Excessive rust, corrosion or force can damage the threads in a tapped hole to the point where the threaded hole will no longer accept a bolt. In these cases, you need to retap the threaded hole by using a special set of tools called a tap and die set. A tap and die set will contain a large variety of taps that can be inserted into the rusted hole and turned to rebore the hole, removing the rust to create a new threaded hole.
Scrub the rusted hole with a steel wire brush to remove any surface rust and corrosion, then spray some penetrating oil into the hole to help loosen up the rust.
Select the tap of the same size as the hole and seat the tap. Use the bolt that would be inserted into the hole to match up to a tap with the same diameter and number of threads. Since corrosion has made the hole unusable, start with a tap of the same size to see if you can restore the threads without increasing the size of the hole.
Place a drop or two of penetrating oil on the tip of the tap for lubrication and Insert the tap into the top of the rusted hole, making sure the tap is level in the hole.
Turn the tap a few turns, then back it out and inspect the tap to insure that it is level, and that it is cleaning the rust out of the threaded hole. Re-insert the tap and continue to clean out the thread. The tap will have a centre groove to collect the rust and metal that it removes from the thread. Once you have cleaned the hole, remove the tap test to see if the cleaned threads will secure the bolt.
Things you need
- Tap and die set
- Steel wire brush
- Penetrating oil