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Grade 2 bolt torque chart and strength

A 1/2-13 Grade 2 bolt takes 49 ft-lb plain and dry or 37 ft-lb oiled, about 35% less than Grade 5. Above 3/4 in, Grade 2 proof strength drops from 55 to 33 ksi, so the torque does too.

By the Torque Reference editors 3 sources cited Last reviewed Method
Contents8 sections

Key takeaways

  • Grade 2 has no head marking, the same as Grade 1 ⁠1.
  • Proof stress is 55 ksi from 1/4 to 3/4 in and 33 ksi from 7/8 to 1-1/2 in ⁠1.
  • A 7/8 in Grade 2 bolt takes less torque than a 3/4 in one: 167 vs 173 ft-lb dry.
  • Grade 5 needs about 55% more torque up to 3/4 in, and about 2.6 times as much at 7/8 and 1 in.

Quick facts

MaterialLow or medium carbon steel ⁠1
Head markingNone
Proof, 1/4–3/4 in55,000 psi
Proof, 7/8–1-1/2 in33,000 psi

Grade 2 strength

SizesProof stressYieldTensile
1/4 to 3/4 in55,000 psi57,000 psi74,000 psi
7/8 to 1-1/2 in33,000 psi36,000 psi60,000 psi

SAE J429 values as tabulated by Fastenal⁠1.

Grade 2 torque chart

SizeDry K 0.20⁠1Zinc K 0.18⁠1Oiled K 0.15⁠1
1/4-2066 in-lb7.4 N·m59 in-lb6.7 N·m49 in-lb5.6 N·m
5/16-1811 ft-lb15 N·m10 ft-lb14 N·m102 in-lb11 N·m
3/8-1620 ft-lb27 N·m18 ft-lb24 N·m15 ft-lb20 N·m
7/16-1432 ft-lb43 N·m29 ft-lb39 N·m24 ft-lb32 N·m
1/2-1349 ft-lb66 N·m44 ft-lb60 N·m37 ft-lb50 N·m
9/16-1270 ft-lb95 N·m63 ft-lb86 N·m53 ft-lb72 N·m
5/8-1197 ft-lb132 N·m87 ft-lb118 N·m73 ft-lb99 N·m
3/4-10173 ft-lb234 N·m155 ft-lb211 N·m130 ft-lb176 N·m
7/8-9167 ft-lb226 N·m150 ft-lb203 N·m125 ft-lb170 N·m
1-8250 ft-lb339 N·m225 ft-lb305 N·m187 ft-lb254 N·m

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.

Material and marking

SAE J429 Grade 2 is a low or medium carbon steel. Unlike Grades 5 and 8, it is not listed as quenched and tempered. Core hardness is Rockwell B80–B100 up to 3/4 in and B70–B100 above ⁠1.

The head carries no grade mark. Grade 1 also has a plain head ⁠1, and Grade 1 is rated at 33 ksi proof in every size from 1/4 to 1-1/2 in. So a plain-head bolt under 3/4 in could be either grade. If you cannot confirm Grade 2 from the packaging or the supplier, the stronger assumption is not safe.

The 3/4 in strength break

Grade 2 has two strength levels ⁠1:

  • 1/4 to 3/4 in: 55 ksi proof, 57 ksi yield, 74 ksi tensile.
  • 7/8 to 1-1/2 in: 33 ksi proof, 36 ksi yield, 60 ksi tensile.

The proof stress falls 40% at the break, which is enough to undo the gain from the larger bolt. A 3/4-10 bolt takes 173 ft-lb dry (13,820 lbf clamp). A 7/8-9 bolt takes 167 ft-lb (11,430 lbf), even though its stress area is 38% larger. A 1-8 Grade 2 bolt reaches only 250 ft-lb. Charts that apply one Grade 2 strength to all sizes overstate the large sizes by two-thirds.

Compared with other grades

At the same size and condition, torque scales with proof stress:

Grade 2 torque compared with other grades, same size and condition
Size range Grade 5 Grade 8
1/4 to 3/4 in 1.55 × Grade 2 2.18 × Grade 2
7/8 and 1 in 2.58 × Grade 2 3.64 × Grade 2

By strength, Grade 2 up to 3/4 in falls between metric classes 4.6 and 8.8. Above 3/4 in it matches class 4.6 closely, with 228 MPa proof against 225 ⁠3.

Never tighten a Grade 2 bolt to a Grade 5 value. At 1/2-13, the dry Grade 5 torque of 75 ft-lb would aim for a clamp load above the Grade 2 bolt’s proof load.

Matching nut

For a Grade 2 bolt, Fastenal lists a low-carbon nut, and SAE J995 Grade 5 or Grade 8 nuts may substitute ⁠1. A stronger nut does no harm. It is a weaker internal thread that strips first.

A plain head cannot prove the grade. When torque matters, a marked Grade 5 bolt removes the doubt. See Grade 5 and the SAE chart.

Sources

  1. 1
  2. 2
  3. 3

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