In a joint for joining a first and a second part so that these first and second parts maintain a predefined rotational position in relation to each other, where the joint comprises an internal thread (33) in the first part (34), which internal thread has an inner end and an outer free end receiving an outer free end of a corresponding external thread (22) on the second part (21) which external thread has an inner end adjacent to the second part, the free end of the ridge and the inner end of the groove forming the threads are bounded by surfaces (27, 39) lying in a plane through the axis of the threads, and the length of the threads are so adapted that a plane end surface of the ridge of one thread engages the plane end surface of the groove of the other thread when the two parts are screwed together and occupy the predefined mutual rotational position.