Abstract: A tamper proof fastening device includes a threaded portion, a tool engageable portion for engagement by a tool for rotation of the device and drive coupling means coupling the engageable portion and the threaded portion for transmission of rotation from the engageable portion to the threaded portion. The drive coupling means is frangible upon application of a torque in excess of a predetermined torque to the engageable portion. After fracture, the engageable portion is rotatable on the threaded portion without application of substantial torque thereto. The engageable portion includes a generally cylindrical bore and an annular inwardly directed flange at one end of the bore. The threaded portion is provided with an abutment adapted to engage with the flange to restrict against axial separation of the threaded portion and the engageable portion in a direction.