I've been thinking about how to fit the wheels for some time ("putting it off" might be more accurate). Seems that people have their own methods, and the instructions touch on this only briefly. I've looked at the design for a jig that fits wheels (and does the quartering) by tightening it a vice. But an Association member who builds his own locos successfully with relatively simple tools has described his method for fitting and quartering wheels which I plan to follow.
First file a chamfer on the end of each wheel axle
I'd already opened up the muffs with a 1.5mm drill. Now I need to drill a 1mm hole through the centre of the muff. This is to allow release of air as the tight-fitting axles are pushed in.
A final check that the chassis sides are still electrically separate, then I fit the first axle by turning each wheel by hand with gentle pressure inwards.
I've put a small washer on this axle to reduce sideways play.
I'd previously calculated that I needed a 0.45mm gap on each side between the wheels and chassis and now I need some spacers.
simple styrene sheet spacer
The key dimension is the distance between the back of the wheel rim and the chassis side. So the gap in the spacers has to be big enough to go around the small raised section visible around the axle, allowing the spacer to lie flat against the wheel. I cut some from 0.5mm thick styrene sheet.
Now with the spacers in place I can gently twist both wheels inwards until they are tight against the spacers. The first axle turns freely so I do the same with the central axle.
spacers on central axle
Now the gears are locking. Even though they had previously been running smoothly, I have to spend quite a while running the wheels along the test track by hand to loosen them up again.
Next to add the final axle, tighten the wheels to 8.5mm back-to-back (or just over) and turn the wheels so that the balance weights are aligned on both sides, with a quarter-turn difference between sides.