Stripped thread: thread chaser, coil insert or bigger bolt?
If the threads are damaged but still there (rusted, burred, partly crossed), chase them with a rethreading tap. If the bolt spins or pulls out because the threads are gone, fit a coil insert of the same size: the original bolt still fits. Go to a bigger bolt only when the part has the wall to spare and the mating part can take the larger bolt too.
The test is simple. Run a clean bolt in by hand. If it goes in and holds, the thread is damaged, not stripped. If it spins, wobbles or comes out with metal on it, the thread is stripped.
When is a thread chaser enough?
Use a chaser when the thread form is still there and only needs cleaning. A rethreading tap follows the existing thread and clears rust, burrs and nicks. Lang describes its rethreading taps as made to "chase out damaged threads and remove nicks and burrs without undercutting good threads". A cutting tap from a tap-and-die set will also clean a thread, but it removes metal as it goes, and a hole that is already worn ends up looser.
A chaser cannot put back metal that is missing. If the bolt was spinning before you started, chasing the hole will not make it hold.
How does a coil insert fix a stripped hole?
You drill out the old thread, tap a larger thread with the special tap in the kit, screw in a stainless wire coil, and break off its tang. The coil's inside is the original thread size, so the original bolt, washer and torque setting still apply. Böllhoff, which makes HELICOIL, says the insert increases the load capacity of the thread compared with the parent material alone, which is why the same method is used in new aluminium parts.
Four details decide whether the repair holds:
- Use the drill and tap from the kit. The insert needs a hole larger than the bolt and a special thread, not a standard one. Böllhoff's own drill sizes are given as a guide only; the matched tap is what sets the fit.
- Drill straight and deep enough. Keep the drill square to the face. The tapped hole must be at least as deep as the insert is long.
- Choose the insert length for the material. Inserts come in lengths of 1, 1.5 and 2 times the bolt diameter. In aluminium, Böllhoff's table starts at 1.5 times the diameter, and asks for more with high-strength bolts in the softest alloys.
- Sit it below the surface. Wind the insert in until its last coil sits at least a quarter turn below the top face, then break off the tang.
Buy the kit for the exact thread, size and pitch. Measure the bolt with a thread gauge before you order: an M10x1.25 fine-thread insert will not fit an M10x1.5 bolt.
| Thread | TOPTUL kit |
|---|---|
| M5x0.8 | JGEW2401 |
| M6x1.0 | JGEW2402 |
| M8x1.25 | JGEW2403 |
| M10x1.5 | JGEW1901 |
| M12x1.75 | JGEW1401 |
| Spark plug | JGEW0901 |
When is a bigger bolt the better fix?
Rarely. A bigger bolt is quick when you have a tap in the next size up, but it changes more than the hole:
- The hole in the mating part, the washer and the socket size all change.
- The torque value in the manual no longer applies.
- The next person to work on it expects the original size.
- It removes more metal than an insert. By Böllhoff's figures, the tapped hole for an M8 insert measures about 9.6 mm across, less than a plain M10 thread.
It makes sense on a bracket or a frame you own and will service yourself, where there is plenty of wall around the hole, the bolt passes through a part you can drill, and no torque figure has to be met. For an engine, a gearbox casing, a brake caliper or anything with a sealing face, use an insert.
What if the bolt broke off in the hole?
Get the broken bolt out first, with a screw extractor and its drill guide (TOPTUL JGEW2501 is a set). Then run a clean bolt in by hand and apply the same test: chase the thread if it survived, fit an insert if it did not.
Find the TOPTUL JGEW coil-insert repair kits by thread size in the Vesprr Tools catalogue, or ask Vesprr Tools for a size not listed.
Sources
- Böllhoff, HELICOIL Kits: repair solutions for damaged threads (catalogue 0180, installation process, construction guidelines and metric installation data).
- Böllhoff, Repair solutions, product page.
- Lang Tools, Rethreader tools, product range.
- TOPTUL, 24PCS Professional Threaded Coil-Insert Repair Kit (M8x1.25), product page.