Rhino,: can you please respond to the question, on the loco pictured in your last post, what is the back to back measurement?
"Fred": yes, technically you can set the back to back to anything.... but it DOES INDEED CHANGE the gauge, and you CANNOT necessarily set the correct gauge if the tread width and flange width are way out of spec....
All of this works together, mess one measurement up, and the others will change, go way off spec on tread width, flange width, back to back and you will have issues.
Not really sure of what the point was other than the back to back is set by the axle shoulder.... then if their 45mm axle has shoulders of 40mm that part is right... if you have issues on 45mm track then either the flange width is out of spec, or the tread width is out of spec, or the track is out of spec, or a combination.
I keep asking questions to locate where the issue is, which will give the answer. So the OP says the back to back is 40mm...
So we need to check that track, but probably little can be done.
So you are down to measuring the gauge, that should be simple... that will tell us the thickness of the flange... which I suspect is very narrow, combined with a tread width of 4mm is not going to work...
Can anyone measure these wheels for these dimensions?
Greg