Guides / Grades and Materials

What Fastener Grades and Property Classes Mean

September 27, 2026

The number stamped on a bolt head or a nut face is a strength rating, not a material name. Reading it correctly tells you the tensile and yield strength, roughly how the part was heat treated, and which coatings are safe to put on it. This guide covers the classes and grades buyers call out most often on a custom fastener print.

How to read 8.8, 10.9 and 12.9 (ISO 898-1)

ISO 898-1 sets the property classes for carbon and alloy steel fasteners in metric threads. The class is written as two numbers separated by a dot, for example 8.8. The first number multiplied by 100 gives the nominal tensile strength in megapascals. The second number, divided by 10, gives the ratio of yield strength to tensile strength for that class.

Class	Nominal tensile	Nominal yield	Typical hardness	Typical use
8.8	800 MPa	640 MPa	22 to 32 HRC	General machinery and structural joints
10.9	1,000 MPa	900 MPa	32 to 39 HRC	Engines, heavy equipment and high-load joints
12.9	1,200 MPa	1,080 MPa	39 to 44 HRC	High-load joints where weight matters

All three classes are quenched and tempered after cold heading or machining. The higher the class, the narrower the furnace tolerance a heat treater has to hold, and the more that process control affects whether the finished part meets spec.

SAE J429 Grade 5 and Grade 8

Equipment and vehicles built to North American practice usually call out an SAE J429 grade in inch sizes rather than an ISO property class. Two grades cover most of what buyers ask for.

• Grade 5, roughly 120 ksi minimum tensile, marked with three radial lines on the head. Close to ISO class 8.8 in strength, though the two standards are not identical and a maker should not substitute one for the other without checking the print.
• Grade 8, roughly 150 ksi minimum tensile, marked with six radial lines. Comparable in strength to class 10.9, and it carries the same hydrogen embrittlement caution after electroplating.

If a customer specification calls out SAE grades, keep the print in inch dimensions with the matching head marking. Converting to a metric class on paper can produce a part that is equivalent in strength but fails an incoming inspection because the head does not carry the mark the customer’s drawing requires.

Stainless: A2-70, A4-80 (ISO 3506)

ISO 3506 covers stainless fasteners in two common families.

• A2 is the 304 family, general purpose corrosion resistance, non-magnetic when annealed and slightly magnetic after cold work.
• A4 is the 316 family, with added molybdenum for better resistance to chlorides, the usual choice for marine and outdoor parts.

The number after the letter, most often 50, 70 or 80, is a strength class built up by cold work during forming, not by heat treatment. A4-80 is stronger than A4-70 because the wire was worked harder, not because the alloy changed.

Stainless threads gall more easily than steel ones, which means they can seize during assembly under heat and pressure. A coating, a dry lubricant or a different material on the mating nut all cut the risk. Say on the RFQ if the part is assembled with an impact tool or at high speed, since that is when galling shows up most often.

Materials without a property class

Titanium, brass, copper and aluminum fasteners are not covered by ISO 898-1, so they do not carry a class number. Instead the print should name the material and its condition directly, for example Ti-6Al-4V annealed, or half-hard brass. Strength and hardness for these materials come from the alloy and how it was worked or aged, so a maker needs the material spec, not a class, to know what you expect.

Nuts and washers must match

A high-grade bolt paired with a low-grade nut can strip the nut thread before the bolt ever reaches its rated clamp load. ISO 898-2 sets property classes for nuts, numbered 04, 05, 8, 10 and 12, built to pair with the equivalent bolt classes. Hardened washers matter for the same reason: a soft washer can crush or dish under a high-grade bolt’s clamp load and quietly lose tension over the life of the joint. Put the nut class and washer hardness on the print, not just the bolt grade, and let the maker confirm the set works together.

Grade changes coating and heat treatment

The property class is not only a number on the head. It decides which finishing processes are safe to apply afterwards.

Hydrogen embrittlement. Electroplating deposits hydrogen into the steel alongside the coating metal. In a low-strength part the hydrogen escapes without doing harm. In a high-strength, heat-treated part such as a 10.9, a 12.9 or an SAE Grade 8, trapped hydrogen can crack the part under load, sometimes days or weeks after assembly. ISO 4042 sets out baking requirements for electroplated fasteners to drive the hydrogen back out before it causes a failure. If a print calls for electroplating on a high-strength grade, the bake step belongs on the print itself, not left to whatever the plater does by default.

Zinc flake instead. Many engineers avoid the embrittlement question altogether by specifying a zinc flake coating to ISO 10683. Zinc flake is applied by dipping and curing rather than plating in a bath, so it adds no hydrogen to the part. It costs more per piece than electroplating but removes a failure mode that is hard to catch by inspection alone.

Passivation on stainless. A2 and A4 parts are usually left bare or passivated instead of plated. Passivation, commonly done to ASTM A967, removes free iron left on the surface after forming or machining, which is what actually gives stainless its corrosion resistance. It is a chemical treatment rather than a coating, so it adds almost no dimension to the part.

Choosing a grade: stronger is not always better

A higher property class is not automatically the right call. Three trade-offs are worth weighing before specifying up.

1. Toughness falls as strength rises. A 12.9 bolt is harder and more brittle than an 8.8 bolt of the same size. In a joint that sees shock loads, or one that might see a slight overload, a lower class with more ductility can outlast a higher class that cracks rather than yields.
2. Embrittlement risk rises with the class. As covered above, a high-strength grade forces a coating decision that a lower grade never raises.
3. Cost rises at every step. Higher classes need tighter furnace control during heat treatment, closer inspection and, if plated, extra bake time. On a part ordered by the tens of thousands, that adds up to real money for strength the joint may never use.

The right grade is the lowest one that gives the joint its needed clamp load and fatigue life with a sensible margin, not the highest one available in the size.

A maker quoting a special grade or coating still needs the rest of the print to price it properly. Our custom fastener RFQ checklist covers everything else to include, and how to source custom fasteners walks through choosing the right manufacturing route for the part.

For service-condition choice, stainless, duplex and high-temperature alloys, read our guide to fastener grades.

When the grade and coating are set, send your RFQ and we match you with up to three makers that run that grade.

Frequently asked questions

Keep reading

• Torque vs Clamp Load: The K Factor on Custom Bolts
• Bolt Head Markings: Grade Marks and Manufacturer ID on Custom Bolts