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How to repair a broken or damaged thread

In the past, a damaged or broken thread would be a major setback to production and likely mean scrapping a job or component. However, this is now no longer the case; thread repair kits can now be used to save those oversized, stripped and damaged threads.

You can also use thread repair kits to strengthen threads, especially in soft or brittle materials like aluminium, plastics and cast iron. This will allow you to tighten the bolt screws further without risking thread stripping.



How do they work?

When you have determined that a tapped or threaded hole is no longer suitable for fair use, you will need to use the thread repair kit to drill the threaded hole to a larger size (yes, you read that right!). The size of the hole and repair tap are pre-determined and all relate to the size thread you would like to finish with (e.g. to achieve an M6 X 1 finished repaired thread, you would drill the hole to 6.2mm dia and use the over sized tap provided in the set).

The main reasons to do this are so:

  • The existing hole can be cleaned.
  • Any sign of the previous thread is removed.
  • We can start the threading process from scratch.

This new, larger hole can now be threaded using a special thread repair tap. Even when tapped, the hole will still be larger than the actual finished size that is required, but don’t worry: it’s an integral part of the process!

To match the intended threaded hole size with our bigger hole, a tough stainless-steel insert is used. This is driven by hand into the newly tapped hole using a special insertion tool, which is screwed to depth via the tang on the stainless thread insert but is later removed. Once the insert is sitting flush or just under the surface of the hole, another special tool called a break tool/stamp will be used to remove the drive tang used to screw the insert to depth. (the broken drive tang will need to be removed from the bottom of the hole.

With the thread repair insert in place, you are now ready to use a freshly revived and strengthened thread.



To summarize...

1. Drill out the damaged bore/thread with the specific drill for the thread you are wanting to restore.

2. Clean the hole with a cloth or cleaning aerosol, ensuring no swarf is left in the hole.

3. Run the tap into the hole to the correct depth either by hand or on a machine tool.

4. Clean the thread with a cloth or cleaning aerosol, ensuring no swarf is left in the hole.

5. Screw in the threaded repair insert using the insertion tool.

6. Break the tang that you have used to drive the wire to depth using the break tool.

7. Clean the thread and remove any dust/debris.

8. You are now ready to use the repaired thread.



Need more help?

Cutwel supply a range of inserts which cover the depths from 1xD, 1.5xD and 2.5xD in metric form only.

We also supply a range of STI thread plug gauges to ensure the tapped hole is correct and within tolerance.

To discuss further please get in touch with the technical team on 01924 869615.