Bolt Head Markings Explained: Property Classes & Grades

1. Why Bolt Head Markings Matter

A bolt is only as good as its property class. The marking on the head - or on the nut face - tells the fitter and the inspector which strength grade the fastener belongs to, so the right bolt can be installed in the right joint. Using an unmarked or wrongly identified bolt in a load-bearing connection can lead to premature loosening, fatigue failure or sudden fracture. Markings also enable traceability: the manufacturer's mark lets the lot be traced back to the mill certificate and heat number.

2. Metric Property Classes: Reading the Two Numbers

The most common metric marking system is defined by ISO 898-1 for carbon steel bolts, screws and studs. It consists of two numbers separated by a dot, for example 8.8, 10.9 or 12.9:

The number before the dot multiplied by 100 gives the approximate minimum tensile strength in MPa

The number after the dot multiplied by 10 gives the yield strength as a percentage of the tensile strength

For class 8.8: tensile strength approximately 800 MPa and yield strength approximately 800 x 80% = 640 MPa. For class 10.9: 1000 MPa tensile and about 900 MPa yield. For class 12.9: 1200 MPa tensile and about 1080 MPa yield. The same logic applies to nuts, where the marking indicates the property class of the nut.

Property class	Min tensile strength (MPa)	Min yield / proof strength (MPa)
8.8	800	640
10.9	1000	900
12.9	1200	1080

3. Stainless Steel Fastener Markings: A2 and A4

Stainless steel bolts and nuts use the designation system of ISO 3506 instead of the dot system. The marking is a letter and a number, for example A2-70 or A4-80:

A2 designates austenitic grade 304 (chromium-nickel) stainless steel

A4 designates austenitic grade 316 (chromium-nickel-molybdenum) stainless steel

The number after the dash is one tenth of the minimum tensile strength in MPa: 70 means about 700 MPa, 80 means about 800 MPa

Both families resist corrosion well; A4 (316) adds molybdenum, giving better resistance in chlorides, seawater and acidic environments, and is specified where the service demands it. Group A4 is also preferred at low temperatures where impact toughness matters.

4. Why Some Bolt Heads Have No Marking

Bolts without head markings are normally the low-strength classes 4.8 or 5.8, which ISO 898-1 does not require to be marked. A 4.8 bolt has a minimum tensile strength of about 400 MPa and a yield of about 320 MPa; 5.8 gives 500 MPa tensile and about 400 MPa yield. These are general-purpose fasteners for light duty: furniture, guards, covers and non-structural connections. If a marked bolt is required for structural or safety-critical joints, specify the property class on the purchase order and verify the marking on delivery.

5. Imperial Markings and Buyer's Guidance

Inch-series fasteners use a different convention. SAE J429 grades are marked with radial lines on the head: Grade 5 has three radial lines, Grade 8 has six. ASTM A325 and A490 structural bolts carry "A325" or "A490" text plus the manufacturer's mark. Metric and imperial markings are not interchangeable; mixing them in one joint is a common field error that inspectors look for.

Select the property class from the design: 8.8 for general structural duty, 10.9 for higher loads, 12.9 only where the design specifically allows it (it is notch-sensitive and prone to hydrogen embrittlement if plated)

Match nut class to bolt class: a class 10 nut on a class 8.8 bolt is wrong

For stainless applications, choose A2-70 for standard corrosion duty and A4-80 for marine and chemical service

Avoid galvanic corrosion: use stainless fasteners on stainless structures; use isolating washers when joining stainless to aluminium

Apply torque per the fastener's class and lubrication state; the same class 10.9 bolt torques differently when lubricated versus dry

Request the mill certificate with heat number for project-critical lots, and inspect markings at goods-in

For non-standard and special-shaped fasteners, drawings, OEM and ODM orders can be produced to customer specification with matching certificates; technical consultation on grade selection should be resolved before ordering, not after delivery.

6. FAQ

What does 8.8 mean on a bolt head?

It is the property class per ISO 898-1: the minimum tensile strength is about 800 MPa (8 x 100) and the yield strength is about 80% of that, roughly 640 MPa.

What do A2 and A4 mean on stainless bolts?

A2 is 304 stainless steel and A4 is 316 stainless steel, per ISO 3506. The number that follows (70 or 80) indicates the minimum tensile strength: A2-70 means about 700 MPa, A4-80 about 800 MPa.

Why is a 12.9 bolt marked but a 4.8 bolt is not?

ISO 898-1 requires marking only for property classes from 8.8 upward. Low-strength classes 4.8 and 5.8 are normally supplied unmarked because they are general-purpose fasteners for light duty.

What is the difference between class 10.9 and class 12.9 bolts?

Class 10.9 has about 1000 MPa tensile and 900 MPa yield; class 12.9 has about 1200 MPa tensile and 1080 MPa yield. Class 12.9 is stronger but more sensitive to notches, stress corrosion and hydrogen embrittlement, so it is restricted to specific designs.

Which stainless bolt should I use outdoors near the sea?

Use A4 (316) stainless steel fasteners, typically A4-70 or A4-80. The molybdenum content resists chloride pitting and crevice corrosion far better than A2 (304) in marine atmospheres.

How do I verify the grade of delivered bolts?

Check the head marking against the order, measure hardness or tensile on samples if required, and request the mill certificate with heat number. Unmarked bolts should be treated as class 4.8/5.8 unless certified otherwise.