Bolt Torque Chart: Tightening Specs by Grade and Size

Reviewed by the ScrewBoltSize Editorial Team

Why Torque Specs Exist (And Why They're Not Optional)

Applying the right torque to a bolted joint does one thing: it creates a predictable clamping force. Under-torque it and the joint can loosen from vibration. Over-torque it and you can stretch the bolt past its yield point — permanently weakening it — or crush a gasket, crack a casting, or strip threads.

The relationship between torque and clamping force isn't direct — friction absorbs roughly 90% of the applied torque. That's why lubrication matters so much: it changes the friction coefficient, and the same torque value produces significantly different clamping force depending on whether the threads are dry, oiled, or coated in anti-seize.

SAE Grade Bolt Torque Chart (Imperial)

Values below are dry torque specifications to 75% of proof load — the standard general-purpose target. Use ft-lb for sizes 1/4" and up. Bolts must be clean, dry, and lightly oiled only where noted.

Values are dry torque to 75% of proof load per ASTM A307, A449, and A354. Fine-thread versions of the same bolt (e.g., 1/2-20 vs 1/2-13) require approximately 9% higher torque for the same clamping force.

Metric Bolt Torque Chart (N·m and ft-lb)

Metric fastener classes follow a different numbering scheme. Class 8.8 is roughly equivalent to SAE Grade 5; Class 10.9 is approximately Grade 8; Class 12.9 exceeds any SAE grade. All values below are for coarse pitch (standard) threads, dry.

Values per ISO 898-1 property classes. For fine-pitch metric threads (e.g., M10 × 1.25 instead of M10 × 1.5), increase torque by approximately 8%.

Dry vs Lubricated Torque: The 25–30% Rule

The torque wrench only measures torque — it has no idea how much clamping force you're producing. That's because friction dominates the torque equation. A simplified model:

T = K × D × F
T = torque (ft-lb or N·m) | K = nut factor (friction) | D = bolt diameter | F = clamp load

The nut factor K is roughly 0.20 for dry zinc-plated steel, 0.15 for oiled steel, and 0.13 for PTFE-coated fasteners. At K=0.15 vs K=0.20, the same torque produces 33% more clamping force. That's why applying dry torque specs to an oiled joint can yield the bolt.

Thread-locking compounds (Loctite) don't reduce torque as much as oil because they're applied in small amounts and their viscosity is higher. Follow the compound manufacturer's specifications when they differ from the bolt maker's.

Torque Sequence Matters: Pattern and Passes

For multi-bolt joints (cylinder heads, flanges, structural plates), achieving uniform clamping load requires both the right torque value and the right tightening sequence. Bolt pattern matters more than most people realize.

Star / cross pattern: On circular bolt patterns (flanges, wheels, cylinder heads), tighten in a star pattern — not sequentially around the circle. Sequential tightening concentrates load on one side, distorting the joint face.

Two-pass minimum: For critical joints, use two passes. First pass at 50% of final torque to seat the joint, second pass at 100%. For critical structural or head gasket joints, add a third pass.

Re-torque after thermal cycling: Gaskets and soft surfaces (aluminum, copper, fiber) relax after the first heat cycle. Many engine head gasket specs call for a re-torque after the first heat-to-operating-temperature cycle. Do not skip this step — you'll likely find torque has dropped 10–20%.

When Torque Specs Don't Apply

The chart values above are general-purpose guidelines. Several situations override them:

OEM service manuals always win. Automotive, aerospace, and heavy equipment manufacturers publish specific torque specs for every fastener in the assembly. If a service manual exists for your application, use it — not a generic chart.

Torque-to-yield (TTY) bolts. Many modern engine head bolts are torque-to-yield: they're intentionally stretched past yield on initial installation and must be replaced, not retorqued. If a service manual specifies "replace bolt after removal," it's TTY. Using a standard torque spec on a used TTY bolt is dangerous.

Flanged fasteners. Bolts with a flange (built-in washer) have different friction characteristics than standard hex bolts. Most flange bolt specs are 10–15% lower than equivalent non-flanged values.

Stainless steel fasteners. Stainless threads gall under the same torque that steel handles fine. Always lubricate stainless threads with anti-seize or PTFE paste, then reduce torque to 65% of dry steel spec.

Identifying Your Bolt Grade Before Torquing

Torquing to the wrong spec because you misidentified the grade is the most common mistake. Before applying any spec, confirm the grade:

• SAE Grade 2: No head markings. Common on hardware-store bolts. Do not substitute where Grade 5 or 8 is specified.
• SAE Grade 5: 3 radial lines on hex head. Medium carbon steel. Most common grade for general structural use.
• SAE Grade 8: 6 radial lines. Alloy steel, quenched and tempered. Required for many automotive and high-load applications.
• Metric Class 8.8: "8.8" stamped on head. Most common metric bolt from hardware stores and automotive supply.
• Metric Class 10.9: "10.9" on head. Alloy steel, similar to SAE Grade 8 in strength category.
• Metric Class 12.9: "12.9" on head, often a socket cap screw. Highest standard metric class; do not substitute a lower grade.

Use our bolt head markings guide and bolt identifier tool if you're uncertain of the grade of a fastener you've pulled from a disassembly.