Further to the reported problems converting to P4, I received my wheels from Alan Gibson this morning (very prompt service) so set about doing my three MDV wagons.
Firstly some comparative measurements done with my digital calipers.
Accurascale; axle length 25.35mm, axle diameter 1.5mm, wheel diameter 11.8mm.
Alan Gibson; axle length 25.90mm, axle diameter 2.0mm, wheel diameter 12.3mm.
With the brake gear removed I found that on my wagons at least the new wheelsets ran quite freely with no adjustments needed. In fact I was surprised how easy they went in considering the longer axle length.
However fitting the alternative brake gear did cause some binding issues. Partly with the brake shoes rubbing on the wheel treads (possibly due to the larger wheel diameter) but I think mainly due to the link joining the two yokes together. This link passes over the top of the axle and as supplied just about clears the 1.5mm diameter axle but binds quite severely on the 2.0mm diameter Alan Gibson axle. A bit of work with a file on this link plus the faces of the brake shoes soon provided the necessary clearance to allow the wheels to turn freely again.
Maybe I've been lucky with mine I don't know, I'll see if my three Coil A wagons turn out the same.
I do wonder if Accurascale actually tried fitting some EM/P4 wheels during the development stage but hope they take note of these comments and make due allowances on future models. Some comment/feedback from them here would also be appreciated.