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M20 bolt torque specifications (8.8, 10.9, 12.9)

An M20 x 2.5 class 8.8 bolt takes 325 ft-lb (441 N·m) dry, and class 10.9 takes 450 ft-lb (610 N·m). Both are for plain steel, dry threads (K 0.20) tightened to a clamp load of 75% of proof load.

By the Torque Reference editors 8 sources cited Last reviewed Method
Contents14 sections

Key takeaways

  • Class 8.8 dry: 325 ft-lb (441 N·m); class 10.9 dry: 450 ft-lb (610 N·m).
  • M20 is the first size in the chart on the higher class 8.8 proof stress, 600 MPa instead of 580 MPa ⁠1.
  • M20 x 1.5 fine carries about 11% more clamp load than M20 x 2.5.
  • Structural joints are not tightened from a torque table ⁠2.

Quick facts

Coarse threadM20 x 2.5 (245 mm²)
Fine threadM20 x 1.5 (272 mm²)
Wrench, ISO 4014/401730 mm
Wrench, DIN 931/93330 mm (same)

M20 bolt specifications

ThreadPitchTensile stress area
M20 x 2.5 (coarse)2.5 mm pitch245.0 mm²
M20 x 1.5 (fine)1.5 mm pitch272.0 mm²
Wrench, ISO 4014/401730 mm⁠3

Stress areas from ISO 898-1 Table 4⁠1.

M20 torque by grade

M20 x 2.5 torque, coarse pitch
GradePlain, dry K 0.20⁠3Lubricated K 0.15⁠3Clamp load
Class 4.6122 ft-lb165 N·m91 ft-lb124 N·m41.3 kN
Class 8.8325 ft-lb441 N·m244 ft-lb331 N·m110.3 kN
Class 10.9450 ft-lb610 N·m337 ft-lb458 N·m152.5 kN
Class 12.9526 ft-lb713 N·m394 ft-lb535 N·m178.2 kN

Calculated with T = K × D × F, where F is 75% of the bolt’s proof load⁠3 and K is the nut factor for each condition. Real preload from a torque wrench scatters by about ±25%⁠4. When the equipment maker gives a torque, use theirs. How these numbers are made.

M20 4.6 torque

Proof 225 MPa · yield 240 MPa · tensile 400 MPa⁠1. Clamp load at 75% of proof: 41.3 kN (M20 x 2.5).

ConditionKM20 x 2.5M20 x 1.5
Plain steel, dryAs-received black or plain finish, clean threads, no lubricant0.20⁠3122 ft-lb165 N·m135 ft-lb184 N·m
Zinc-plated, dryClean zinc-plated threads, no added lubricant0.18⁠3110 ft-lb149 N·m122 ft-lb165 N·m
Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head0.15⁠391 ft-lb124 N·m102 ft-lb138 N·m
Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize0.16⁠598 ft-lb132 N·m108 ft-lb147 N·m
Moly pasteMolybdenum disulfide paste0.11⁠667 ft-lb91 N·m74 ft-lb101 N·m

M20 8.8 torque

Proof 600 MPa · yield 660 MPa · tensile 830 MPa⁠1. Clamp load at 75% of proof: 110.3 kN (M20 x 2.5).

ConditionKM20 x 2.5M20 x 1.5
Plain steel, dryAs-received black or plain finish, clean threads, no lubricant0.20⁠3325 ft-lb441 N·m361 ft-lb490 N·m
Zinc-plated, dryClean zinc-plated threads, no added lubricant0.18⁠3293 ft-lb397 N·m325 ft-lb441 N·m
Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head0.15⁠3244 ft-lb331 N·m271 ft-lb367 N·m
Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize0.16⁠5260 ft-lb353 N·m289 ft-lb392 N·m
Moly pasteMolybdenum disulfide paste0.11⁠6179 ft-lb243 N·m199 ft-lb269 N·m

M20 10.9 torque

Proof 830 MPa · yield 940 MPa · tensile 1040 MPa⁠1. Clamp load at 75% of proof: 152.5 kN (M20 x 2.5).

