Stripped Threads, Rebuilt
A stripped bolt hole is rarely the end of the part. Three honest fixes exist: a thread insert (the strongest repair), an oversize tap (the simplest), or an epoxy rebuild (light-duty only). Epoxy-rebuilt threads will hold a cover or a bracket for years. They must never hold an engine, a brake, or a wheel. If the bolt's failure could hurt someone, reach for the insert.
Every mechanic knows the feeling. The wrench turns, resistance fades to nothing, and the bolt spins in place — free, useless, mocking. The threads inside the hole have given up. What was once a precise spiral of metal is now a smooth, tired bore, and the bolt has nothing left to climb.
A thread is only a ramp wrapped around a hole — a tiny inclined plane that turns rotation into clamping force. Strip the ramp and the whole system quits. But the hole is not dead. The parent metal around it is still there, still strong, still willing. You only need to give the bolt a new ramp.
This guide ranks the three real repairs by strength, walks through the epoxy rebuild step by step, and draws the line — clearly, twice — around where glue threads belong and where they never go.
Why threads strip
Threads rarely die of old age. They die of violence or neglect, usually both.
Over-torque is the classic killer. A bolt tightened past its limit stretches, and the threads in the softer parent metal shear first. Aluminum and pot metal surrender long before the steel bolt does — which is why stripped holes cluster in aluminum housings, transmission cases, and cast brackets.
Corrosion is the slow killer. Rust and oxidation weld bolt to hole over the seasons; the threads tear on the way out, sometimes years after the damage was done. Cross-threading is the impatient killer: a bolt started crooked and forced straight cuts a new, wrong path on its very first turn. And repeated assembly wears the ramp a little every time, especially in soft metals that see frequent service.
The prevention follows the cause: a torque wrench instead of a strong arm, anti-seize where dissimilar metals meet, every bolt started by hand. But this guide is for the hole that already lost.
Option one: the thread insert
When a stripped hole must hold real torque, the professional answer is a thread insert — a coil or bushing of hardened steel that goes into the drilled-out hole and gives the bolt new threads, often stronger than the original.
The method demands the kit: drill to the insert's specified size, tap with the matching special tap, wind the insert in, break off the tang. The bolt then threads into steel even when the parent part is soft aluminum — and the joint can be torqued to specification without apology.
This is the repair for anything load-bearing or safety-adjacent — engine accessories with real torque values, transmission housings, structural brackets. It costs a kit and an afternoon of care, and it is the only option on this page that restores full strength. If your fastener has a published torque spec, start here and stop here.
Option two: tap it oversize
Sometimes the honest answer is a bigger bolt. Drill the stripped hole out, tap it to the next thread size up, and fit a larger fastener. Fast, cheap, permanent — and fully metal, so it holds real torque.
The catch is meat. You need enough parent metal around the hole for the larger threads to bite: enough wall thickness, enough edge distance. On a thin casting, or a hole sitting near an edge, oversize is not an option — you would break through the wall and trade a stripped hole for a cracked part.
Where it fits — a thick bracket, a sturdy housing with room to grow — it is the working person's favorite. Keep the larger bolt, and write the new size on the part — future-you will not remember.
Option three: the epoxy rebuild
And then there is the glue. A metal-filled epoxy can rebuild the thread body inside a stripped hole: fill the bore, let it cure into a hard, machinable solid, then cut new threads into it. The bolt gets its ramp back — this time made of cured resin instead of metal.
Be clear about what this is. Cured structural epoxy — even a good one at Shore D 85–90 — is not steel. It will not hold engine torque. It will not survive a torque wrench set to specification. What it will do, faithfully, is hold light-duty fasteners: covers, trim, brackets, housings — bolts that only need to stay put, not clamp hard.
The honest test fits in one sentence: if the original bolt had a published torque spec above "snug," epoxy is the wrong repair. If the bolt just needs to not fall out, epoxy is a fine one.
The rebuild, step by step
Do it in this order. Every step exists because skipping it is how rebuilds fail.
Step 1 — Assess. Light duty only: cover, trim, bracket, housing, panel. If the fastener is safety-critical or carries a real torque spec, stop here and buy an insert kit. Choosing the right repair matters more than performing it well.
Step 2 — Clean like the repair depends on it. Degrease the hole — brake cleaner or acetone on a swab, worked all the way down — then roughen the walls with a small round file or a twist of coarse sandpaper. Epoxy bonds to clean, rough metal and peels off oily, smooth metal.
Step 3 — Mask the neighborhood. Tape around the hole. Plug any passage the epoxy must not enter — a dab of wax or a foam plug you can pull out later. Epoxy goes where liquid goes; decide in advance where that is.
