A locking mechanism for a rotary power tool that includes an elongated locking member that is retained by, and is at opposite first and second end portions within, at least one of a motor housing anda gearbox end casting and being slideable between unlocked and locked positions, the locking member first end portion being accessible by a user to move the locking member to the locked position. Thelocking member also includes a locking portion intermediate the first and second end portions that is configured to engage the non-circular configured portion of a rotatable armature shaft and preventrotation thereof when the locking member is in its locked position. A biasing element is also included and configured to bias the locking member toward said unlocked position.