ConditionKM20 x 2.5M20 x 1.5
Plain steel, dryAs-received black or plain finish, clean threads, no lubricant0.20⁠3450 ft-lb610 N·m500 ft-lb677 N·m
Zinc-plated, dryClean zinc-plated threads, no added lubricant0.18⁠3405 ft-lb549 N·m450 ft-lb610 N·m
Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head0.15⁠3337 ft-lb458 N·m375 ft-lb508 N·m
Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize0.16⁠5360 ft-lb488 N·m400 ft-lb542 N·m
Moly pasteMolybdenum disulfide paste0.11⁠6247 ft-lb336 N·m275 ft-lb373 N·m

M20 12.9 torque

Proof 970 MPa · yield 1100 MPa · tensile 1220 MPa⁠1. Clamp load at 75% of proof: 178.2 kN (M20 x 2.5).

ConditionKM20 x 2.5M20 x 1.5
Plain steel, dryAs-received black or plain finish, clean threads, no lubricant0.20⁠3526 ft-lb713 N·m584 ft-lb792 N·m
Zinc-plated, dryClean zinc-plated threads, no added lubricant0.18⁠3473 ft-lb642 N·m525 ft-lb712 N·m
Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head0.15⁠3394 ft-lb535 N·m438 ft-lb594 N·m
Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize0.16⁠5421 ft-lb570 N·m467 ft-lb633 N·m
Moly pasteMolybdenum disulfide paste0.11⁠6289 ft-lb392 N·m321 ft-lb435 N·m

Coarse or fine pitch

M20 x 2.5 coarse has 245 mm² of stress area ⁠1; M20 x 1.5 fine has 272 mm² ⁠3, about 11% more. In class 8.8 dry the fine thread takes 361 ft-lb instead of 325 ft-lb and clamps 122.4 kN instead of 110.2 kN. The ISO 262 table also lists M20 x 2 ⁠7; by the ISO stress-area formula it has about 258 mm², about 5% more than coarse, and falls between the two columns.

Class 8.8 above and below M16

ISO 898-1 raises the class 8.8 proof stress from 580 MPa to 600 MPa above 16 mm, and minimum tensile strength from 800 to 830 MPa ⁠1. M20 is the first size in this chart on the higher values. The step adds about 3.4%: on the old 580 MPa basis the 8.8 dry torque would be 314 ft-lb rather than 325.

Class 10.9 stays at 830 MPa ⁠1, so its lead over 8.8 shrinks from about 43% at M16 to about 38% here. The 8.8 clamp load is 110.2 kN (about 24,790 lbf); 10.9 is 152.5 kN (about 34,290 lbf).

M20 next to 3/4 inch

M20 is noticeably larger than 3/4 in (19.05 mm). On the same basis an M20 class 8.8 takes about 22% more torque than a 3/4-10 Grade 5 and clamps about 16% more.

Wrench size

M20 hex heads are 30 mm in both ISO 4014/4017 and DIN 931/933 ⁠3. The in-between size M22, not in this chart, is the one large size where they differ, and the other way round: 34 mm ISO against 32 mm DIN ⁠3.

Which class you have

The class symbol and the maker’s mark are on the head ⁠1; see the bolt grades page. Class 12.9 dry is 526 ft-lb (713 N·m).

When the chart does not apply

Structural joints built to the RCSC Specification use ASTM F3125 bolts, the standard that replaced A325, A325M, A490 and A490M ⁠8, rather than the ISO 898-1 classes in this chart. For calibrated-wrench installation RCSC says torque values from tables or equations shall not be used without verification ⁠2. See structural bolts. For machinery, zinc-plated class 8.8 M20 is 293 ft-lb at K 0.18 ⁠3.

Sources

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