Like our 5202 Series Yellow Jackets (which have a 6mm D-Shaft), these 5203 Series Yellow Jackets feature an FTC legal motor with the goBILDA Pattern on the face, a high-quality encoder, and convenient bullet connectors. 
This series of Yellow Jackets features the primary shaft profile of the goBILDA® build system - 8mm REX™. This already strong shaft goes up to a 9mm diameter inside the head with dual ball-bearing support, and the 36mm diameter gearbox contains all steel gears. The screws mounting the gearbox come in from the sides, which gives them even more resistance to loosening over time. Additionally, the head's shape provides space for nearby components.