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1/4 inch bolt torque specifications (Grade 2, 5, 8)

A 1/4-20 Grade 5 bolt takes 101 in-lb (11 N·m) and a Grade 8 takes 12 ft-lb (16 N·m). Both values are for plain, dry steel threads (K 0.20) tightened to 75% of proof load.

By the Torque Reference editors 4 sources cited Last reviewed Method
Contents11 sections

Key takeaways

  • Grade 2 and Grade 5 stay under 10 ft-lb dry, so set the wrench in in-lb: 66 in-lb and 101 in-lb.
  • Oiled threads need 25% less torque for the same clamp load (K 0.15 instead of 0.20) ⁠1; Grade 5 drops to 76 in-lb.
  • 1/4-28 fine thread has 14.5% more stress area than 1/4-20, the biggest coarse-to-fine gap of any size from 1/4 to 1 in.
  • One 7/16 in wrench fits both the hex head and the hex nut ⁠1.

Quick facts

Coarse thread1/4-20 UNC
Fine thread1/4-28 UNF
Wrench (head and nut)7/16 in
Grade 5 clamp load, coarse2,030 lbf

1/4 in bolt specifications

ThreadPitchTensile stress area
1/4-20 (UNC)20 threads per inch0.0318 in²
1/4-28 (UNF)28 threads per inch0.0364 in²
Wrench, hex cap screw7/16 in⁠1

Stress areas from ASME B1.1 as tabulated by Fastenal⁠1.

1/4 in torque by grade

1/4-20 torque, coarse thread
GradePlain, dry K 0.20⁠1Lubricated K 0.15⁠1Clamp load
Grade 266 in-lb7.4 N·m49 in-lb5.6 N·m1,310 lbf
Grade 5101 in-lb11 N·m76 in-lb8.6 N·m2,030 lbf
Grade 812 ft-lb16 N·m107 in-lb12 N·m2,860 lbf

Calculated with T = K × D × F, where F is 75% of the bolt’s proof load⁠1 and K is the nut factor for each condition. Real preload from a torque wrench scatters by about ±25%⁠2. When the equipment maker gives a torque, use theirs. How these numbers are made.

1/4 in Grade 2 torque

Proof 55,000 psi · yield 57,000 psi · tensile 74,000 psi⁠1. Clamp load at 75% of proof: 1,310 lbf (1/4-20).

ConditionK1/4-201/4-28
Plain steel, dryAs-received black or plain finish, clean threads, no lubricant0.20⁠166 in-lb7.4 N·m75 in-lb8.5 N·m
Zinc-plated, dryClean zinc-plated threads, no added lubricant0.18⁠159 in-lb6.7 N·m68 in-lb7.6 N·m
Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head0.15⁠149 in-lb5.6 N·m56 in-lb6.4 N·m
Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize0.16⁠352 in-lb5.9 N·m60 in-lb6.8 N·m
Moly pasteMolybdenum disulfide paste0.11⁠436 in-lb4.1 N·m41 in-lb4.7 N·m

1/4 in Grade 5 torque

Proof 85,000 psi · yield 92,000 psi · tensile 120,000 psi⁠1. Clamp load at 75% of proof: 2,030 lbf (1/4-20).

ConditionK1/4-201/4-28
Plain steel, dryAs-received black or plain finish, clean threads, no lubricant0.20⁠1101 in-lb11 N·m116 in-lb13 N·m
Zinc-plated, dryClean zinc-plated threads, no added lubricant0.18⁠191 in-lb10 N·m104 in-lb12 N·m
Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head0.15⁠176 in-lb8.6 N·m87 in-lb9.8 N·m
Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize0.16⁠381 in-lb9.2 N·m93 in-lb10 N·m
Moly pasteMolybdenum disulfide paste0.11⁠456 in-lb6.3 N·m64 in-lb7.2 N·m

1/4 in Grade 8 torque

Proof 120,000 psi · yield 130,000 psi · tensile 150,000 psi⁠1. Clamp load at 75% of proof: 2,860 lbf (1/4-20).

ConditionK1/4-201/4-28
Plain steel, dryAs-received black or plain finish, clean threads, no lubricant0.20⁠112 ft-lb16 N·m14 ft-lb19 N·m
Zinc-plated, dryClean zinc-plated threads, no added lubricant0.18⁠111 ft-lb15 N·m12 ft-lb17 N·m
Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head0.15⁠1107 in-lb12 N·m10 ft-lb14 N·m
Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize0.16⁠3114 in-lb13 N·m11 ft-lb15 N·m
Moly pasteMolybdenum disulfide paste0.11⁠479 in-lb8.9 N·m90 in-lb10 N·m

Working in inch-pounds

At 1/4 in most of the chart sits below 10 ft-lb, which is why it is printed in in-lb. The Grade 5 dry value of 101 in-lb is only 8.4 ft-lb; Grade 8 at 143 in-lb is just over 12 ft-lb. To convert, divide in-lb by 12, or use the ft-lb to N·m converter.

Small numbers do not mean a small margin. A torque wrench delivers preload with about ±25% scatter ⁠2, so 101 in-lb on a 1/4-20 Grade 5 can leave the joint anywhere from roughly 1,520 to 2,540 lbf instead of the 2,030 lbf the chart aims for. At this size an extra few in-lb from a lubricated thread or a heavy hand is a large fraction of the total, so set the wrench for the actual thread condition.

Coarse or fine thread

The 1/4-20 coarse thread has a tensile stress area of 0.0318 in²; the 1/4-28 fine thread has 0.0364 in² ⁠1. That is 14.5% more steel carrying load, so at the same grade the fine thread clamps 14.5% harder: 2,320 lbf against 2,030 lbf in Grade 5. The calculated torque rises by the same ratio (116 in-lb against 101 in-lb dry). Use the column that matches the thread you have; a 1/4-28 torque on a 1/4-20 bolt overloads it by that 14.5%.

Which grade you have

Grade 2 has no head marking, Grade 5 has three radial lines and Grade 8 has six ⁠1. Proof stress at 1/4 in is 55,000 psi for Grade 2, 85,000 psi for Grade 5 and 120,000 psi for Grade 8 ⁠1, so a Grade 8 bolt carries 41% more clamp load and torque than a Grade 5, and a Grade 5 carries 55% more than a Grade 2. Pair a Grade 5 bolt with a Grade 5 nut and a Grade 8 bolt with a Grade 8 nut ⁠1. The grades page shows the markings, and the grade identification tool walks through them.

When the chart does not apply

These values are sized to the bolt’s proof load. They say nothing about whether the parts being clamped or a tapped thread can carry 2,000 to 3,000 lbf, and NASA warns that the common K of 0.20 “should not be used blindly” ⁠2. Fastenal adds that published K values vary between sources and should be measured for critical joints ⁠1. When the equipment maker gives a torque for the fastener, use the maker’s value. For other lubricants see dry vs lubricated torque.

Sources

  1. 1
  2. 2
  3. 3
  4. 4

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