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A325 vs Grade 5 and A490 vs Grade 8: why they are not interchangeable

On paper A325 equals SAE Grade 5 up to 1 in (120 ksi minimum tensile), and A490 equals Grade 8 (150 ksi). They are still not interchangeable. Structural bolts have larger heavy hex heads, shorter threads and an A490 strength ceiling, and RCSC covers only ASTM F3125 and F3148 bolts.

By the Torque Reference editors 3 sources cited Last reviewed Method
Contents7 sections

Key takeaways

  • Up to 1 in, A325 and Grade 5 share 120 ksi tensile, 85 ksi proof, 92 ksi yield; above 1 in A325 is about 14% stronger ⁠1 ⁠2.
  • A490 and Grade 8 share 150 ksi tensile and 120 ksi proof; A490 also has a 173 ksi maximum ⁠1 ⁠3.
  • A 3/4 in structural bolt takes a 1-1/4 in wrench, 1/8 in larger than a 3/4 in hex cap screw ⁠3 ⁠2.
  • Fastenal says a Grade 5 must not replace a specified A325, nor a Grade 8 a specified A490 ⁠2.

Quick facts

A325 vs Grade 5, 1/2–1 inSame 120 / 85 / 92 ksi
A325 vs Grade 5, over 1 in120 vs 105 ksi tensile
3/4 in thread length1-3/8 in structural vs 1-3/4 in cap screw

Strength: where they match and where they do not

SAE J429 vs ASTM F3125 strength
Property Grade 5, 1/4–1 in Grade 5, over 1–1-1/2 in F3125 A325, 1/2–1-1/2 in Grade 8, 1/4–1-1/2 in F3125 A490, 1/2–1-1/2 in
Min. tensile 120 ksi 105 ksi 120 ksi 150 ksi 150 ksi
Max. tensile — — — — 173 ksi
Proof (length method) 85 ksi 74 ksi 85 ksi 120 ksi 120 ksi
Yield (SAE) / proof by yield method (F3125) 92 ksi 81 ksi 92 ksi 130 ksi 130 ksi

Scroll the table sideways to see every column.

SAE values are from the Fastenal guide ⁠2, F3125 values from F3125-15 Table 4 as published by St. Louis Screw & Bolt ⁠1, and the A490 maximum from RCSC ⁠3.

Up to 1 in, A325 and Grade 5 have the same minimums. Above 1 in they split. Grade 5 drops to 105 ksi tensile and 74 ksi proof; F3125 A325 stays at 120 and 85 ⁠2 ⁠1. That makes A325 about 14% stronger in tensile and 15% in proof at 1-1/8 to 1-1/2 in. On a 1-1/4-7 bolt (0.969 sq in stress area), the A325 full-size tensile minimum is 116,300 lbf ⁠1; Grade 5 at 105 ksi works out to about 101,700 lbf.

A490 and Grade 8 share the 150 ksi minimum and 120 ksi proof ⁠1 ⁠2. A490 adds a 173 ksi maximum and a 38 HRC hardness cap. Both are there to keep production bolts clear of the 200 ksi range where hydrogen embrittlement becomes a risk ⁠3.

Head and nut size

Structural bolts are heavy hex. RCSC explains that the head is one of the two features that set them apart from general-purpose bolts. Its width across flats equals the heavy hex nut’s, so one wrench fits both ends ⁠3. Grade 5 and Grade 8 bolts are usually made as standard hex cap screws ⁠2.

Width across flats
Diameter Structural bolt and heavy hex nut Hex cap screw (Grade 5/8)
1/2 in 7/8 in 3/4 in
5/8 in 1-1/16 in 15/16 in
3/4 in 1-1/4 in 1-1/8 in
7/8 in 1-7/16 in 1-5/16 in
1 in 1-5/8 in 1-1/2 in

Structural dimensions are from RCSC Table C-2.1 (ASME B18.2.6) ⁠3; cap screw dimensions are from ASME B18.2.1 via Fastenal ⁠2. The heavy hex is 1/8 in wider across flats at every size from 1/2 to 1 in, giving a larger bearing surface under the head ⁠2. The socket size chart lists both.

Thread length

The second difference is thread length. Heavy hex structural bolts have shorter threads than general-purpose bolts. Body length is the controlled dimension, so the threads can be kept out of the shear plane ⁠3.

Thread length
Diameter Structural bolt thread length Hex cap screw thread length (6 in and shorter)
1/2 in 1 in 1-1/4 in
3/4 in 1-3/8 in 1-3/4 in
1 in 1-3/4 in 2-1/4 in

Sources: RCSC Table C-2.1 ⁠3, ASME B18.2.1 via Fastenal ⁠2.

This matters in shear. Fastenal’s example uses two 3/4-10 × 4 bolts and an estimated 72 ksi shear strength. With the shear plane through the body (0.417 sq in), capacity is nearly 30,000 lb. Through the threads (0.302 sq in) it is about 21,740 lb, roughly 8,260 lb less ⁠2. Structural drawings mark this choice as X (threads excluded) or N (threads may be included) ⁠2. The shorter thread also means the installer must check there is enough thread between nut and run-out so the nut does not jam ⁠3.

Why they are not interchangeable

  • The Specification does not cover them. RCSC defines a high-strength bolt as an ASTM F3125 or F3148 bolt, or an approved alternative design under Section 2.12 ⁠3. Its pretensions, rotations and design strengths are written for those bolts.
  • Direct statement. Fastenal asks whether a Grade 5 can be used where an A325 is specified and answers “definitely not”. The same goes for a Grade 8 where A490 is specified, because of the head and thread-length differences above ⁠2.
  • Different behavior under load. Fastenal reports a test of A325, A490, Grade 5 and Grade 8 bolts at different pretension levels. Grade 5 and 8 bolts used in place of A325 and A490 gave larger deformations at failure; their longer threads spread the plastic strain ⁠2.
  • Matched components. Structural bolts go with ASTM A563 heavy hex nuts of the grades in RCSC Table 2.2 and F436 washers of the same type as the bolt ⁠3. Galvanized A325 bolts and nuts must be supplied and rotational-capacity tested as an assembly ⁠3.

Torque and tension differ too

A Grade 5 or Grade 8 torque chart does not install a structural bolt. Charts on this site target 75% of proof load ⁠2. For a 3/4-10 that is about 21,360 lbf for Grade 5 and 30,150 lbf for Grade 8. RCSC requires 28 and 35 kips for the 3/4 in A325 and A490 ⁠3, about 31% and 16% more. The RCSC pretension is 70% of tensile strength, not 75% of proof ⁠3. Structural bolts are tensioned by the methods on the A325 and A490 pages; for Grade 5 and 8 in machinery, see Grade 8 bolt torque and the Grade 5 page.

Sources

  1. 1
  2. 2
  3. 3

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