Bolt Grades Explained: 8.8 vs 10.9 vs 12.9

Look at a bolt head and you will usually see two numbers separated by a dot — 8.8, 10.9, or 12.9. These are not model numbers or batch codes. They are ISO property class designations that precisely define the bolt's minimum mechanical strength. Specifying the wrong grade is one of the most common — and costly — mistakes in industrial procurement. This article explains exactly what the numbers mean and when to use each grade.

What the Numbers Actually Mean

The property class system is defined in ISO 898-1:2013 (Mechanical properties of fasteners — Bolts, screws and studs). It applies to metric fasteners made from carbon or alloy steel.

The designation has a simple mathematical structure:

• First digit(s) × 100 = nominal minimum tensile strength in MPa
• Second digit × 10% = the yield-to-tensile ratio (as a percentage)

So for 8.8: tensile strength = 8 × 100 = 800 MPa. Yield ratio = 8 × 10% = 80%. Minimum yield = 0.80 × 800 = 640 MPa.

For 10.9: tensile = 1000 MPa nominal (actual minimum 1040 MPa per ISO 898-1). Yield ratio = 90%. Minimum yield = 940 MPa.

For 12.9: tensile = 1200 MPa nominal (actual minimum 1220 MPa). Yield ratio = 90%. Minimum yield = 1100 MPa.

The first digit gives you the strength. The second digit tells you how close the bolt is to yielding at its maximum load — a useful measure of reserve ductility.

Grade Comparison Table

Q&T = quenched and tempered. Values from ISO 898-1:2013.

Grade 8.8 — The Structural Workhorse

Grade 8.8 is the most widely used high-strength bolt in construction and general engineering. It is referenced directly in EN 1993-1-8 (Eurocode 3) for structural steel connections and in equivalent codes worldwide. At 800 MPa tensile strength, it provides a substantial safety margin for most applications without the brittleness concerns of higher grades.

Use grade 8.8 when:

• You are connecting structural steelwork (beams, columns, base plates)
• The joint is loaded primarily in shear or bearing
• Hot-dip galvanizing is required for corrosion protection
• The specification calls for "high-strength bolts" without a more specific grade
• Field installation is done by general contractors (8.8 tolerates moderate installation variability)

Grade 10.9 — Preloaded and High-Tension Applications

Grade 10.9 is the standard for high-strength friction-grip (HSFG) bolts used in preloaded structural joints (EN 14399-4). It is also the dominant grade in automotive manufacturing and heavy machinery. At 1040 MPa tensile strength, a 10.9 bolt of a given diameter generates significantly more clamping force than an 8.8 — allowing smaller bolts or fewer of them for the same joint.

Use grade 10.9 when:

• The joint design requires preloading (tensioned bolting to slip-critical connections)
• Vibration resistance is required — high preload reduces bolt loosening under dynamic loads
• Space constraints mean fewer or smaller bolts are needed
• The specification explicitly calls for 10.9 or HSFG bolts

Important: grade 10.9 requires controlled torque installation with a calibrated torque wrench. Over-torquing a 10.9 bolt can induce plastic deformation or fracture. Do not use hot-dip galvanizing on 10.9 (see coatings section below).

Grade 12.9 — Maximum Strength, Maximum Care

Grade 12.9 is the highest standardised property class and should be specified only when genuinely necessary. At 1220 MPa, it provides extreme clamping force in a compact fastener — which is why it dominates in precision machinery, die tooling, automotive engine components, and aerospace sub-assemblies. However, it is the most brittle of the common grades and the most susceptible to hydrogen embrittlement and stress-corrosion cracking.

Use grade 12.9 when:

• Space is severely constrained and maximum clamp load per bolt is critical
• The design is verified by a mechanical or structural engineer
• Precise installation torque and lubrication will be applied
• The application is in controlled-environment machinery (not exposed weather or corrosive environments)

Never use grade 12.9 when:

• Hot-dip galvanizing is required
• The fasteners will be installed by workers without torque control tools
• The joints will experience impact loading or vibration without preload retention measures

Head Markings and Identification

ISO 898-1 requires every fastener to carry two permanent markings on the head (or on the thread end of studs):

1. Property class — embossed or indented: 8.8, 10.9, or 12.9
2. Manufacturer's identification mark — a letter, symbol, or number registered to the manufacturer for full traceability

Some bolt heads also carry clock-position radial lines as an alternative identification method — three lines at 120° intervals on 12.9 bolts is a common convention, though the property class number takes precedence.

Never install an unmarked bolt in a structural or safety-critical joint. The absence of a property class marking means the fastener's properties are unverified and should be treated as unknown.

Coatings and Grade Compatibility

Coating choice is not independent of bolt grade. Hydrogen embrittlement — absorbed during acid pickling before hot-dip galvanizing or electroplating — becomes a catastrophic risk at higher strength levels because the material is less able to accommodate the trapped hydrogen.

Guidance per ISO 4042 (electroplating) and ISO 10684 (hot-dip galvanizing). Where electroplating is used on 10.9, a hydrogen embrittlement relief bake (190°C for ≥4 hours, within 4 hours of plating) is required per ASTM B849.

Mill Test Certificates and Supply from TERNS EXIM

Every bolt shipment from TERNS EXIM is accompanied by a Mill Test Certificate (MTC) per EN 10204 Type 3.1 (or 2.2 where specified). The MTC records:

• Heat / melt number for full material traceability
• Chemical composition (carbon, manganese, phosphorus, sulphur, boron/chromium for alloy grades)
• Tensile strength, yield strength, and elongation test results
• Hardness test results (Vickers HV)
• Property class confirmation and standard reference (ISO 898-1)
• Manufacturing plant name and test laboratory

We supply grades 8.8, 10.9, and 12.9 in all standard metric sizes — including hex bolts and anchor bolts — in plain, zinc-plated, hot-dip galvanized, and mechanically plated finishes. (Hot-dip galvanizing is suitable for grade 8.8; it is not recommended for 10.9 and is prohibited for 12.9 due to hydrogen embrittlement risk — see the coating section above.) Third-party pre-shipment inspection (SGS, Bureau Veritas, or TÜV) is available on request for high-value or safety-critical orders.