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Description
FINISHED THREADM14 × 1.25
TOLERANCE4H
PRE-DRILLED HOLEØ14.3 mm
LENGTH L1102 mm
M14x1.25 thread repair tap – STI tap for V-Coil M14x1.25 inserts
Tool dimension table and technical data
| V-COIL M14x1.25 THREAD REPAIR TAP | |
|---|---|
| Thread to be repaired | M14 × 1.25 |
| Tolerance class | 4H |
| Tap diameter B | 15.6 mm |
| Overall length L1 | 102 mm |
| Flute length L2 | 32 mm |
| Shank diameter D2 | 12.5 mm |
| Square drive a | 10 mm |
| Pre-drilled hole | Ø14.3 mm |
The V-Coil M14x1.25 thread repair tap cuts an STI (Screw Thread Insert) thread for a Helicoil-type insert. The tool restores a load-bearing seat in the damaged hole without increasing the bolt size. Once the insert is installed, the hole again accepts a standard M14x1.25 bolt, and the joint retains its original size. The 12.5 mm shank with a 10 mm square drive fits standard hand tap wrenches.
Repairing stripped M14x1.25 threads in steel, stainless steel, cast iron, and aluminum
Materials, tips, and limitations
The tool works in steel, stainless steel, cast iron, and aluminum. Threads strip most often in soft metals, while in steel the insert improves the fatigue life of the joint. A typical example is restoring a stripped M14x1.25 spark plug hole in an aluminum cylinder head, where the soft material is easily damaged during servicing. Drill the pilot hole with a drill bit Ø14.3 mm for the STI tap, drill at the upper tolerance limit, and tap with coolant.
The M14x1.25 thread repair tap cuts an STI thread with dimensions conforming to DIN 8140-2. The tool's overall length L1 is 102 mm, and its flute length L2 is 32 mm. The tool is compatible with V-Coil inserts in lengths of 1.5xD, 2xD, 2.5xD, and a correctly repaired thread carries loads comparable to or greater than those of the original thread.
Repairing an M14x1.25 thread step by step
From drilling to the nominal thread
Repairing with an insert is faster and less expensive than replacing the entire engine cylinder head, and the steps below show the installation procedure.
Thread repair M14x1.25 with a wire insert
1Drilling
Hole: Ø14.3 mm
2Chamfering the hole entrance
Angle: 90–120°
3STI tapping
With coolant, 4H
4Insert installation
Tang: break off
✓Nominal thread
Screw: M14 × 1.25
In a blind hole, remove the broken-off tang before installing the screw
M14x1.25 Helicoil tap (STI) – why it is larger than a standard tap
Compensating for the wire thickness of the thread insert
The STI thread retains the pitch P = 1.25 mm and the 60° thread angle of the ISO metric thread, but its diameter is intentionally enlarged. The increase compensates for the insert wire thickness on both sides of the profile, according to the following relationship: D STI = 14 + 1.3 × P = 14 + 1.3 × 1.25 = 15.62 mm. Therefore, the tool's cutting diameter B is 15.6 mm — a nonstandard value that neither an M16 nor an M18 tap can provide. The 4H tolerance corresponds to inch thread class 3B and is also suitable for screw-locking inserts; once the insert is installed, the finished thread achieves the standard 6H class.
Tap compatibility with the V-Coil insert
What must match before starting the repair
✓Matching size: V-Coil M14x1.25 insert with a 1.25 mm pitch.
✓Matching hole: prepare a Ø14.3 mm diameter before tapping.
✓Matching version: available M14x1.25 insert lengths: 1.5xD, 2xD, 2.5xD.
▲Required inspection: depth and wall thickness of the component being repaired.
Tool exclusively for insert threads
The thread cut by this tool is larger than nominal and is intended solely for installing a V-Coil insert. Use a standard tap to cut a standard M14x1.25 thread — never to prepare a seat for an insert.
Frequently asked questions
For the STI tap, use a drill bit Ø14.3 mm, rather than the 12.8 mm sufficient for a standard M14x1.25 thread. Drill at the upper limit of the hole tolerance. Chamfer the hole entrance at 90–120° to help guide the tap into the hole.
No, the STI thread has a nonstandard diameter 15.62 mm, which neither M16 nor M18 can reproduce. An incorrect profile prevents the insert from locking properly in the hole. The cutting diameter of this tool is B = 15.6 mm.
The designation 4H indicates a close-fit STI thread tolerance class, corresponding to inch thread class 3B used in aerospace and screw-locking inserts. After the insert is installed, the finished thread meets class 6H.
Aluminum has low shear strength, and in a cut thread the first turn carries approximately 50% of the load. The spark plug is removed at every service, often while the engine is hot. The insert distributes the force evenly across successive thread turns.
Product accessories
Thread repair insert M14x1.25 2.5xD V-Coil
Reference: 0000006046 In stock > 10 items
€1.05
€1.29 brutto
Thread repair insert M14x1.25 2xD V-Coil
Reference: 0000007177 In stock > 10 items
€1.05
€1.29 brutto
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| Courier GLS | Przelew24 / Prepayment | 14,76 zł gross | Delivery time 1 working day |
| Courier GLS | Cash on delivery | 17,26 zł gross | Lead time 1 working day |
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