Technical Tables

mm / in

Millimeter / inch. 1 in = exactly 25.4 mm.

M / d / P

M identifies an ISO metric thread; d is the nominal outside bolt diameter and P is the axial distance between adjacent crests, both in mm. M10 ×1.5: diameter 10 mm and pitch 1.5 mm. Fine pitch is an alternative, not the pitch used for every column in this row.

d₂ / d2

Bolt pitch diameter: theoretical cylinder where thread thickness and gap width are equal, in mm.

d₃ / d3

Bolt minor/root diameter at the root of the external thread, in mm. Not nominal diameter or tap drill size.

D₁ / D1

Internal thread (nut) minor diameter, in mm; a basic geometric value, not a selected drill size.

h₃ / H

h₃: basic radial external thread height. H: height of the fundamental 60° profile triangle. Both in mm; H is not overall bolt height.

Aₛ / As (mm²)

Effective tensile stress area of the threaded section, in square millimeters. Written A subscript s, not A2. Converts stress into force; not the nominal-diameter circle area.

Broca / Llave / Allen

Tap drill: indicative hole diameter before tapping. Wrench: across-flats size of the head/nut. Hex key: across-flats size of the internal hex socket. Check pitch, head style and manufacturer.

Reading the values: Each row uses its diameter and primary pitch. d₃ and Aₛ are reference geometric values; mm² means area, not length. Confirm tolerances and head type before selecting tools.

8.8 / 10.9 / 12.9

ISO bolt property classes, not units or sizes. First number ×100: nominal Rm in MPa; second number ÷10: nominal yield-to-Rm ratio. Example 8.8: 800 MPa and 0.8 ×800 = 640 MPa. Standard minimums may depend on diameter.

Usos / Uses

8.8: common in machinery and structures; 10.9: mechanical joints requiring higher preload; 12.9: common in high-strength screws, including certain socket-head screws. Examples, not design selection: higher strength is not a safe automatic substitution; verify load, fatigue, corrosion, nut, lubrication and manufacturer.

T / N·m

T is tightening torque: turning effect, in newton-meters. N·m is neither force in N nor stress in MPa.

F / kN

F is estimated axial tightening preload (bolt tension), not maximum allowable external load. Shown in kilonewtons; 1 kN = 1000 N.

K = 0,20

Dimensionless nut/torque factor combining thread friction, bearing-surface friction and geometry. Assumed 0.20 here; NOT a universal constant. In temperature, K means kelvin: a different context.

T = K·d·F

Use d in meters and F in newtons to obtain T in N·m. Preload used is F = 0.7·Rp0.2·Aₛ: MPa and mm² give N. The 0.7 is the reference fraction of yield strength.

Reading the values: In “8.8 · N·m (kN)”, 8.8 is the class; the number outside parentheses is T and inside is F. For example 50 (25) means 50 N·m and 25 kN preload, not ultimate capacity. Lubrication changes K: the ≈25% reduction is indicative, never universal; follow the manufacturer procedure.

8.8 / 10.9 / 12.9

ISO bolt property classes, not units or sizes. First number ×100: nominal Rm in MPa; second number ÷10: nominal yield-to-Rm ratio. Example 8.8: 800 MPa and 0.8 ×800 = 640 MPa. Standard minimums may depend on diameter.

Usos / Uses

8.8: common in machinery and structures; 10.9: mechanical joints requiring higher preload; 12.9: common in high-strength screws, including certain socket-head screws. Examples, not design selection: higher strength is not a safe automatic substitution; verify load, fatigue, corrosion, nut, lubrication and manufacturer.

Rm

Ultimate tensile strength of the material: maximum test stress, not an allowable working load.

Rp0,2 / Rp0.2

0.2% proof/yield stress: stress corresponding to 0.2% permanent plastic strain, not fracture.

MPa / N/mm²

Megapascal: stress or pressure unit. 1 MPa = 1 N/mm²; it is not a force on its own.

kgf/mm² / ksi

Kilogram-force per square millimeter / thousand pounds-force per square inch. 1 kgf/mm² ≈ 9.80665 MPa; 1 ksi ≈ 6.894757 MPa. kgf is not kg of mass.

Reading the values: Stress (MPa) alone does not tell how much weight a bolt supports: stress area, load, joint and safety must be considered. Nominal class values do not replace the certificate or diameter-specific minimums.

mm / in

Millimeter / inch. 1 in = exactly 25.4 mm.

M / d / P

M identifies an ISO metric thread; d is the nominal outside bolt diameter and P is the axial distance between adjacent crests, both in mm. M10 ×1.5: diameter 10 mm and pitch 1.5 mm. Fine pitch is an alternative, not the pitch used for every column in this row.

d₂ / d2

Bolt pitch diameter: theoretical cylinder where thread thickness and gap width are equal, in mm.

d₃ / d3

Bolt minor/root diameter at the root of the external thread, in mm. Not nominal diameter or tap drill size.

D₁ / D1

Internal thread (nut) minor diameter, in mm; a basic geometric value, not a selected drill size.

h₃ / H

h₃: basic radial external thread height. H: height of the fundamental 60° profile triangle. Both in mm; H is not overall bolt height.

Aₛ / As (mm²)

Effective tensile stress area of the threaded section, in square millimeters. Written A subscript s, not A2. Converts stress into force; not the nominal-diameter circle area.

Broca / Llave / Allen

Tap drill: indicative hole diameter before tapping. Wrench: across-flats size of the head/nut. Hex key: across-flats size of the internal hex socket. Check pitch, head style and manufacturer.

T / N·m

T is tightening torque: turning effect, in newton-meters. N·m is neither force in N nor stress in MPa.

F / kN

F is estimated axial tightening preload (bolt tension), not maximum allowable external load. Shown in kilonewtons; 1 kN = 1000 N.

K = 0,20

Dimensionless nut/torque factor combining thread friction, bearing-surface friction and geometry. Assumed 0.20 here; NOT a universal constant. In temperature, K means kelvin: a different context.

T = K·d·F

Use d in meters and F in newtons to obtain T in N·m. Preload used is F = 0.7·Rp0.2·Aₛ: MPa and mm² give N. The 0.7 is the reference fraction of yield strength.

Reading the values: · means multiplication, / division, √ square root and ² squaring. Pitch and diameters use mm for geometry; only T =K·d·F uses d in m and F in N. These are basic dimensions, not all manufacturing limits.