How to Drill and Retap

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When you're installing a bolt into a threaded part, sometimes the bolt will break. Should that happen, you will be stuck with a broken bolt inside a part, and in worst-case situations the bolt will break flush with the part into which it's bolted. To fix the problem, you have to remove the bolt, then you need to drill and retap the threads so that you can install a new bolt. This process takes some time and can be quite frustrating, but you should still be able to get the job done with a bit of patience.

Things You'll Need

  • Rotary tool with grinding stone
  • Automatic center punch
  • Drill
  • Carbide-tipped drill bit set
  • Penetrating oil
  • Screw extractor set
  • Tap
  • Tap handle
  • Look at the bolt in relation to the part into which it's threaded. Grind the top of the bolt using the rotary tool and the grinding stone so the surface of the broken bolt is level with the surface of the part.

  • Put the automatic center punch on the center of the bolt. Push down the automatic center punch to mark the center of the bolt. Insert a small drill bit into the drill and place the tip of the bit on the center punch mark. Drill into the center of the bolt with the drill, keeping the drill parallel to the bolt so that it doesn't drill into the surrounding part.

  • Step up to progressively larger drill bits until you have a hole in the center of the bolt that's large enough to fit one of the screw extractors. Spray the bolt with the penetrating oil.

  • Put the screw extractor into the drill. Insert the tip of the screw extractor into the hole in the middle of the bolt. Turn on the drill and pull the broken bolt out of the part with the screw extractor.

  • Install a tap into the tap holder that is the same size and thread pitch as the broken bolt. Apply some penetrating oil to the tap. Thread the tap into the part by turning it clockwise 1-1/2 rotations, then back it off and turn the tap handle counterclockwise 1/2 rotation. Repeat this process until the tap either bottoms out on the bottom of the bolt hole or you can't use the tap any further.

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