The rings 33, 34, which are held in the indicated clamping position by corresponding clamping means, such as clamping screws 44 or clamps, during application of the seam 17 (FIG. 4), are expediently guided on the stand 51 with the aid of guide columns 37 but can be secured on the latter in the position lowered towards the retaining bush 32 with the aid of adjusting rings 43 which can be moved alon