One bearing per wheel. So it takes 8 per car along with one pack of Kato wheels per car.
Here is the hardest (and scariest part
)
That bracket is in the way of the file. I've done this about 12 times and never broke one yet. But I also try to be VERY careful to manage the flexing and go slowly. Obviously this is "dangerous".
Also, I found I had to not cut too high (away from the track) when filing the plastic. Lower was better- pretty much retaining most of the top surface.
As long as everything is aligned well, the wheelset should spin freely. A micro MICRO drop of superglue on the outer rim of the bearing holds the bearing to the plastic.