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Screw Extractors: Sizes, Grit Numbers and Broken Screw Removal
A screw extractor is a tapered, fluted tool that bites into the inside of a drilled hole and turns the remains of a fastener out. The range here covers two families: extractors sized by the screw thread they suit, quoted as a span such as M4-12, and extractors sized by number, from 1 grit up to 9 grit. Both work the same way. What changes is the range of screw sizes each tool covers and how much torque it can transmit before the flutes slip.
What decides which extractor you need
- Screw size. A thread-size extractor is quoted as a span, from the smallest to the largest screw it will remove. A number-size extractor is quoted as a grit.
- Broken screw diameter. The extractor has to fit the drilled hole, which in turn is set by the diameter of the screw remains.
- Flute count. More flutes spread the bite and grip more evenly. Fewer, deeper flutes cut into harder material more aggressively.
- Taper. The taper is what makes the tool bite as it is turned anticlockwise. A long taper grips gradually; a short one bites hard immediately.
- Material of the screw. Hardened and stainless fasteners need a harder extractor and a properly drilled pilot hole.
- Access for the drill. The extractor needs a drilled hole first, so the job is really about whether there is room to drill.
Size against the broken screw
| Extractor | Screw range | Pilot drill | Notes |
|---|---|---|---|
| M1-8 | Very small screws | Small, matched to the screw | Delicate; easy to snap the extractor |
| M1-10 | Small screws | Small | Good for instrument and appliance work |
| M3-12 | Common machine screws | Mid size | Low cost entry into the range |
| M4-12 | General workshop screws | Mid size | The most used span |
| M9-27 | Larger bolts and studs | Larger | Needs a drill with the capacity to match |
| M13-32 | Large bolts | Large | Heavy torque; use a tap wrench |
| Grit sets | Numbered series 1 to 9 | Matched drills | Buying the set covers the range |
The size of the extractor has less to do with the size of the screw than with the size of the hole that can be drilled into it. That is why a set is often the better purchase: the correct grit is decided by the hole, not by the fastener, and a broken screw rarely fits neatly into the middle of any single extractor’s range.
The screw extractors in this guide
Every option in this range
Seventeen options: thread-span extractors, numbered grit sizes and sets. Prices are per unit or per set as described.
| Option | Type | Supplied as | Best for | Price |
|---|---|---|---|---|
| Thread extractor | M1-8 span | One extractor | Very small screws | $8.99 |
| Thread extractor | M3-12 span | One extractor | Common machine screws | $9.92 |
| Thread extractor | M1-10 span | One extractor | Small screw work | $10.31 |
| Thread extractor | M4-12 span | One extractor | General workshop screws | $18.77 |
| Thread extractor | M9-27 span | One extractor | Larger bolts and studs | $26.97 |
| Thread extractor | M13-32 span | One extractor | Large bolts | $26.97 |
| Grit extractor | Grit 1 (M4) | One extractor | Small screws | $10.18 |
| Grit extractor | Grit 2 (M5) | One extractor | Small to medium screws | $8.99 |
| Grit extractor | Grit 3 (M6) | One extractor | Everyday screws | $9.78 |
| Grit extractor | Grit 4 (M8) | One extractor | General workshop use | $10.58 |
| Grit extractor | Grit 6 (M12) | One extractor | Larger screws and bolts | $14.67 |
| Grit extractor | Grit 7 (M14) | One extractor | Bolts | $18.24 |
| Grit extractor | Grit 8 (M16) | One extractor | Larger bolts | $21.95 |
| Grit extractor | Grit 9 (M20) | One extractor | Heavy bolts and studs | $26.97 |
| Five-piece set | Grit 1 to grit 5 | Set of five | Small to medium screws | $26.97 |
| Six-piece set | Grit 1 to grit 6 | Set of six | General workshop coverage | $26.97 |
| Nine-piece set | Grit 2 to grit 10 | Set of nine | The widest coverage in the range | $26.97 |
The sets cost the same as a single large extractor, which makes the set the obvious choice unless the work is confined to one size. Beyond grit 4 the single extractors get progressively more expensive because there is more steel and more grinding in each one.
Removing a broken screw step by step
- Centre punch the remains. The hole has to be in the middle of the screw. Off centre and the extractor will walk into the thread of the surrounding part.
- Drill the pilot hole to the right size. Roughly half to two thirds of the screw diameter for a thread extractor, and matched to the grit for a numbered one.
- Drill deep enough. The extractor needs a hole at least as deep as its cutting length. A shallow hole gives the flutes nothing to bite.
- Tap the extractor in. A light tap seats the flutes in the hole so the tool starts to bite rather than skating.
- Turn anticlockwise with a tap wrench. A tap wrench applies even torque on the square shank. Pliers on the shank round it over.
- Add heat and penetrating oil if it resists. A seized screw is seized in the thread, not in the hole, and oil plus a controlled warm is what frees it.
- Stop rather than snap the extractor. A snapped extractor is hardened and far harder to remove than the original screw.
Why extractors fail
The most common failure is a pilot hole that is too small. The flutes cannot bite, the tool skates in the hole and the corners of the flutes round over. When the extractor finally does bite, it bites hard, and the torque then snaps it. The second most common failure is the opposite: a hole so large that the flutes have nothing to grip and the extractor spins freely.
A snapped screw that has been damaged by repeated attempts is harder to remove than one that failed cleanly. If the head has been chewed by pliers or the remains have been hammered, the screw may have expanded into the thread, and no extractor will shift it. At that point the correct methods are heat, drilling out and retapping, or a thread repair insert.
Hardened fasteners are a separate problem. A grade 12.9 cap screw or a stainless fastener that has galled into its thread will resist nearly any extractor. In those cases the sequence is heat first, then a left-hand drill bit, then the extractor, and finally drilling out if nothing else works.
Technique that saves the thread
Everything about extractor work is about protecting the thread in the part, because the screw is already scrap. Centring the pilot hole is the single most important step: an off-centre hole takes the extractor into the parent material, and the damage then has to be repaired with an insert rather than a new screw.
Torque is the second consideration. An extractor is designed to be turned with a tap wrench, which keeps the load axial and even. Gripping the square shank with pliers applies load to one corner and rounds it, and once the shank is round there is no way to turn the tool at all.
Where the screw is small, the safest approach is often a left-hand drill bit on its own. On a screw that is not badly seized, the drill bites and winds the screw out without needing an extractor at all. The extractor is the fallback when the drill alone will not do it, and working in that order keeps a small screw from being damaged further.
Frequently Asked Questions
How do I choose a screw extractor size?
Choose the pilot drill first, based on the diameter of the broken screw, then pick the extractor that suits that hole. A set covers the range.
What size pilot hole do I drill?
Roughly half to two thirds of the screw diameter for a thread extractor. The hole has to leave enough wall for the flutes to bite.
Why does my extractor keep slipping?
Usually a hole that is too large, or flutes that have rounded over. Check the hole size before buying a replacement.
Can I use pliers to turn an extractor?
No. Use a tap wrench on the square shank. Pliers round the shank over and the tool then cannot be turned at all.
What if the extractor snaps in the hole?
It is hardened and very difficult to remove. The usual answer is to drill it out with a carbide drill or take the part to a machinist.
Buying the right one
Buy a set rather than a single extractor, drill the pilot hole to the correct size, and turn the tool with a tap wrench. The screw extractors in this range cover thread-span sizes and numbered grits from 1 to 9, singly and in sets.
