Drawing interpretationReading time 11 min
What every character of M8×1.25-6H is telling you
Everyone recognises M8. Rather fewer can say what separates M8×1-6g from M8-6H, or why the tapping drill is 6.8 and not 6.9.
01The structure of a metric designation
A full ISO metric thread callout looks like this: M8 × 1.25 − 6H − LH.
| Position | Contents | What it asks |
|---|---|---|
| M | Thread form | M = ISO metric triangular thread |
| 8 | Nominal diameter | Major diameter of the external thread, in mm |
| × 1.25 | Pitch | Omitted means coarse pitch |
| − 6H | Tolerance grade and position | Number = grade, letter = position |
| − LH | Hand | Omitted means right hand; LH means left hand |
Tolerance grade and position
| Code | Typical use | What it asks |
|---|---|---|
| 6H | Internal thread (nut) | The usual general-purpose choice |
| 6g | External thread (bolt) | The usual choice; leaves room for plating |
| 6h | External thread | Zero fundamental deviation, tighter |
| 4H5H / 4h | Internal / external | Precision fits |
| 7H / 8g | Internal / external | Loose; suits threads after hot-dip galvanising |
The smaller the number the tighter the tolerance. Internal and external threads each carry two tolerances, one for the pitch diameter and one for the major or minor diameter. Written 5g6g, it means grade 5 on the pitch diameter and grade 6 on the major. A single group (6g) means both are the same.
02Length of engagement: the letter nobody notices
The same tolerance grade means a different actual value at different lengths of engagement. ISO 965 divides engagement into S (short), N (normal) and L (long), written after the tolerance class as 6H-L. Omitted, it defaults to N.
Why it matters: the longer the engagement, the more accumulated pitch error, so a long engagement needs a wider pitch-diameter tolerance to go together at all. In deep tapped holes, beyond about 1.5 times diameter, an N tolerance can seize.
03How to work out the tapping drill
The approximation is drill ≈ nominal diameter − pitch. For M8×1.25 that is 8 − 1.25 = 6.75, and in practice 6.8 mm.
| Thread | Pitch | Theoretical drill | Drill used |
|---|---|---|---|
| M3 | 0.5 | 2.50 | 2.5 |
| M4 | 0.7 | 3.30 | 3.3 |
| M5 | 0.8 | 4.20 | 4.2 |
| M6 | 1.0 | 5.00 | 5.0 |
| M8 | 1.25 | 6.75 | 6.8 |
| M10 | 1.5 | 8.50 | 8.5 |
| M12 | 1.75 | 10.25 | 10.2 |
The trade-off on thread engagement
Open the hole and the percentage of thread falls: tapping gets easier and the risk of snapping a tap drops, but so does the tensile strength of the joint. Experience shows 75% thread reaches close to 100% of the stripping strength while halving the tapping torque, which is why most shops choose a slightly larger drill.
04Depth callouts on the drawing
“M8↧16” and “M8↧16 / Ø6.8↧20” are two different statements. The first gives only the effective thread depth and leaves the drill depth to manufacturing; the second specifies the drilled depth as well.
- Effective thread depth is the length of full-form thread, usually 1.5 to 2 times diameter in steel and 2 to 2.5 in aluminium.
- Drill depth must allow for incomplete threads and chip space beyond the effective depth, usually three to five pitches more.
- Through holes are written M8 THRU or shown in section, and need no depth.