Adding MR522Z / MR52ZZ ball bearings to LEGO train wheels to reduce rolling resistance of my LEGO MOC trains. I also used DIN 913 M5x10 grub screws to fix the ball bearings inside a standard LEGO train wheel holder (part number 2878) … a method invented by Maciej Drwiega.
I bought the ball bearings and the crub screws on aliexpress.com
https://www.holgermatthes.de/bricks/