A thread damage is a quite rare failure type that may require the use of a heli-coil insert that is not based on an international standard.

A thread damage is a quite rare failure type characterized by specific features, especially if, as in the case of the present study, it concerns a threaded coupling between screw and nut of large sizes – equal to ca. 80 mm – that requires the use of a heli-coil insert. Because of its dimensions it is not ruled by any international standard.

Equipment used during repairing was a U-tube heat exchanger, where the tube plate is welded to the header, composed by a forged hollow cylinder and two nozzles – inlet and outlet nozzles – for the fluid that has to be cooled. The header is closed by a flat lid, through threaded coupling with tie-rod stud.

Repairing was necessary, because an interior inspection for maintenance purposes pointed out a thread damage of the tenth hole of the header (starting from the upper part of the device and proceeding clockwise). Therefore, a (spiral) self-tapping heli-coil insert has been used in order to ensure once again the coupling between the lid and the header, through the original tie-rod.

The main steps for the repairing procedure that has been carried out after in-depth computation tests – that ensured correctness and feasibility from a design point of view – are resumed as follow:

1. I Phase: Boring of the threaded hole on the nut side with a matrix bit drill.

2. II phase: Surface magnetic particle examinations for defects testing.

3. III Phase: New tapping of the hole on the nut side through a device provided by the helicoid manufacturer.

4. IV Phase: Installing an heli-coil insert with the specific equipment.

5. V Phase: Tie rod clamping after its greasing.

6. VI Phase: Pressure testing to be carried out.


Each phase has been monitored by Inail, inspection agency, which monitored the repair activity.

The successful outcome of the repair activity is due to the safe implementation and use of the equipment, after it has undergone the controls provided by norms and by the repair procedure.

The inspection agency, the local office as well as the central office, has proved itself to be very available and with great expertise. This, together with the user’s important experience in the field of metal equipment, have ensured a successful repair activity, which provided several challenges and uncertainties.