Mechanical Reference

Metric Threads & Fasteners

ISO metric screw threads, thread geometry, standard sizes, bolt property classes, and fundamental fastening equations.

ISO Metric Thread

Metric screw threads use a 60° flank angle. Thread size is specified by nominal diameter and pitch, for example M8 × 1.25.

Thread Designation

M8 × 1.25

M8Nominal diameter = 8 mm

1.25Pitch = 1.25 mm

Thread Geometry

d / D

Major Diameter

Nominal external or internal thread diameter.

Pitch

Axial distance between corresponding thread points.

d₂ / D₂

Pitch Diameter

Theoretical diameter controlling thread fit.

d₃ / D₁

Minor Diameter

Diameter measured at the thread root region.

Metric Thread Sizes

Common ISO metric coarse and fine thread sizes.

Thread	Coarse Pitch	Fine Pitch	Tap Drill	Pitch Dia.	Minor Dia.
M3	0.50 mm	0.35	2.5 mm	2.675	2.459
M4	0.70 mm	0.5	3.3 mm	3.545	3.242
M5	0.80 mm	0.5	4.2 mm	4.480	4.134
M6	1.00 mm	0.75	5.0 mm	5.350	4.917
M8	1.25 mm	1	6.8 mm	7.188	6.647
M10	1.50 mm	1.25 / 1	8.5 mm	9.026	8.376
M12	1.75 mm	1.5 / 1.25	10.2 mm	10.863	10.106
M14	2.00 mm	1.5	12.0 mm	12.701	11.835
M16	2.00 mm	1.5	14.0 mm	14.701	13.835
M18	2.50 mm	2 / 1.5	15.5 mm	16.376	15.294
M20	2.50 mm	2 / 1.5	17.5 mm	18.376	17.294
M22	2.50 mm	2	19.5 mm	20.376	19.294
M24	3.00 mm	2	21.0 mm	22.051	20.752
M27	3.00 mm	2	24.0 mm	25.051	23.752
M30	3.50 mm	2	26.5 mm	28.051	26.211
M33	3.50 mm	2	29.5 mm	31.051	29.211
M36	4.00 mm	3	32.0 mm	34.052	31.670

Tap drill values (D ≈ d - P) are general references. Tapping drill selection depends on required thread percentage, material, and applicable standards.

Bolt Property Classes

Property Class	Tensile Strength	Yield Strength	Typical Application
4.6	400 MPa	240 MPa	General low-strength fastening
5.8	500 MPa	400 MPa	General machinery applications
8.8	800 MPa	640 MPa	Common high-strength structural fastening
10.9	1000 MPa	900 MPa	High-strength machinery and automotive applications
12.9	1200 MPa	1080 MPa	Very high-strength applications

Fastener Assembly & Torque

Torque to Preload Formula

T = K × F × d

T Tightening torque (N·m)

K Nut factor / friction coefficient (typically 0.15 - 0.20)

F Target bolt preload force (N)

Fastening Equations

Thread Geometry

Basic Thread Height (H)H = 0.8660 × P
Pitch Diameter (d₂)d₂ ≈ d - 0.6495 × P
Tensile Stress Area (As)As ≈ (π/4) × (d - 0.9382P)²

Preload Estimation

Yield Load (Fy)Fy = As × Rp0.2Rp0.2 is the nominal yield strength from bolt class.
Recommended Preload (F)F = 0.65 to 0.75 × FyTypically targeted at 65-75% of yield strength for static loads.

Engineering Safety Note

Torque-based preload estimation is highly sensitive to friction (lubrication, coatings, temperature). For safety-critical joints, use specific standards (VDI 2230) and precise tightening methods (yield-control or tensioners).