How to Drill and Tap a Broken Bolt: The Thread Repair Guide (Drill, Tap, Insert)
By WhatSizeBoltEdited 11 min read
Once the broken bolt is out, you still have to fix the hole. The decision tree for drill-and-tap, Helicoil and TIME-SERT, the tap-drill and Helicoil STI charts, and the two mistakes that turn a repair into a bigger job.
This is a guide to re-threading a hole — drilling it to the correct oversize (or Helicoil/TIME-SERT insert size), cutting fresh threads with a tap, and fitting an insert or a larger fastener so the joint holds like new. It picks up once the broken piece is out of the hole and you’re deciding how to restore the threads underneath: four real repair methods, when each one is actually correct, and the two mistakes that turn a straightforward thread repair into a much bigger job.
Decision tree: what are you actually dealing with?
Four questions get you to the right method before a drill ever touches the hole.
- Is a broken bolt still in the hole? Extract it first — see the guide linked above. Everything below assumes the hole is now empty.
- Are the threads mostly intact, or gone? Cross-threaded or lightly damaged but still there — a plug tap or thread chaser re-cuts them with no drilling at all. Stripped, torn out, or pulled out with the bolt — you need to enlarge the hole; go to question 3.
- Blind hole or through hole? A blind hole (doesn’t go all the way through) needs a bottoming tap, and a wire insert deep enough to exceed the fastener’s diameter for real pull-out resistance — don’t drill past the original hole depth, or you risk breaking into a water jacket, wiring or a fuel passage on the other side. A through hole gives you the full menu below.
- What’s the material, and will this fastener come out again? Aluminum, or limited material depth — use an insert (Helicoil or TIME-SERT), not oversizing; aluminum runs out of meat fast. Steel or cast iron with plenty of depth — oversizing to the next thread size is the simplest fix if the original size doesn’t matter. Removed and reinstalled repeatedly — spark plugs, drain plugs — use TIME-SERT’s solid bushing; a Helicoil wire insert is the documented failure mode for spark-plug holes.
Material matters: aluminum, cast iron, steel
Same damage, different repair — aluminum runs out of material fast; steel and cast iron give you more room to choose.
| Material | Best repair | Why |
|---|---|---|
| Aluminum (soft) | Helicoil or TIME-SERT insert | Limited material depth; oversizing runs out of meat quickly |
| Cast iron | Insert or oversizing | Enough material depth for either method |
| Steel | Any method; insert for critical loads | High material strength supports all approaches |
If a bolt is still stuck: extract first, and skip the easy-out on hardened bolts
One gotcha worth knowing before you pick up an extractor.
A spiral-fluted easy-out (screw extractor) works well on mild steel, brass, aluminum and Class 4.6 bolts or softer. It does not work on Class 8.8 bolts and harder: the extractor is hardened tool steel, but so is the bolt, and when it snaps off inside the hole it’s harder than a standard HSS drill bit — often unrecoverable without EDM. For a hardened bolt, reach for a left-hand drill bit in reverse instead; the counter-clockwise bite frequently spins the bolt straight out, and even when it doesn’t, it loosens the bolt without adding a second, harder broken piece to the problem. Full technique in the drilling-out guide linked above.
Method 1: drill and tap to the next size up
The simplest fix when the original fastener size doesn’t matter and there’s enough surrounding material.
- Best for: steel or cast iron with material to spare, a through hole, and a job where stepping up to the next standard bolt size is acceptable.
- Not ideal for: aluminum or thin material (you’ll run out of meat), or anywhere the original bolt size is fixed by a mating part.
- Drill out the damaged threads to the tap-drill size for the next bolt size up, with cutting fluid throughout.
- Cut fresh threads with a tap at the standard 75% thread engagement — about 98% of full thread strength, with a low risk of snapping the tap.
- Fit the new, larger bolt. If the original size is fixed by a mating part, this method isn’t an option — use Method 2 or 3 instead.
