An actuating assembly for an aircraft engine thrust reverser includes a drive shaft bearing a master pinion; a ball screw bearing a slave pinion engaging with the master pinion; a nut capable of translating over the ball screw as a result of the rotation of the ball screw; an extension tube rigidly secured to the nut and a lock assembly to block the rotation of the drive shaft in a so-called direct jet position of the reverser. The extension tube includes, at its free end, a member for connecting to a portion of the reverser prior to being actuated. In particular, the lock assembly includes an abutment secured to the drive shaft, and a cotter moveable between a locking position and an unlocking position. The reduction ratio (R) between the slave and master pinions is even.