Abstract: An anti-jackknifing device for a tractor and trailer combination is provided having a pressure cylinder with an axial bore which mounts to the lower surface of a fifth wheel plate of the tractor. A piston locates within the axial bore and has a stop pin which project into a stop bore formed in the fifth wheel plate and into a slot formed in a trailer plate of the trailer. An end plug couples to the pressure cylinder for sealing the axial bore and is removable to allow the interior of the axial bore to be accessed. Also provided on the piston is a socket which allows a piston removal tool to be inserted therein so that the piston can be removed from the axial bore when the end plug is removed. The structure of the anti-jackknifing device prevents stresses on the fasteners or bolts used in securing the pressure cylinder to the fifth wheel plate as a result of lateral forces being exerted on the stop pin.