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Description
FINISHED THREADM8 × 1.25
TOLERANCE4H
PRE-DRILLED HOLEØ8.3 mm
LENGTH L180 mm
M8 thread repair tap – STI tap for V-Coil M8x1.25 inserts
Tool dimension table and technical data
| V-COIL M8 THREAD REPAIR TAP | |
|---|---|
| Thread to be repaired | M8 × 1.25 |
| Tolerance class | 4H |
| Tap diameter B | 9.6 mm |
| Overall length L1 | 80 mm |
| Flute length L2 | 24 mm |
| Shank diameter D2 | 10 mm |
| Square drive a | 8 mm |
| Pre-drilled hole | Ø8.3 mm |
The V-Coil M8 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 M8 × 1.25 bolt without modification. The 10 mm shank with a 8 mm square drive fits standard hand tap wrenches and T-handle tap wrenches.
Repairing a stripped M8 thread in steel, stainless steel, cast iron, and aluminum
Materials, tips, and limitations
The tool works in all common engineering materials: steel, stainless steel, cast iron, and aluminum. Threads strip most frequently in soft metals, while in steel the insert improves the fatigue life of the joint. It is suitable, for example, for repairing a stripped accessory mounting thread in an aluminum engine or gearbox housing. The pilot hole is made with a drill bit Ø8.3 mm for the STI tap. Drill at the upper tolerance limit and tap with coolant to obtain a clean thread profile.
The dimensions of the cut STI thread are specified by DIN 8140-2. The M8 thread repair tap has an overall length L1 of 80 mm and a flute length L2 of 24 mm, and is compatible with V-Coil inserts in lengths of 2xD and 3xD. A correctly installed insert restores a thread with a load capacity comparable to or greater than the original.
M8 thread repair step by step
From drilling to the nominal thread
Repairing with an insert is faster and less expensive than replacing the entire aluminum housing, such as an engine housing with a stripped accessory mounting thread.
Thread repair M8 with a wire insert
1Drilling
Hole: Ø8.3 mm
2Chamfering the hole entrance
Angle: 90–120°
3STI tapping
With coolant, 4H
4Insert installation
Tang: break off
✓Nominal thread
Screw: M8 × 1.25
In a blind hole, remove the broken-off tang before installing the screw
Helicoil M8 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 = 8 + 1.3 × P = 8 + 1.3 × 1.25 = 9.62 mm. Therefore, the tool's cutting diameter B is 9.6 mm — a nonstandard value that neither an M9 nor an M10 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 M8 insert with a 1.25 mm pitch.
✓Matching hole: prepare a Ø8.3 mm diameter before tapping.
✓Matching version: available M8 insert lengths: 2xD, 3xD.
▲Required inspection: depth and wall thickness of the component being repaired.
This is not a substitute for an M8 tap
The tool cuts only an oversized thread for an insert. A standard M8 bolt will not engage in a hole tapped this way without an insert, and a standard tap will not prepare the hole for an insert.
Frequently asked questions
For an STI insert, drill a hole with a diameter of Ø8.3 mm, rather than 6.8 mm as for a standard M8 thread. Drill the hole at the upper tolerance limit so the insert can enter freely. Chamfering the entrance at 90–120° is recommended.
The STI tap cuts a larger thread with a major diameter of approximately 9.62 mm instead of 8 mm, as it accommodates an insert with a wire thickness of 0.81 mm. The 1.25 mm pitch and 60° profile remain identical to those of a standard M8 tap. Thanks to DIN 8140, Helicoil, V-Coil, Recoil, and BaerCoil inserts are interchangeable.
Yes, once the insert is installed, the inner edge of the wire restores the nominal thread M8 × 1.25. The minor diameter is then approximately 6.65 mm and complies with ISO 724. You can therefore install a standard bolt without any modification.
Yes, the same inserts are installed at the factory to reinforce threads in cast iron and aluminum as a preventive measure. The thread then benefits from the hardness of wire with a minimum tensile strength of 1400 MPa and an even load distribution. This also improves the fatigue life of the joint.
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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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