An M8 × 1.25 cutting tap commonly starts with a 6.8 mm drilled hole. That answer is not valid for every M8 thread. Fine pitches and forming taps require different holes, and the drawing, material condition and selected tap must be checked before machining.
First read the complete thread callout
M8 describes an 8 mm nominal metric thread diameter. The pitch tells us how far the thread advances in one revolution for a single-start thread. M8 × 1.25 and M8 × 1.0 are different threads; the smaller pitch leaves a shallower radial thread profile and normally requires a larger starting hole. Confirm the pitch, tolerance class, handedness, required full-thread depth and whether a thread insert is specified.
Why does the workshop rule give 6.8 mm?
For conventional cutting taps, a useful first estimate is tap-drill diameter ≈ nominal diameter − pitch. For M8 × 1.25, the arithmetic is 8.00 − 1.25 = 6.75 mm. A commonly selected nearby drill is 6.8 mm. This is an estimation rule for selecting a practical cutting process, not the mathematical definition of an internal thread and not a replacement for the tap manufacturer’s recommendation.
OSG’s M8 × 1.25 cutting-tap example 1981401 lists a 6.647–6.912 mm tap-drill range. Its M8 × 1.0 example 1982500 lists 6.917–7.153 mm. The common starting choices of 6.8 and 7.0 mm fall inside those respective published ranges. These are examples for named tools, not permission to apply their full ranges to any material or tap.
| Thread and process | Starting point | What must be checked |
|---|---|---|
| M8 × 1.25, cutting tap | Commonly 6.8 mm | Tap series, material, hole accuracy and specified thread class |
| M8 × 1.0, cutting tap | Commonly 7.0 mm | Fine-pitch tool and corresponding inspection gauge |
| M8 × 1.25, forming tap | Use the forming-tap chart | OSG 2868905, for example, lists 7.363–7.490 mm |
| M8 screw clearance | Not a tap-drill operation | Clearance class, assembly stack and drawing requirements |
Why a forming tap needs a different hole
A cutting tap removes material. A forming or roll tap displaces suitable ductile material into the thread shape. Giving a forming tap the same 6.8 mm hole intended for a cutting tap can create excessive forming load and a failed process. The material’s formability, lubrication and the maker’s specified pre-hole window matter. Do not use an approximate forming formula as the final instruction for a hardened mold component.
The diameter is only one part of the machining plan
Blind-hole depth: full usable thread depth is shorter than total drilled depth. Allow for the drill point, the tap’s entry chamfer, chip space where applicable, and safe bottom clearance. A callout for 15 mm of full thread does not mean drilling only 15 mm.
Material condition: quote the steel grade and current hardness. Tapping before heat treatment, tapping hardened steel and thread milling after hardening are different process decisions. Avoid breaking through into cooling channels or leaving an inadequate wall beside the thread.
Inspection: verify the finished thread with the specified GO/NO-GO gauge and inspect the required usable depth. Drill diameter alone does not establish pitch diameter, thread class or load capacity. Chamfers must not remove too much of the available engagement.
Ordering threaded mold components
Send the controlled drawing, thread callout, steel and hardness, quantity, full-thread depth and any nearby cooling or fitting surfaces. NewwaySmart can review the thread together with replacement inserts, mold cores and other precision tooling features. Start with a component drawing review, rather than a drill diameter in isolation.
Technical references
OSG 1981401: M8 × 1.25 cutting tap; OSG 1982500: M8 × 1.0 cutting tap; OSG 2868905: M8 × 1.25 forming tap. Reference values checked 24 October 2026. Final machining and inspection instructions must follow the drawing and the selected tool documentation.