an apparatus for preventing disengagement of an axially movable and rotatable flexible core of a flexible shaft attachment , which has a driven end and a handpiece end , and the driven end has a ferrule for attachment to a rotary power tool . the rotary power tool has an exteriorly threaded hollow output shaft with a first predetermined interior depth for receiving the base end of tool bits that can be used with the power tool , and the flexible shaft attachment has a drive cap attached to the output shaft , and the drive cap has an aperture sized and shaped for receiving the core in fixed rotating engagement . the apparatus includes a generally cylindrical stop plug having an open end portion and a closed end portion , and a cylindrical chamber extending from said open end portion to said closed end portion . the open end portion has an outward extension adapted to engage the end of the output shaft and be held in place by the attached drive cap , and the chamber is capable of receiving the core therein . the distance from the open end portion to said closed end portion is less than the first predetermined interior depth , and the stop plug thereby limiting the depth of penetration of the core in the output shaft .