Metric tap drill chart
| Thread | Pitch (mm) | Tap drill |
|---|---|---|
| M3×0.5 | 0.5 | 2.50 mm |
| M4×0.7 | 0.7 | 3.30 mm |
| M5×0.8 | 0.8 | 4.20 mm |
| M6×1.0 | 1.0 | 5.00 mm |
| M8×1.25 | 1.25 | 6.80 mm |
| M10×1.5 | 1.5 | 8.50 mm |
| M12×1.75 | 1.75 | 10.20 mm |
| M14×2.0 | 2.0 | 12.00 mm |
| M16×2.0 | 2.0 | 14.00 mm |
Formula: tap drill (mm) = thread OD (mm) − pitch (mm), for standard 75% engagement.
SAE / UNC-UNF tap drill chart
| Thread | TPI | Tap drill | Decimal |
|---|---|---|---|
| #6-32 UNC | 32 | #36 | 0.1065” |
| #8-32 UNC | 32 | #29 | 0.1360” |
| #10-24 UNC | 24 | #25 | 0.1495” |
| #10-32 UNF | 32 | #21 | 0.1590” |
| 1/4-20 UNC | 20 | #7 | 0.2010” |
| 1/4-28 UNF | 28 | #3 | 0.2130” |
| 5/16-18 UNC | 18 | F | 0.2570” |
| 5/16-24 UNF | 24 | I | 0.2720” |
| 3/8-16 UNC | 16 | 5/16” | 0.3125” |
| 3/8-24 UNF | 24 | Q | 0.3320” |
| 7/16-14 UNC | 14 | U | 0.3680” |
| 1/2-13 UNC | 13 | 27/64” | 0.4219” |
75% thread engagement is the standard here too. Some materials tap easier at a lower 65–70% engagement — both are industry-accepted.
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Tap and drill bit set (metric or SAE)
A matched tap-and-drill set pairs the right pilot bit with the right tap for each thread size, so Method 1 doesn’t come down to guessing between two drill bits that look almost the same.
Method 2: Helicoil wire thread insert (STI)
Restores the original thread size in aluminum or any hole that’s lost its threads — if you drill it to the right size.
- Best for: aluminum and other soft material (an insert is the preferred repair there), moderate-to-severe thread damage in otherwise sound material, and jobs where the original bolt size has to come back.
- Blind or through hole: both work. In a blind hole, the wire coil’s own spring tension holds it in place, but depth must exceed the fastener’s diameter for real pull-out resistance.
Metric Helicoil STI drill sizes
| Thread | Standard tap drill | Helicoil STI drill |
|---|---|---|
| M3×0.5 | 2.50 mm | 2.8 mm† |
| M4×0.7 | 3.30 mm | 4.2 mm† |
| M5×0.8 | 4.20 mm | 5.2 mm† |
| M6×1.0 | 5.00 mm | 6.3 mm |
| M8×1.25 | 6.80 mm | 8.3 mm |
| M10×1.5 | 8.50 mm | 10.4 mm |
| M12×1.75 | 10.20 mm | 12.4 mm |
| M14×2.0 | 12.00 mm | 14.4 mm‡ |
| M16×2.0 | 14.00 mm | 16.4 mm‡ |
† M3–M5 STI sizes are single-source figures (AmesWeb’s and EngiCalc’s Helicoil drill calculators) with no second independent source found. ‡ M14–M16 STI sizes are marked approximate by their source (AIMS Industrial’s drill reference), calculated from the general STI formula rather than published directly by an insert manufacturer. Confirm both against your specific insert kit’s instructions before drilling.
SAE Helicoil STI drill sizes (ASME B18.29.1)
| Thread | Standard tap drill | Helicoil STI drill |
|---|---|---|
| #6-32 | #36 (0.1065”) | #25 (0.1495”) |
| #8-32 | #29 (0.1360”) | #17 (0.1730”) |
| #10-24 | #25 (0.1495”) | 13/64” (0.2031”) |
| 1/4-20 | #7 (0.2010”) | 17/64” (0.2656”) |
| 5/16-18 | F (0.2570”) | 21/64” (0.3281”) |
| 3/8-16 | 5/16” (0.3125”) | 25/64” (0.3906”) |
| 1/2-13 | 27/64” (0.4219”) | 33/64” (0.5156”) |
Installing a Helicoil
- Drill to the STI size from the table above — not the standard tap drill.
- Tap with the matching STI tap — same pitch as a standard tap for that thread, but cut to the larger STI diameter.
