A rotary locking cylinder for a safety lock has a housing and a crosspiece. A recess is incorporated in the crosspiece from the top, a locking element being inserted into this recess. The recess connects an upper outer side of the crosspiece with a transverse bore hole in an arm of the crosspiece. In the assembled state, the crosspiece is inserted into a recess of the cylinder pocket and connected with a transverse pin. In order to secure the transverse pin against longitudinal displacement in the cylinder pocket, this transverse pin has a circumferential groove in which the locking element engages from above. In order to exchange the crosspiece, the rotary locking cylinder is rotated about its longitudinal axis in such a way that the cylinder pocket is on top. During rotation, the locking element falls downward due to the force of gravity and releases the transverse pin. The rotary locking cylinder can be lengthened or shortened at the installation site without a reduction in strength by exchanging the crosspiece.