What the Numbers on the Bolt Head Mean

Per GB/T 3098.1-2010, property class marks on bolts follow a two-number code. The number to the left of the decimal is one-hundredth of the nominal tensile strength in MPa. The number to the right is the yield-to-tensile ratio. So 8.8 means 8 x 100 = 800 MPa tensile, with a yield ratio of 0.8, giving 640 MPa yield. A 10.9 bolt has 1040 MPa tensile and 940 MPa yield. A 12.9 bolt has 1220 MPa tensile and 1100 MPa yield. The higher the numbers, the stronger and harder the bolt.

These marks are raised on the top of the bolt head for bolts of M6 and larger. Smaller bolts (under M6) may not have room for raised marks and may use other identification. The marks are mandatory for bolts of 8.8 class and above; they are how the user verifies the bolt class on site and how the manufacturer traces responsibility.

Mechanical Properties from 4.6 to 12.9

The 4.6 and 5.6 classes are low-carbon grades used for light sheet-metal covers, nameplates, and non-critical brackets. The jump from 4.8/5.6 to 8.8 is not a gradual step: it crosses from low-carbon as-formed bolts to quenched-and-tempered bolts. The 8.8 and above classes require heat treatment (quench and temper), which is why they are stronger and more brittle. The 10.9 and 12.9 classes use alloy steel for hardenability through the cross-section.

Manufacturer Marks, Left-Hand Thread, Stainless Grades

Next to the property class number, most bolts also carry a manufacturer's identification mark (a logo or letters). This is required by GB/T 3098.1 for bolts of 8.8 class and above, and it enables traceability: if a bolt fails, the manufacturer mark identifies who made it. Left-hand thread bolts have an arrow or notch on the head to indicate the reverse thread direction. Stainless steel bolts use a different marking system, defined in GB/T 3098.6: A2-70, A4-80, etc., where the letter-number is the austenitic grade and the trailing number is one-tenth of the tensile strength in MPa. The bolt class must also be paired with a matching nut class per GB/T 3098.2 (for example, an 8.8 bolt uses a class 8 nut and a 10.9 bolt uses a class 10 nut).

Why Markings Are Mandatory: Traceability and Anti-Counterfeit

Markings are not optional decoration. In automotive and aerospace supply chains, a bolt without a clear property class and manufacturer mark is rejected on incoming inspection. The marking system enables two things: traceability (which heat lot, which factory, which shift) and anti-counterfeit (counterfeit bolts often skip proper heat treatment and do not carry correct marks). When a bolt fails in service, the head marks are the first evidence investigators examine. A bolt stamped "12.9" that tests at 8.8 hardness is a counterfeit and a safety hazard.

Three Common Mistakes When Checking Markings

First, confusing the two numbers: some users read "8.8" as "880 MPa" or "8.8 grade" without understanding the yield ratio. Second, ignoring the manufacturer mark and assuming any bolt stamped "10.9" is genuine - counterfeit bolts are a known industry problem, especially in gray markets. Third, mixing metric and inch markings: inch bolts use SAE grades (Grade 5, Grade 8) that look similar but are not interchangeable with metric property classes. High-strength 10.9 and 12.9 bolts that are electroplated also carry hydrogen-embrittlement risk and must receive bake-out and pre-load testing per GB/T 3098.17. When in doubt, verify with a hardness test or a tensile test rather than relying on the head stamp alone.

Counterfeit fastener risk. In price-driven procurement, counterfeit high-strength bolts are a real problem. A bolt stamped "10.9" that is actually made from low-carbon steel will fail under load. Buy from suppliers who can provide material certificates (mill certs), who run incoming hardness and tensile tests, and who mark their bolts with a manufacturer's identification. If the price seems too good to be true, it probably is - a genuine 10.9 alloy steel bolt costs a certain amount to heat-treat and test, and counterfeiters skip those steps. For safety-critical applications, third-party inspection or material spectroscopy on incoming lots is a worthwhile investment.

Field identification when markings are unclear. On a used bolt where the head marking is worn or painted over, you can estimate the class by hardness. An 8.8 bolt measures about 250-320 HV (about 24-32 HRC). A 10.9 bolt measures 320-380 HV. A 12.9 bolt measures 380-430 HV. A portable hardness tester or file test gives a rough indication. A magnetic test separates stainless (non-magnetic, austenitic) from carbon steel (magnetic). If you cannot identify a bolt on a safety-critical joint, replace it with a known-good bolt of the specified class rather than reusing an unknown one. The few cents saved by reusing an unidentified bolt is not worth a safety failure.

Marking requirements by size. Per GB/T 3098.1, property class marks are mandatory on bolts of M6 and larger for 8.8 class and above. Bolts smaller than M6 may not have room for raised marks, and manufacturers use packaging and lot labels to identify them. Studs use a different marking system. Nuts have their own marking (a number indicating the nut class). When incoming inspecting, verify that the marks match the purchase order. A bolt that arrives without the required marks should be quarantined pending material verification. In safety-critical applications, do not accept unmarked high-strength bolts regardless of price.

Counterfeit fastener risk. In price-driven procurement, counterfeit high-strength bolts are a real problem. A bolt stamped "10.9" that is actually made from low-carbon steel will fail under load. Buy from suppliers who can provide material certificates (mill certs), who run incoming hardness and tensile tests, and who mark their bolts with a manufacturer's identification. If the price seems too good to be true, it probably is - a genuine 10.9 alloy steel bolt costs a certain amount to heat-treat and test, and counterfeiters skip those steps. For safety-critical applications, third-party inspection or material spectroscopy on incoming lots is a worthwhile investment.

Field identification when markings are unclear. On a used bolt where the head marking is worn or painted over, you can estimate the class by hardness. An 8.8 bolt measures about 250-320 HV (about 24-32 HRC). A 10.9 bolt measures 320-380 HV. A 12.9 bolt measures 380-430 HV. A portable hardness tester or file test gives a rough indication. A magnetic test separates stainless (non-magnetic, austenitic) from carbon steel (magnetic). If you cannot identify a bolt on a safety-critical joint, replace it with a known-good bolt of the specified class rather than reusing an unknown one. The few cents saved by reusing an unidentified bolt is not worth a safety failure.

Marking requirements by size. Per GB/T 3098.1, property class marks are mandatory on bolts of M6 and larger for 8.8 class and above. Bolts smaller than M6 may not have room for raised marks, and manufacturers use packaging and lot labels to identify them. Studs use a different marking system. Nuts have their own marking (a number indicating the nut class). When incoming inspecting, verify that the marks match the purchase order. A bolt that arrives without the required marks should be quarantined pending material verification. In safety-critical applications, do not accept unmarked high-strength bolts regardless of price.