How to Tell What Grade a Bolt Is

Quick visual markers to identify bolt grade

The fastest, most reliable first step is a visual inspection of the bolt head. Manufacturers and standards use standardized head markings—lines, numbers, or stamped class codes—that directly indicate grade (imperial/SAE) or property class (metric). Look for stamped numbers (e.g., "8.8", "10.9"), radial lines, or manufacturer logos. If markings are missing or obscured by paint, rust, or plating, proceed with measurement and testing.

Interpreting SAE (imperial) head markings

SAE (United States) bolts follow simple visual marks on the head:

• No radial lines (often only a manufacturer's logo): usually unfinished low-strength bolts commonly called Grade 2.
• Three radial lines on a hex head: Grade 5 (medium-strength, case-hardened/tempered).
• Six radial lines: Grade 8 (high-strength, quenched & tempered). Count the lines carefully — orientation shouldn't matter.

Reading metric property class markings

Metric bolts use two-digit (or two-part) property-class stamps like 8.8, 10.9, 12.9. This notation directly encodes mechanical properties:

• The first number ×100 = approximate ultimate tensile strength in MPa (e.g., 8.8 → 8×100 = 800 MPa).
• The second number ×10% = yield-to-tensile ratio (e.g., 8.8 → yield ≈ 0.8 of tensile → 640 MPa).
• Higher class numbers mean higher strength and usually a different heat treatment or alloy.

Tools and tests to confirm a bolt's grade

When markings are missing, ambiguous, or suspect, use tools and simple tests to confirm grade. Use the least-destructive tests first.

Essential tools

A small toolkit for identification:

• Vernier caliper or micrometer — measure diameter and thread dimensions.
• Thread pitch gauge — verify metric vs. imperial threads and pitch size.
• Hardness tester (Rockwell or portable tester) — gives comparative hardness values to infer heat treatment.
• Magnet and simple chemical spot tests — check for stainless vs carbon steel or plating presence (magnetic response varies).

Non-destructive confirmation steps

1) Verify thread type and size with a gauge. 2) Read any faint head stamp with good lighting, a magnifier, or solvent to remove grime. 3) Measure hardness — high-strength quenched & tempered bolts will be noticeably harder than low-grade bolts. 4) Compare to a known-good sample of the same size and finish.

Step-by-step identification workflow

Follow this practical sequence on the bench or in the field to identify most bolts.

• Clean the head: remove rust, paint, or grime so markings are visible.
• Read the stamp: if it shows 8.8, 10.9, or similar — record that as metric grade. If it shows radial lines, note how many for SAE.
• Measure diameter and pitch to confirm standard size and whether the bolt is metric or imperial.
• Perform a hardness test if marks are missing — compare to values from standards or a known sample to estimate grade.
• If critical, send a sample for lab tensile testing or chemical analysis (required for safety-critical components).

Quick reference: head marks and likely identification

Common pitfalls and safety notes

Do not rely solely on appearance for safety-critical applications. Re-stamping, head replacements, or counterfeit fasteners exist — a bolt can be re-marked to appear stronger than it is. Plating or heavy corrosion can hide markings. When in doubt for load-bearing or safety-critical use, verify with hardness/tensile testing or replace the fastener with one of known grade and traceable certification.

When to send bolts for lab testing

If the bolt will carry critical loads (structural connections, suspension, pressure vessels, or aerospace use), or if identification is unclear and failure would be catastrophic, send representative samples for tensile testing, hardness profiling, and chemical composition analysis. Labs provide traceable results and certificates necessary for compliance.

Summary checklist

• Clean the bolt head and read any stamp. Metric stamps (8.8, 10.9) are definitive.
• Count radial lines for SAE (0 → Grade 2, 3 → Grade 5, 6 → Grade 8).
• Measure threads and diameter to confirm standard; use hardness testing if needed.
• Replace unknown bolts in critical assemblies or obtain lab certification.