Motor Questions

[flatcar51]

Hi All.
Do all shaft gears on Williams trains motors measure 9mm. 
I have three motors that have 8mm gears on them  which I thought were Williams but did not fit the trucks properly.
Got new motors from Bachmann and they have 9MM gears and fit perfectly in all my 6 wheel trucks.
Do four wheel power trucks also have 9MM gear just a different shaft length?

Thanks for all replies