Malcolm,
The "O" wheels you speak of are mfg. by both Jeff (imports ), and ours, which are from the old MEW molds, made here in the U.S. I can not speak for Jeff's, but ours do not use any kind of spacers before mounting the gear. When I mount the gears , I use a mechanics socket to 'tap' down the gear FLAT over the wheel gear hub, then peen around the square hub w/ a ball peen end hammer. Probably the biggest error made is in mounting the axle to the wheel. We use an axle tool in a press to guarantee the axle being mounted straight into the wheel. Many of the customers try using a vise, and that can be very frustrating trying to mount straight. A large drill press can be used to mount the axle into the wheel also. Note, it has to be like a 'floor model' to be able to apply enough pressure. The drill press is also great to mount the other wheel. We do not use spacers on the plain wheel side, but only press on far enough so that the gears mesh and side play is only a couple of thousands, ( very slight side play).
If you are re-using older wheels, We use a diagonal cutters to place 2 45 degree burrs on the axle end. This will normally suffice to remount the old wheels.
We also have new brass compound gears to replace the original, also new brass motor spacers, axle bearings, axles & armature gears & bearings collectors & cloth covered solid wire, if you want to go 100% rebuilt.
Harry 
