Threaded Cap for 24 mm retainer. 24 mm retainer set is made of a base unit (MRB-24) glued to the motor mount, and a threaded cap (MRA-24). You can install a base for each rocket and use a single cap for all of them. A threaded coned retainer (MRC-24) can be used instead of the cap to hold the motor in place. 24mm retainers fits Aerotech 24mm RMS motors and Estes motors.