A thread insert is a new steel thread fitted into a damaged hole. The common kind is a coil of diamond-shaped steel wire that screws into a hole tapped with a special oversize tap. Solid inserts, a steel sleeve threaded inside and out, are also sold. Both give a thread of the original size, often stronger than the aluminium it replaces.
What it does
It lets you keep a casting such as a crankcase, cover or cylinder head when its thread has stripped. The bolt then screws into steel instead of soft aluminium.
Signs you need one
- A bolt or screw turns but never tightens: A bolt hole thread is stripped.
- A drain plug will not seal or tighten: Oil drain plug thread is stripped.
- Bright aluminium in the thread of a bolt you removed.
What to match when buying
- The thread size and pitch of the original bolt, for example metric 6 mm by 1.0. Measure with a pitch gauge: Identify a thread standard and pitch.
- The insert length. Longer inserts hold more; choose one at least as long as the bolt engagement.
- A kit with the matching drill, tap, inserting tool and tang breaker. Inserts and taps from different makers may not match.
- For a drain plug or spark plug hole, buy a kit made for that job.
Fitting notes
- Drill straight with the kit drill only. A crooked or oversize hole cannot be saved with the same insert.
- Keep chips out of the engine.
- High load holes (head studs, engine mounts, brake parts) are workshop jobs. See Repair a stripped thread in a bolt hole.