How to Drill Out a Broken Bolt or Stud

How to drill out a broken and clean out the threads using a tap. This is one of the many methods for dealing with broken bolts, you can also weld on a nut, drill the center and use an extractor, grab the remaining stud from the backside using interlocking pliers, etc.

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Tools/Supplies Needed:
-drill
-drill bit set
-tap set
-center punch
-hammer
-file
-safety glasses
-cutting fluid

Procedure:
Starting with the first one, you may need to flatten out the surface which can help when making a center mark. This isn’t always possible, depending on where the bolt broke. Considering this is on a bracket that isn’t very rigid, I placed a metal wedge behind the bracket to prevent any flexing or vibrating.

Using a center punch, mark the center of the bolt. This can be sometimes tricky, especially when you’re working in awkward positions. If the mark is off, angle the punch to push the center mark over.

First starting with a center drill. A smaller drill bit can be used, but there may be too much flex so you can risk breaking the bit or having it wander. A center drill is much more rigid which allows for accuracy. A cutting oil or fluid does help with lubrication, keeping the bit cool and preventing it from burning out.

Once that center has been made, move up the correct sized drill bit. You can use an indexing chart that determines the size of the drill bit required for the size of the bolt used. Use an indexing chart to determine what size drill bit is required for the threaded hole. This is where you’ll need to know the bolt size. You can go with a slightly smaller bit just to ensure you’re centered. Left-handed drill bits can also be used, these would turn in the counterclockwise direction. Sometimes the forced from these will help pull the remaining stud out. When the hole is perfectly centered, then finish up with the appropriate size based on what is required for the tap.

Using cutting oil again, drill the center of the broken bolt. Unfortunately, this can be tricky due to the awkward workspace, and depending on the material, sometimes the drill will cut into the softer material, other words not being the remaining bolt so that would damage the existing threads.

Take your time when drill and try to keep the drill square with the hole.

If it starts to wander in one direction, you can push it over to the opposite direction or pull the bit slightly. Be careful not to elongate the hole or break the drill bit when doing so. Apply more cutting oil when needed. As mentioned earlier, the cutting fluid keeps the bit cool and reduces the chance of it burning out, keeping it sharp longer.

Once done, when inspecting the hole you can sometimes see the edge of the threads.

Apply more cutting fluid and using the correct tap which matches the existing threads, the hole can be cleaned up. Here I am using a straight fluted plug tap because I don’t have much room in behind if I was using a tapered tap. If you’re working with a blind hole, a bottoming tap would be the best option. When using a tap, try to keep it square with the hole. You’ll need to run it in slightly, then loosen slightly, and then run it in some more and repeat. This will help reduce binding, apply more cutting fluid when needed, and don’t force the tap as you can risk breaking it.

And once done, the threads are as good as new.

If you’re dealing with a stripped head or in this case, the bolts were seized and the brackets are too soft, so it was causing them to twist. The head of the bolt can be drilled, this would separate the head from the stud so you’re able to remove whatever is being held down. The same process can be used, marking the center with a punch, use a center drill, and then follow up with the correct sized drill bit.

When done, then you can continue with disassembly and in this case, cleaned up the rust and repainted everything. I do have a video for the full process on that, so be sure to check it out.

If the bit wanders and you end up damaging the existing threads, this can be fixed but it does require more work. The hole can be welded up and then re-drilled and then tapped again, a threaded insert can be used, or you can go up to a larger sized fastener.

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