ceramic bearings

before them.. we used dentist drill bearings... super high speed.. stainless steel..

on the 27 turn Stock motors.. it was possible to alter the brush hood for the brushes to change timing....
cut the commutator down small.. more rpms... and power..
as there was more brush to commutator contact.. but cut the brush face to reduce friction. and the comm drops.
then drill a very small hole into face of brush.. insert cotton.. so you have more comm drops on the brush longer.

not about motors... but the 1/10 scale carpet pan car

1/2 inch thick by 6 inches by 6 inches.. machined True flat steel plate
1000 and 2000 grit wet/dry sand paper.
to flat polish the diff rings..
then get from US bearing..... 1/8 inch ball bearings..... grade 8 steel, ground to 0.00001... true.. 100 in a box.
for a super smooth diff... a smoother diff has more speed.

I could spend 10 hours per week.. just to get ready for the STOCK class pan car Saturday night Race.. for years...
I started RC racing in 1991...