Constructed of glass-filled nylon, these units have positive up-and-down locks, ... Constructed of glass-filled nylon, these units have positive up-and-down locks, a smooth, dependable cam operation, and a tempered-steel, 1/8' (3.2mm) wire strut. The 1-oz (28.4 grams) micro retracts can be operated with standard or retract servos. F-RB123/4 are for small electric and I/C models.