A locking spacer assembly for securing adjacent rotor blades includes a first end piece having a platform portion and a root portion that define a first inner surface of the first end piece. The root portion defines a first projection and an opposing second projection of the first end piece. The first projection has an outer profile adapted to project into a first lateral recess of the attachment slot. The second projection has an outer profile adapted to project into a second lateral recess of the attachment slot. A second end piece fits between the first inner surface of the first end piece and a sidewall portion of the attachment slot and includes a platform portion and a root portion. A borehole extends continuously through the first end piece and the second end piece. A fastener configured to engage with a sidewall portion of the attachment slot extends through the borehole.