4 ball bearings, v4 60mm dia, others 43mm dia.

use 6mm plastic bb (will not rust in the rain or bath room!). big one for my sliding door, smaller ones for my sliding windows. v7 use 5mm bolt, v7 6mm. they all ride on 12mm dia aluminium tube which acts as guide rail. printed with 0.3 or 0.1 mm hight.