Dimensional and assembly data for M8 Hex Head Bolts with Reduced Shank to ISO 4015. Includes head size and width across flats, clearance-hole and tapping-drill diameters, tightening torque per property class, and the sizes to step up or down to when the joint requires it.
M8 Dimensional Row (ISO 4015)
| Size | Pitch P (mm) | b1 (mm) | b2 (mm) | c_max (mm) | d_a_max (mm) | d_s (mm) | d_w_min (mm) | Width across corners e (mm) | Head height k (mm) | k_max (mm) | k_min (mm) | k_w_min (mm) | r_min (mm) | Width across flats s (mm) | s_min (mm) | x_max (mm) | Length range l (mm) | Reference standard |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| M8 | 1.25 | 22 | 0 | 0.6 | 9.2 | 7.1 | 11.47 | 14.2 | 5.3 | 5.54 | 5.06 | 3.54 | 0.4 | 13 | 12.57 | 3.2 | 30~80 | ISO 4015 |
Spanner & Drive for M8 Hex Head Bolts with Reduced Shank
M8 Hex Head Bolts with Reduced Shank have a width across flats of 13 mm — fit a 13 mm open-ended spanner or socket. A 6-point socket grips all six flats of the 13 mm hex and is what to reach for once a fastener has been on and off a few times; a 12-point ring spanner starts rounding a worn head. For repeatable preload, torque rather than feel.
Hole Sizes for M8
| Coarse-thread pitch (ISO 724) | 1.25 mm |
|---|---|
| Through-hole / clearance (ISO 273 medium) | 9 mm |
| Tapping drill, coarse thread | 6.75 mm |
Tightening Torque for M8
| Class 8.8 (dry, ~µ 0.125) | ≈ 24 Nm |
|---|---|
| Class 10.9 (dry, ~µ 0.125) | ≈ 36 Nm |
| Class 12.9 (dry, ~µ 0.125) | ≈ 42 Nm |
Indicative dry-joint values. Lubrication can lower the required torque by 15–25%. Always confirm against the joint design, especially when going up a strength class.
Proof Load & Strength for M8
| Tensile stress area As (ISO 724) | 36.6 mm² |
|---|---|
| Proof load, class 8.8 | 21 kN |
| Proof load, class 10.9 | 30 kN |
| Proof load, class 12.9 | 36 kN |
| Min. ultimate tensile load, class 8.8 | 29 kN |
Proof load = tensile stress area × the proof stress Sp of the class, per ISO 898-1 — Sp is a lower figure than the yield stress Rp0.2 and the two are not interchangeable. The stress area is computed from this size's own diameter and pitch, so a fine-pitch designation gives the fine-pitch figure. Design the joint so the working preload stays below the proof load of the class selected.
M8 Hex Head Bolts with Reduced Shank Weight
A steel M8 hex head bolts with reduced shank at the listed 3,080 mm length weighs approximately 1,221 g each — about 1,221 kg per 1,000 pieces. Stainless A2/A4 weighs roughly the same (density 7.9–8.0 g/cm³).
Actual weight varies slightly with head form, thread and finish; use this figure for freight estimation only.
Installation Tips for M8 Hex Head Bolts with Reduced Shank
- Use a 6-point socket where access allows — 12-point sockets are more prone to rounding the corners on smaller sizes.
- On flanged or serrated variants, do NOT add a separate flat washer — the flange already spreads the load and the washer can defeat the locking serrations.
When to Step Up or Down from M8
When the joint preload approaches the proof load of M8 class 8.8, step up to M10 class 8.8 (or move to M8 class 10.9). When the joint is over-specified, M6 often saves weight and cost without losing the safety margin.
Custom manufacturing · Made to drawing
Need hex head bolts with reduced shank in a size this table does not list?
This page covers M8 hex head bolts with reduced shank. Material and finish are usually what drives a special — a grade the standard part is not stocked in, or a coating chosen against a salt-spray figure instead of a default. Made by cold heading or hot forging.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
