Lego axles in the most popular sizes available, including:
2, 3, 4, 5, 6, 8, 10, and 12.
These are all measured from real Lego axles and printed to check if they work correctly. They do.
The ones printed in ABS seem to be the best choice strength-wise. I sheared a few original axles making torque devices out of gears and axles, and the ABS is best. However, Taulman's 618 nylon filament also works wonders for these.
Print at 100% infil, 0 shells for best results.
Three ways to print:
-Print on end, with rafting.
-Print on side, with support and rafting.
-Print on side, rotated 45 degrees, no support, no rafting. (Best option)
-Print at a lower temperature than you usually print.
-If able, pause briefly between layers.