- Thread the insert in to about ¼–½ turn below the surface.
- Break off the installation tang with a punch, on coiled inserts that use one.
Helicoil 5334-14 Save-A-Thread Thread Repair Kit (M14 x 1.25)
A complete M14 x 1.25 kit — STI drill, STI tap and inserts in one box. M14 x 1.25 also happens to be the standard spark-plug thread, but for a spark-plug hole specifically, use TIME-SERT’s solid bushing instead (see below) — this kit is for the M14 x 1.25 holes everywhere else: mounting bosses, brackets and housings.
A Helicoil holds up fine almost everywhere — except a fastener that gets removed and reinstalled often, like a spark plug. That’s a documented failure mode, and it’s exactly what TIME-SERT was built to fix. See Method 3 below before you Helicoil a spark-plug hole.
Method 3: TIME-SERT solid bushing (spark plugs and anything removed often)
A solid threaded bushing instead of a wire coil — the standard fix where a Helicoil is known to back out.
Repairing a spark-plug hole specifically? We have a dedicated walkthrough that compares the options head to head for that job: Stripped Spark Plug Thread Repair: Helicoil vs. TimeSert vs. Rethreader. The rest of this section covers TIME-SERT as a general-purpose repair.
A TIME-SERT insert is a solid threaded bushing, not a wire coil. The installation driver cold-rolls and expands the bottom of the insert into the parent material — it’s not hammer-swedged the way some competing inserts are — which is why it holds up better on a fastener that gets removed and reinstalled often. Most kits are also “over the fender” designs, meaning the repair doesn’t require pulling the cylinder head.
Common TIME-SERT thread sizes
| Thread | Typical use |
|---|---|
| M14 x 1.25 | The most common spark-plug thread; also general-purpose M14 repairs |
| M12 x 1.25 | Less common spark-plug thread |
| M10 x 1.0 | Specialty and glow-plug repairs |
For a hole where a previous repair has already failed, an oversized insert — sold as “Big-Sert” — is built for exactly that case.
Blind hole vs. through hole
The hole type changes your tap, your depth math, and your insert choice.
| Aspect | Blind hole | Through hole |
|---|---|---|
| Insert type | Wire coil — spring tension holds it | Wire coil or solid bushing |
| Tap | Spiral-flute (chips don’t pack the hole) | Spiral-point (ejects chips forward) |
| Depth | Critical — must exceed fastener diameter for pull-out resistance | Not critical — full depth is easy to reach |
| Risk | Over-drilling can break into a water jacket, wiring or a fuel passage | Lower risk; depth is easy to monitor |
Frequently asked questions
Is a Helicoil as strong as the original threads?
For most uses, yes — correctly sized and installed in sound parent material, a Helicoil meets or exceeds the original thread’s strength. It’s not the right choice for a fastener removed and reinstalled often, like a spark plug; use TIME-SERT there instead.
Can I use a standard tap drill for a Helicoil?
No — that’s the most common field failure. A Helicoil needs the larger STI drill size in the table above, then a matching STI tap; a standard tap drill leaves the insert loose and poorly seated.
Do I need a special tap for a Helicoil?
Yes. STI taps share the same pitch as a standard tap for that thread size, but cut to the larger STI diameter, giving the wire coil more material to grip.
Can a hole that’s already been repaired once be repaired again?
Often, yes — an oversized insert, sold as “Big-Sert” for TIME-SERT, is built for exactly that: a hole where a previous Helicoil or TIME-SERT repair has already failed.
Why did my easy-out snap off in the bolt?
Easy-outs are hardened tool steel — but so are Class 8.8+ bolts, and on a hardened bolt the extractor can snap before it turns the bolt, leaving a harder problem behind. Use a left-hand drill bit in reverse on hardened bolts instead; full technique in our drilling-out guide.
Keep going
Bottom line: once the hole is empty, match the repair to the material and the job — re-chase intact threads with a plug tap, drill and tap to the next size up when the original size doesn’t matter, fit a Helicoil (drilled to the STI size, never the standard tap drill) for aluminum or when the size has to stay the same, and reach for TIME-SERT’s solid bushing on spark plugs and anything else removed often. Get the drill size right for the method you’re using, and the repair will outlast the original.