Steel bolts often go into aluminium castings such as crankcases, covers and cylinder heads. The female thread in the soft metal is the weak side, and the course notes that small crankcase screw threads strip easily. A stripped hole can often be saved by cleaning the thread with a tap, or by fitting a steel thread insert. This page helps you decide and gives the general steps. Holes for head studs, engine mounts, axles or brake parts carry high loads: have a workshop do those.
Safety
- Wear eye protection when drilling and tapping.
- Keep metal chips out of the engine. On an open engine, plug passages with clean rag and use grease on the drill to catch chips.
- A wrong drill size or a crooked hole can ruin a casting. If unsure, stop and ask a workshop.
Decide what the hole needs
| What you find | What to do |
|---|---|
| Thread dirty, burred or slightly damaged | Clean it with a tap of the same size and pitch |
| Some threads gone, bolt still bites | Thread insert of the original size |
| Hole badly torn or cracked, or a stud hole | Workshop repair; the casting may need welding or a specialist insert |
Steps
- Identify the thread size and pitch on the bolt with a pitch gauge: Identify a thread standard and pitch.
- Clean the hole of oil, sealant and loose metal. Blow it out with eye protection on.
- To clean a lightly damaged thread, start the tap by hand, square to the face. Turn it in a little, back it off to break the chip, and go on. Use cutting oil.
- To fit an insert, drill the hole with the drill from the insert kit only, keeping the drill straight. Use the drill depth the kit gives.
- Cut the new thread with the special tap from the kit, square to the face, with cutting oil.
- Clean out every chip.
- Screw the insert onto the inserting tool and wind it in until it sits just below the surface.
- Break off the drive tang if the insert has one, using the tang tool, and remove it from the hole.
- Test the bolt by hand. It should screw in smoothly to full depth.
- Fit the part and tighten to the torque in the manual: Use a torque wrench.
Good to know
- Do not cut extra thread on a bolt unless you must. Rolled bolt threads are stronger than cut ones.
- A thread that keeps stripping often means the bolt is too long and bottoms out, or it was tightened with an impact gun. Start all screws by hand.
- Fine and coarse threads look alike. A tap of the wrong pitch ruins the hole on the first turn.
Check your work
- The bolt reaches full depth by hand and tightens to torque without turning further.
- No leak at the cover or plug after a warm run.
If it did not fix the problem
- The insert turns with the bolt: the hole was drilled oversize. Workshop repair.
- Leak from a drain plug thread: Oil drain plug thread is stripped.