanism.

Keeping it all neat, accurate, straight and square with invisible joints could be a challenge, but such things are key to a good final model, along with a good paint finish. Of course, if you don't try, you never find out if you can do it.

I'm not sure you could use a 101A boiler from a B1 or O1. The cladding on this boiler is in line with the outer diameter of the smokebox, while on the 15A boiler of the O4/4 the smokebox has a larger diameter and there is a definite step down to the cladding.

I could possibly extend my O4/5 boiler to produce an O4/4 one, but the hard part is still going to be extending the Bachmann frames.

Good point about the cladding diameter. I missed that one as I was only thinking about the diameter of the barrel itself.

If it were me tackling this project, not that I intend to attempt it, I'd probably find it just as easy to form up a new boiler from layers of pre-curved plastic as it would be to extend an existing boiler that wasn't long enough, and I save on cost of materials too - although not necessarily on cost of time.