Use a 6.8 mm Drill for an M8 × 1.25 Cutting Tap
An M8 × 1.25 cutting tap commonly uses a 6.8 mm drill. Check the pitch and whether the tap cuts or forms threads before drilling.
Use a 6.8 mm drill bit for an M8 × 1.25 cutting tap. This is the direct answer for the common coarse-pitch M8 thread: AutoDrill’s metric cut-tap chart specifies 6.80 mm or a letter H drill for M8 × 1.25 (AutoDrill).
Do not select the drill from “M8” alone. Confirm the complete marking on the tap because pitch and tap type change the required hole.
| Tap | Tap type | Starting drill size |
|---|---|---|
| M8 × 1.25 | Cutting tap | 6.8 mm or letter H |
| M8 × 1.0 | Cutting tap | 7.0 mm or letter J |
| M8 × 1.25 | Forming/roll tap | 7.4 mm |
| M8 × 1.0 | Forming/roll tap | 7.5 mm or 19/64 in |
AutoDrill lists 7.00 mm or J for an M8 × 1 cutting tap. Haas lists substantially larger starting holes for form taps: 7.4 mm for M8 × 1.25 and 7.5 mm or 19/64 inch for M8 × 1 (Haas Automation). A form tap displaces material rather than cutting it, so using the smaller cutting-tap hole can overload the tool.
Should you use 6.8 mm, 6.9 mm or 17/64 inch?
Choose 6.8 mm when the tap maker provides no different instruction. It is 0.2677 inch nominally. A letter H drill is 0.2660 inch and is the inch-system alternative in AutoDrill’s chart.
You may also encounter 6.9 mm or 17/64 inch. These are documented recommendations rather than transcription errors: another supplier chart pairs M8 × 1.25 with 6.9 mm or 17/64 inch (Schaefer & Peters). The difference reflects choices about thread engagement as well as the reality that drill runout, tool condition and the workpiece affect the finished hole.
For a repair or general workshop job, use the specified 6.8 mm metric bit rather than treating a nearby fractional bit as an exact equivalent. For a controlled fit or production work, the tap manufacturer’s hole range and the required thread class take priority over a general chart. Haas also advises checking the actual hole and notes that reaming may be needed for tighter tolerance holes.
Drill and tap the hole
- Read the tap marking. Verify M8 × 1.25 or M8 × 1.0 and determine whether it is a cutting or forming tap.
- Secure and mark the work. Clamp the part, locate the center and keep the drill square to the surface.
- Drill the pilot hole. Use a sharp bit suited to the material. For a blind hole, allow extra depth for the drill point, tap chamfer and chip space without breaking through the part.
- Deburr the entrance. A small chamfer removes the raised edge and helps the tap start.
- Lubricate and align the tap. Use cutting fluid appropriate to the workpiece, start perpendicular and avoid forcing the tool. Manage chips according to the tap design.
- Check the thread. Clean the hole and test it with the intended M8 fastener or the specified thread gauge.
A 7 mm or 8 mm bit is not a generic substitute for an M8 × 1.25 cutting tap: 7 mm corresponds to the finer M8 × 1 cutting-tap recommendation, while 8 mm is the nominal fastener diameter and leaves no material to cut the internal thread. The same need to separate cutting taps from forming taps applies to inch threads, as shown in our 3/8-16 tap drill guide.