Abstract: A supporting and centering arrangement in the form of a star the arms of which are radial with respect to a tapping to be machined or checked. These arms have elements for centering and fastening them to the tapped wall, as well as a centering element at the projecting end of the tapping. The support and the centering element carry bearings. A tool holder bar connected to two rotating stub shafts is mounted off-set from the axis of the bearings in the radial direction of the tapping. The bar supports a tool guide, and is rotatable about the axis of the tapping. The device is especially useful for machining and checking tappings formed in a pressurized water nuclear reactor tank.