So, why do you need the spacer? Why can't you just put the thrust bearing on the bearing directly? The thrust bearing itself is balls between two spacers
You got the point following the link.
In the short way, the spacer is a must use because it won't fit directly to the shaft. The shaft diameter is 1/4inch (6.35mm) and the inner thrust bearing is 4mm diameter so the thrust bearing is located on the axle screw and can't go further.
Sparks by a metal-to-metal contact: are you joking?
Yes, Nylon has got some interesting friction properties, but why is that important here?
It's a kind of joke. In this case, sparks won't appears but who know. Basicaly, one metallic part moving relatively to another metallic is not the greatest adjustment, especially when both are iron or aluminum made, at one moment or another, depending on speed, pressure ... those to parts will solder one with the other. But as said, I use plastic in first instance because it's easier to machine and that was the material I had access. In aluminum, that will do the job too.
Thanks, this last bit is clear, in fact this would explain the role of the spacer, however this makes unclear to me why you made the outer diameter of the spacer larger (10,5 mm).
I really want to understand this, this is why I am asking.
I made the spacer 10.5mm to fit correctly with the outer ball bearing ring. Smaller it would have been in contact with the ball bearing shield. So I make myself sure the lip will touch this outer ring.
If you have a vernier and that sort of bearing, check the outer diameter of the ball bearing. The bigger diameter is around 10.5mm and the smaller diameter of the outer ring is around 9.5mm (recalling data). The lip is about 1mm thick so it touch fully the outer ring.