Fastener Parameter Quick Reference: Strength, Thread, Tolerance and Coating on One Page

1. How to use this reference: three conventions first

Fastener data has a common problem. It is all in the standards, but spread across seven or eight of them, so when you need a number you either spend half an hour searching or copy a table of unknown provenance that nobody can trace to an edition or a size range.

This reference gathers the parameters used most often on the floor, ordered strength → thread → tolerance → material → coating, with the standard and edition named for every table. Before you start, three conventions — get these wrong and every table after them will be misread:

2. The nine property classes (ISO 898-1 table 3)

This is the foundation of everything else. The table gathers the core figures from ISO 898-1:2013 table 3, covering coarse threads M1.6 to M39 and fine threads M8×1 to M39×3. The standard excludes set screws, which are covered by ISO 898-5.

Several things to hold on to when reading it:

• Class 3.6 was withdrawn in the 2009 edition. The current list is nine: 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, 12.9. A drawing still showing 3.6 is quoting a very old standard.
• 4.8, 5.8 and 6.8 are cold-work hardened classes. Their strength comes from work hardening during cold heading, with no quench and temper, so the standard does not specify machined-specimen elongation. It judges full-size elongation A_f instead — 0.24d, 0.22d and 0.20d of the original length — together with full-size 0.0048d proof stress R_pf. The dashes in the elongation row are not an omission; the assessment method differs.
• Class 9.8 is easily overlooked. Its tensile minimum matches 8.8 at a higher level (900 MPa against 830 MPa) but its proof stress is clearly higher (720 MPa against 660 MPa); the 9 in the marking stands for R_p0.2,nom = 900 MPa. It suits joints that need strength without the extreme yield ratio of 10.9, and asks slightly less of hardenability.
• 8.8, 9.8 and 10.9 all carry a hidden gate relative to 12.9. These classes must be quenched and tempered with a tempering temperature not below 425 °C. For 12.9 the standard additionally forbids a metallurgically visible white phosphorus-rich layer at the surface, and its removal must happen before heat treatment.

3. Reading a property class marking

The marking is not arbitrary. Two rules generate it: the number before the point × 100 = nominal tensile R_m,nom, and the number after the point ÷ 10 = nominal yield ratio R_p0.2,nom / R_m,nom.

Beyond the two digits, the full marking system has several add-ons, applied to the product or the packaging:

• Left-hand thread adds LH after the class marking, as in 8.8 LH.
• Nuts are marked with a single number, 8, 10 or 12, with no decimal point.
• Stainless works completely differently: steel group plus property class, as in A2-70 or A4-80, covered in section 9.

The most direct way to identify a class is to look at the head marking. Steel bolts normally carry the class digits stamped on the head top for hexagon heads or on the side for socket heads. The "clock face" convention distinguishes manufacturer identity marks — the maker's symbol sits at a defined position on the clock face and, with the class digits, forms the complete identification. When specifying on a drawing, the safest form is "class plus standard number", for example ISO 4017-M12×40-8.8.

4. Stress area A_s and minimum tensile load

Every strength calculation starts from one quantity: the nominal thread stress area A_s. It is not the circle of the major diameter but an equivalent area derived from pitch and minor diameters, about 20–25 % smaller than the nominal circle. With A_s, the minimum tensile load is one line:

Stress areas for common coarse threads, with minimum tensile load for the four most-used classes, rounded to the nearest hundred newtons:

Two cautions:

• For class 8.8 above 16 mm, R_m,min is 830 MPa rather than 800 MPa. The 8.8 column above uses 800 MPa and is therefore conservative; the real value for large sizes is slightly higher.
• Hot-dip galvanised parts must be reduced. Hot-dip galvanised fasteners with 6az thread tolerance per ISO 965-4 have their minimum tensile load reduced per ISO 10684. The table above does not apply to them.

5. Nut proof load and class pairing

Nuts are not tested for tensile strength but for proof load: under the specified proof stress the nut must neither strip nor fracture. The purpose is to make the bolt break before the nut strips, so failure is predictable.

Two rules worth memorising:

• A nut may be a higher class than its bolt; it may never be lower. When it is lower, the failure mode changes from the bolt breaking to the thread stripping — sudden, and often taking the whole thread length with it, with no warning.
• Thin nuts use a reduced proof stress. Nuts with nominal height between 0.5d and 0.8d have their own marking system and proof stress, such as class 04 at 380 MPa and class 05 at 500 MPa. They cannot borrow the values for a standard-height nut, and they suit only light loads or restricted space.

6. Hardness: three scales that do not compare

Three hardness scales are used for fasteners: Vickers HV across the full range, Brinell HBW for softer materials, and Rockwell HRB or HRC, HRB for soft and HRC for hard. ISO 18265 gives the conversions but states explicitly that they are valid only for carbon steel, low-alloy steel and cast steel in the hot-formed or heat-treated condition; high-alloy and cold-worked steels, including classes 6.8, A2 and A4, deviate substantially.

