How to Repair Stripped Threads in a Bolt Hole or Nut
Striping a bolt hole or nut's threads can ruin an otherwise easy repair. A bolt hole or nut's internal threads strip when the bolt or nut is over tightened. Soft aluminum parts are notorious for stripped bolt holes but any bolt hole or nut can strip. In many cases a stripped threads can easily be repaired.
Bolt threads are available in both metric and standard, sometimes called SAE or USS, threads. SAE threads are sometimes called fine threads and USS are sometimes called coarse threads. It is important to determine which thread type, thread pitch and the diameter of the bolt hole before trying to repair the stripped threads. Metric and standard look similar but are not interchangeable.
An example would be a standard thread 1/4 - 13 X 2 bolt's diameter is 1/4 inch and has 13 threads per inch and is 2 inches long. A similar looking bolt, but not interchangeable, is the metric thread M6 - 1.75 X 50 bolt. It has a 6-mm diameter with a thread pitch, the distance between threads, of 1.75 mm and is 50 mm long. Before repairing threads always match a bolt to the hole then repair the hole using that bolt as a guide.
Most stripped threads are only partially stripped. These can be repaired with a tap and die set. A tap cuts female threads in a hole and a die creates male threads on a post. To repair a partially stripped bolt hole; clean the hole with a light oil then dip a tap into a heavy oil.
Place the tap into the hole and start screwing the tap into the hole with fingers. Use care to avoid cross-threading the tap with the stripped threads. Once the tap has finger tightened place the tap's handle or a wrench on the tap's end and turn two full turns, then remove the tap. Clean the tap in oil and repeat the process until the hole has solid threads.
If a hole's threads are badly damaged, then the hole needs an overhaul. There are two easy solutions. The first solution requires drilling the hole to the next largest size. Then create new threads with the next sized tap. Always keep the tap well oiled and clean. This process takes patience but the finished product is as strong as a factory created thread.
The second solution involves using an after market thread repair kit. An example would be a Heli-Coil kit. These kits include a tap, a threaded insert and a setting tool. Using these kits still involves drilling the hole with a larger sized drill bit; the kit will specify the correct sized bit.
Run the included tap into the hole. Place an insert onto the setting tool and screw the insert into the hole. When complete the hole will accept the same sized bolt as the original.
Above all, always remain calm when repairing stripped threads. Trying to rush through this job can destroy expensive parts. On the bright side, these repairs often are not as bad as they look like at first.