gearbox for WPL B series

This is my custom gearbox for B series WPL trucks (straight chassis). The gears aren't in one row, the motor is in lower position so the center of gravity is lower = better for crawling. For geared DC motors with 2x M2 mounting holes with 18mm distance.

The gearbox hasn't mounting holes, you can drill they where you want.

What you need:
4x ball bearing (3x7x3mm)
2x 3mm steel shaft
3x 21T pinion (D13,8 x L11)
2x M2x20 screw (for motor mount)
3x M2x16-20 screw

21T pinion:
diamet