Step 4 — Mix truly. Two-part epoxy, mixed until one uniform color with no streaks. Half-mixed epoxy cures half-strong. You have about 30 minutes of working time at room temperature — plenty, but don't wander off mid-job.
Step 5 — Fill from the bottom. Press the epoxy in with a toothpick or small stick, working out air pockets. Slightly overfill; you'll face it flush after the cure.
Step 6 — Cure fully. Initial set in 3–4 hours; full strength after 24 hours at room temperature. Do not rush this. Threads cut into half-cured epoxy crumble like dry cake.
Step 7 — Face flush, then re-tap. File the surface flat, start the tap straight — a tap guide or drilled block keeps it true — and turn a quarter-turn forward, then back off to break the chip, with cutting fluid or light oil. Epoxy cuts easily; never lean on the tap.
Step 8 — Test modestly. Run the bolt in by hand first — it should turn smoothly, with even resistance. Snug it gently: "snug," not "spec." If it binds, back out and re-run the tap. Forcing it strips your new threads the way the old ones went.
The release-agent trick
There is a second way to form the threads: coat the actual bolt with a release agent — paste wax or grease — fill the hole with epoxy, and thread the coated bolt in. The epoxy cures around it; the release agent lets you back the bolt out afterward, leaving molded threads behind.
It works — but molded threads are weaker than cut ones. A tap shears clean, dense ridges; molding leaves the as-cast surface. Use the mold trick where you have no tap or face an odd thread size. Where you own the tap, cut.
Whichever way you form them, keep the bolt lightly oiled in service — dry steel wears epoxy threads faster.
Where epoxy threads belong
Read this section twice. The epoxy is the same in both columns below — only the consequence of failure changes, and consequence chooses the repair.
The three options at a glance
| Thread insert | Oversize tap | Epoxy rebuild | |
|---|---|---|---|
| Strength | Full — torque to specification | Full, in the larger size | Light duty only — snug, not spec |
| Skill needed | Moderate — requires the kit | Basic tapping | Basic, plus patience for the cure |
| Cost | Kit plus inserts | One tap and a larger bolt | Epoxy you may already own |
| Best for | Load-bearing, safety-adjacent, specified torque | Thick parts with room to grow | Covers, trim, brackets, housings |
| Watch out for | Drill straight; follow the kit's drill size exactly | Needs enough wall metal and edge distance | 24-hour cure; modest torque; the NEVER list |
One honesty that covers the whole table: an insert installed crooked is worse than epoxy done right. The method sets the ceiling; your technique decides whether you reach it.
Frequently asked questions
Can I just use a longer bolt to catch good threads deeper in the hole?
Sometimes — if the hole is blind and the damage sits near the mouth. Try it: if a longer bolt of the same size threads in smoothly and snugs up, you have bought a repair for the price of a bolt. If it binds or the damage runs deep, stop — forcing it strips the good threads too.
Will an epoxy rebuild hold a spark plug?
No. A spark plug lives in heat, vibration, and real torque — three things epoxy threads cannot promise. A damaged plug hole wants a thread insert installed properly, or professional help. This is exactly what the NEVER list is for.
Can I cut threads in epoxy with a normal tap?
Yes — fully cured structural epoxy machines cleanly. Use cutting fluid or light oil, start straight, advance a quarter turn and back off to clear the chip. Never lean on the tap.
How much torque can epoxy threads take?
Modest. Think "snug," not "spec." If you are reaching for a torque wrench, that is the fastener telling you it wants an insert instead. The honest ceiling is finger-tight plus a little; anything that needs more chose the wrong repair.
Is molding threads with a greased bolt as strong as re-tapping?
No. Molded threads work, but cut threads are stronger — the tap shears clean, dense ridges, while molding leaves the epoxy's as-cast surface. Mold when you lack the tap or face an odd thread size; cut whenever you have the tap.
What about stripped screw holes in wood or plastic?
Different materials, different answers. In wood, an epoxy fill works well — so does the old trick of gluing in a hardwood dowel and re-drilling. In plastic, use threaded inserts made for plastic (heat-set or press-in); epoxy threads in plastic are a short-term fix at best.
Sources
- FAA Advisory Circular AC 43.13-1B, Acceptable Methods, Techniques, and Practices: Aircraft Inspection and Repair — the free public reference for bonded and mechanical repairs: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43.13-1B_w-chg1.pdf
- West System, Instruction Manuals and Technical Guides — surface preparation and epoxy handling, free from the boatyard standard-setter: https://www.westsystem.com/