RepairMachineryMedium

Repairing a Stripped Threaded Hole

An M8 tapped hole has lost its thread. The permanent repair is to go up one size: drill to the M10 tapping size, cut a new thread, and fit a bolt one size larger.

Designthe dimensions everything else is derived from

Damaged thread
M8 x 1.25
Tapping drill, M10 x 1.5
8.5 mm
New thread
M10 x 1.5 coarse
Engagement
the bolt should engage at least one diameter of thread
Check
the parent part must have the wall thickness for the larger thread

A repair has no cut list. What has to be checked instead is that the parent part can take the larger thread, which is a wall thickness measurement, not an allowance.

Materialswhat each part is made from, and why

All materials
Carbon Steel · the part being repaired
Worth confirming before cutting. A hole in a casting with 1.25 mm of wall left around the new thread will crack the casting, and the repair becomes a replacement.
Alloy Steel · the replacement bolt
The replacement should be no stronger than the thread it sits in. A very high class bolt in a mild steel hole strips the hole rather than the bolt, which is the opposite of what a repair wants.

Fastenersquantities and lengths that follow from the joints

All standards
ItemQtyGripStandard
M10 x 30 hex bolt1—ISO 4017
M10 hex nut1—DIN 934

Grip is the sum of the clamped thicknesses. A bolt has to clear the nut by two full threads, which is what sets the length that appears in each row — not a preference.

M10 x 30 hex bolt
One size up from the stripped M8. The 1.5 mm pitch is the coarse series for M10, which is what the tapping drill of 8.5 mm corresponds to. ISO 4017 →
M10 hex nut
If the stripped hole is in an inaccessible place, the practical repair is sometimes a clearance hole and a nut on the far side rather than a new thread. DIN 934 →

Toolsand what each one is for on this job

All tools
Tap and Die Set
Cutting the M10 x 1.5 thread in the enlarged hole.
Twist Drill Bits
8.5 mm is the tapping size for M10 x 1.5. Undersize and the tap will jam; oversize and the thread will be shallow and strip again.
Vernier & Dial Caliper
Checking the wall thickness around the hole before committing to the larger size.
Thread Pitch Gauge
Confirming what the damaged thread actually is. Repairing to the wrong size is worse than no repair.
Outside Micrometer
Or a caliper depth rod, for confirming the bolt will not bottom out and that the new thread has enough depth left to engage.

Order of workthe sequence, and where it is not free to change

  1. Identify the thread. A gauge costs less than the part you are about to drill.
  2. Drill to 8.5 mm, the tapping size for M10 x 1.5, using the existing hole as the pilot.
  3. Chamfer the hole mouth lightly so the tap starts square.
  4. Tap through, backing the tap a quarter turn every half turn to break the chips.
  5. Check the bolt runs in by hand. A bolt that needs a spanner to go in has a damaged thread and will strip the new one.

Where it goes wrongthe three failures worth designing out

  • Skipping the wall thickness check. Going up one size removes material all the way round, and the part that fails next is the part, not the bolt.
  • Using the wrong pitch. M10 comes in 1.5 and 1.25 mm; the tapping drill for 1.25 is 8.8 mm, and drilling 8.5 mm for a fine thread gives a thread that is too shallow.
  • Forcing a bolt in with a spanner. It cuts the new thread as it goes, and the repair is then weaker than the hole it replaced.

Frequently asked questions

Why not just fill the hole with weld or epoxy and re-tap it?

Both are done, and both depend on the parent material and the load in a way that a generic guide cannot assess. Going up a size is the repair whose result is fully predictable, which is why it is the one described here.

How much thread should the bolt engage?

At least one diameter, so 10 mm of thread for an M10 in steel. In a softer material such as aluminium or a casting the rule of thumb is closer to two diameters, because the thread in the parent material is what fails.

Is a threaded insert better?

For a hole that will be taken apart repeatedly, yes. An insert also puts a harder thread in a soft parent. Inserts are not carried in this database, so the sizes are not published here.