Michael Clifford
I have solved the problem of the excessive back-to-back.
I used an old OO Romford axle, cut of the threaded portion from one end so that it would not protrude beyond a mounted wheel.
I mounted a troublesome wheel ‘inside-out’ on the shortened axle and set the axle in a Dremel vertical drill. Checked the wheel was on squarely, placed a file under the wheel, ran the Dremel at its lowest speed setting and gently lowered the wheel onto the file.
After about 30 seconds I removed the wheel and checked the thickness measurement, and mounted the wheel on an EM axle and checked the back-to-back.
By trial and error, I have removed sufficient of the wheel boss on the inner face to obtain three sets with a back-to back of 16.5 mm rather than 17.0 mm.
Inspection of the wheels reveals that the tread width is slightly more than the width of the cast wheel centre, such that the tread slightly overhangs the centre. Sometimes the overhang is greater on the inner side of the wheel, sometimes on the outer. If the inner side of the tread aligns with the boss, the wheel is probably OK with regards to the back-to-back. If it doesn’t align, i.e. the tread overhangs the outer edge of the cast wheel centre, then the back-to-back is excessive.
Mike