I'm also interested in 7mm Narrow Guage and am putting together an old Wrightlines 0-4-0 Chassis
Having put the first pair of Alan Gibson Wheels on the Chassis we checked the back to back and it seemed 1mm greater than my colleagues competed Chassis and the OO gauge back to back Gauge
Have you come across these longer axles and have you any ideas how to get around it as I cant see how to easily remove the wheels
Well I'm no great expert on Gibson wheels but wonder if the axles were wrongly packed and you have an EM set ? That would give just over 1mm too much back to back. If not it may be a manufacturing fault and I wouldn't put any more wheels on before checking !
Anyway, to remove the wheel, assuming no superglue has been used to stick the centre to the axle you need a wheel puller. Assuming one isn't available, and lets be honest it's not the sort of tool most us buy, try supporting the wheel in a vice with the axle loosely gripped in the jaws. Then tap the end of the axle with a small hammer and punch (a small screwdriver or scribber will do). The support in the vice ought to mean that the pressure from the punch is taken on the centre of the wheel NOT on the spokes. There may even be some mileage in sticking the wheel in the freezer over night as the metal axle will shrink slightly compared to the plastic centre. No idea if the plastic an handle the cold though.
With the wheels off then check the axles for length. It might be possible to fix the 1mm error by countersinking the back of the plastic wheels to move them in a bit. No guarantees here (or anywhere else since you ask) and getting the wheel on perpendicular to the axle might be interesting. Better still, use a set of Romfords...
One thing I do know about Gibson wheels - they are allergic to Loctite. To stick them to axles use superglue. Loctitie makes the plastic crumble which is interesting but not all that useful.