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Description
FINISHED THREADM12 × 1.25
TOLERANCE4H
PRE-DRILLED HOLEØ12.3 mm
LENGTH L195 mm
M12x1.25 thread repair tap – STI tap for V-Coil M12x1.25 inserts
Tool dimension table and technical data
| V-COIL M12x1.25 THREAD REPAIR TAP | |
|---|---|
| Thread to be repaired | M12 × 1.25 |
| Tolerance class | 4H |
| Tap diameter B | 13.6 mm |
| Overall length L1 | 95 mm |
| Flute length L2 | 30 mm |
| Shank diameter D2 | 11.2 mm |
| Square drive a | 9 mm |
| Pre-drilled hole | Ø12.3 mm |
The V-Coil M12x1.25 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 screwed in, the hole accepts a standard M12x1.25 bolt without modifying the fastener. The 11.2 mm shank with a 9 mm square drive fits standard hand tap wrenches, making work at a service station easier.
Repairing stripped M12x1.25 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 frequently in soft alloys, while in steel the insert improves the fatigue life of the joint. M12x1.25 threads are found, among other applications, in spark plug holes in smaller engines. Downsizing reduced the spark plug diameter there from 14 to 12 mm and weakened the thin cylinder head wall. Make the pilot hole with a drill bit Ø12.3 mm for the STI tap, drill at the upper tolerance limit, and tap with coolant.
The M12x1.25 thread repair tap cuts an STI thread with dimensions conforming to DIN 8140-2. The tool's overall length L1 is 95 mm, and its flute length L2 is 30 mm. The tool is compatible with V-Coil inserts in lengths of 2xD, and a properly repaired thread carries loads comparable to or greater than the original thread.
Repairing an M12x1.25 thread step by step
From drilling to the nominal thread
Repairing with an insert restores the spark plug hole thread faster and at a lower cost than replacing the entire aluminum cylinder head.
Thread repair M12x1.25 with a wire insert
1Drilling
Hole: Ø12.3 mm
2Chamfering the hole entrance
Angle: 90–120°
3STI tapping
With coolant, 4H
4Insert installation
Tang: break off
✓Nominal thread
Screw: M12 × 1.25
In a blind hole, remove the broken-off tang before installing the screw
M12x1.25 Helicoil tap (STI) – why it is larger than a standard tap
Compensating for the wire thickness of the thread insert
The STI tool differs from a standard M12x1.25 tap only in profile size — the 60° angle and 1.25 mm pitch remain metric. The insert wire takes up space on both sides of the profile, so the diameter is D STI = 12 + 1.3 × 1.25 = 13.62 mm; hence the nonstandard 13.6 mm cutting section, which neither an M14 nor an M16 tap can provide. Tolerance class 4H (equivalent to inch class 3B) is also used for screw-locking inserts, while the finished thread after insert installation is class 6H.
Tap compatibility with the V-Coil insert
What must match before starting the repair
✓Matching size: V-Coil M12x1.25 insert with a 1.25 mm pitch.
✓Matching hole: prepare a Ø12.3 mm diameter before tapping.
✓Matching version: available M12x1.25 insert lengths: 2xD.
▲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 M12x1.25 thread — never to prepare a seat for an insert.
Frequently asked questions
The STI insert requires a hole Ø12.3 mm, rather than 10.8 mm as for a standard M12x1.25 thread. Drill at the upper tolerance limit and chamfer the entrance at 90–120°.
No, the STI thread has a nonstandard diameter 13.62 mm, which neither a standard M14 nor an M16 tap can reproduce. The tool's cutting diameter is 13.6 mm. An incorrect profile prevents the insert from locking properly in its seat.
Yes, the inner edge of the wire restores the nominal M12 × 1.25 thread. The minor diameter after installation is approximately 10.65 mm, in accordance with ISO 724. You can install the bolt without any modifications.
Aluminum has low shear strength, and in a cut thread the first thread turn carries approximately 50% of the load. The spark plug may be removed at every service, often while the engine is hot. The insert distributes the force more evenly across successive thread turns.
Product accessories
Thread repair insert M12x1.25 2xD V-Coil
Reference: 0000009464 In stock > 10 items
€0.79
€0.97 brutto
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Shipping
All orders are delivered by courier company GLS. For orders exceeding 369 PLN gross shipping is FREE
| 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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