Paul Willis
@Ray Byde said:
Briefly going back to the original post from Bob about whether it is worth kit producrs catering for EM and P4, surely the main diffeence would be in frame spacer width and diffferent wheel profiles? How hard is it to supply alternative spacers for each gauge?
Yes, and no…
The simple answer is that it is not hard. All that you do is draw another set into the artwork at a different width. However that in turn will push up the cost. Brass or nickel is not at all cheap at the moment, and is likely to become more expensive as the pound declines in value versus the dollar (metals are globally priced in USD). I have been watching this carefully as an occasional kit producer 
The second issue is whether the spacings would work out properly. For an inside cylindered 0-4-0, it’s a no-brainer – no problems at all. If there are tight clearances, you may run into more problems in EM than P4. I’ve built a successfully running High Level Y5 GER tank in P4, and and there is literally half a Rizla in space between the front face of the wheel and the back of the crosshead. If you had a wider wheel like a Romfor/Markit type, I’m not sure that it would even fit, even if the frames were moved closer together. And moving the frames closer together would start to cause problems with where the backs of the splashers fell, and so on…
Thinking about it, I’m not sure that I have ever seen a High Level Y5 built in EM, although I know of several completed models in P4.
So you have to consider whether EM-spaced frames + EM thickness wheels is >= P4 width frames + P4 wheels.
Of course, there are finescale EM wheels like Gibsons or Ultrascales, but not everyone uses them.
Anyway, I hope that this gives you a bit of an answer to your question.
Cheers
Paul