T' diameter o' a hobby rocketry motor casing. Ahoy! Most commercially-manufactured motors are made in standard sizes. Motor diameters are most commonly expressed in millimeters. Arrr! T' most commonly seen motor diameters are 10.5mm, 13mm, 18mm, 24mm, matey, 29mm, 38mm, 54mm, ya bilge rat, 75mm, and 98mm.