Repairing an internal thread

Part 1. Preparation and safety

Objective

• Use a tap to repair damaged threads in an automotive component.

Personal safety

Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:

• Work clothing - such as coveralls and steel-capped footwear
• Eye protection - such as safety glasses and face masks
• Ear protection - such as earmuffs and earplugs
• Hand protection - such as rubber gloves and barrier cream
• Respiratory equipment - such as face masks and valved respirators

If you are not certain what is appropriate or required, ask your supervisor.

Safety check

• Use safety eyewear when using cutting equipment.
• The teeth of a tap are sharp. Be careful when handling the tap so that you do not injure yourself.
• Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.

Points to note

• There are three different types of thread taps: starting, intermediate and finishing.
• A starting tap has a well tapered end, which is why it is sometimes called a "taper" tap. This allows the tap gradually to cut deeper threads as it passes through the job. It can be used to cut a thread in work that has a thin enough section to allow the tap to pass through it. It is also used to perform the first cut in a blind hole.
• An intermediate, or second tap is used for the second cut in a blind hole. It has fewer tapers than a tapered tap, which allows the threads at the bottom of the hole to be more complete.
• A finishing, bottoming or plug tap is designed to cut the final thread into a blind hole. It has almost no taper, so the threads it cuts extend to the bottom of the hole.

• Use a thread cutting compound with the thread tap. It will keep the cutting edge of the tap sharp, allowing it to be used many times.
• When cutting a new thread into a hole, make sure the tap is square to the work piece. This will help lead the teeth to cut squarely onto the shank.
• When cutting or repairing a thread, once the tap has started to cut, turn it about a quarter of a turn then back off, then cut another quarter thread and back off again. Continue until the thread has been cut. This action clears the cutting teeth of any scrap and gives a better finish.
• Once the thread has been cut and the tap removed, clean the new thread with an air duster.
• Do not use an impact wrench on the tap.
• Practice on a scrap component before attempting a real one.

Part 2: Step-by-step instruction

1. Choose the correct tap
   Choose the correct size and type of tap you need to repair the thread, either in Metric or Inch.
2. Determine the thread size
   Determine the thread size of the screw that should fit the damaged hole. Use a thread pitch gauge to confirm the size.
3. Select corresponding tap
   Select the corresponding tap size and type: either a taper, intermediate or bottoming tap.
4. Fit the tap to the tap wrench
   Select either a T-handle wrench or a hand-held tap wrench, and fit the square end of the tap shank into the chuck on the wrench then tighten the chuck.
5. Apply thread cutting compound
   Apply a small amount of thread cutting compound to the cutting teeth of the tap. Position the tap in the damaged hole, making sure that it’s square to the hole and not at an angle.
6. Rotate the tap clockwise
   Slowly and carefully turn the tap wrench in a clockwise direction into the damaged hole. You will feel the tap removing the metal from inside the hole, making the thread. Continue to turn the tap until it has bottomed in the hole or has passed all the way through.
7. Remove the tap counterclockwise
   To remove the tap, turn the wrench counter-clockwise up and out of the hole. Clean any filings or particles from the cutting threads and the flute of the tap before you remove it from the wrench.
8. Clean out the hole and test it
   Clean out any filings or particles from the hole you have just repaired. You can then test the repair by using the correct size screw or bolt and tightening it by hand. Sometimes you’ll find the damage was so severe that the tap wrench procedure fails to fix the problem. If this happens refer the job to your supervisor.