Compiled from the ISO 18265 relationships and rounded; the tensile column is reference only and is not used for a verdict.

The practical point: ISO 898-1 sets HV limits for each class and also gives corresponding HRB or HRC bands. Prefer HV for acceptance, since it covers the widest range. When using HRB or HRC, confirm the part sits within that scale's sensible band — class 12.9 cannot be measured on HRB. The standard also caps how far surface hardness may exceed core hardness, measured as the difference in HV0.3, which is how carburisation and decarburisation are detected.

7. Thread limits and tolerance classes

External threads normally use tolerance class 6g, previously marked 6h; internal threads use 6H. The major and pitch diameters below are external thread limits and can be used directly on a drawing or for inspection, per ISO 965-1 and -2.

Coating consumes tolerance. Electroplating has thickness, and a thread machined to the top of the 6g band may not fit after plating; conversely an over-thick coating can bind an internal thread solid. That is why ISO 4042 requires the pre-plating fundamental deviation to be shifted according to coating thickness, as covered in section 12. How much allowance to leave before plating should be written on the drawing as a coating code, with the supplier allocating it per the standard. Omit it and responsibility cannot be assigned when production goes wrong.

8. Tapping holes and clearance holes

Pre-tapping hole size and bolt clearance hole size are the two numbers looked up most often in design and assembly. Note that tapping hole size depends on pitch and thread form, not on property class — never derive it from the bolt class.

Tapping drill sizes for common coarse threads

⚠ These figures apply only to plain thread tapping. Where an insert such as a wire thread insert is fitted, the hole must be enlarged to the dedicated STI value and cut with a dedicated tap: for M6 × 1.0 the plain tapping drill is 5.0 mm, while the insert hole is about 6.3 mm. The two must never be confused.

Clearance holes per ISO 273

Which band to use depends on assembly accuracy and the clamped material. Fine is for close location with controlled clearance; coarse suits welded structures and painted parts that must absorb larger hole position errors.

9. Stainless fasteners: a different system

Stainless cannot be looked up against steel property classes. ISO 3506 uses a two-part marking of steel group plus property class, such as A2-70 or A4-80. The group identifies the composition family, the number the strength level.

Stainless elongation is a full-size figure, expressed as a multiple of d, rather than measured on a machined specimen. That is not the same thing as the carbon steel A value, and the two must not be compared directly.

Small stainless screws are also commonly accepted on minimum breaking torque M_B,min per ISO 3506:

The first principle for choosing a group is environment, not strength. A2 and A4 differ mainly in molybdenum: A4 contains it, and resists chloride pitting and acids markedly better. Coastal, chemical and chloride-bearing cleaning environments should go to A4, the 316 family. Choose A2-70 on strength alone and rust spots may appear within months.

10. Non-ferrous fasteners

Aluminium screws are used where weight matters, where galvanic corrosion must be avoided, or where the fastener should match the clamped material. Strength is far below steel, so design must use the real aluminium R_p0.2 and cannot borrow steel torque values.

Two safety notes. High-copper 2xxx alloys such as 2017A are sensitive to stress corrosion cracking and need care in wet or chloride service. 7xxx alloys must be used in an overaged condition such as T73, because underaged T6 carries SCC risk at high stress. Aluminium has about one third the elastic modulus of steel, around 70 GPa, so it deflects far more at the same torque, which makes tightening by feel especially dangerous on aluminium.

11. Chemical composition limits

A property class is not only a mechanical rating; it also implies a set of composition constraints, which is a useful corroboration of whether a shipment really is class 8.8. From ISO 898-1 table 2:

Constraints that are easily missed:

• From class 8.8 upwards, the P and S limits drop from 0.05/0.06 to 0.025/0.025. Ordinary quality carbon structural steel often cannot meet that, which explains why many "class 8.8 bolts made from plain 45 steel" are non-compliant on composition alone.
• Boron is capped at 0.003 % for classes 8.8, 9.8, 10.9 and 12.9.
• Alloy steel has a definition. It must contain at least one of Cr 0.30 %, Ni 0.30 %, Mo 0.20 % or V 0.10 %, and where several are added, the total alloy content must be at least 70 % of the sum of the individual minimum contents.
• Class 10.9 restricts low-carbon grades. The 425 °C minimum tempering temperature rules out some low-carbon alloy steels, such as 20MnTiB, that only reach class 10.9 with a lower tempering temperature.

12. Coating thickness limits and the hydrogen line

Thicker coating is not better. ISO 4042 sets coating thickness ceilings from the thread tolerance position, so that the thread still assembles after plating. Common ceilings for coarse external threads in 6g:

Extract only; actual values vary with tolerance position and nominal length, so consult the ISO 4042 table before processing. Threads in tolerance classes f and e permit thicker coatings.

Hydrogen embrittlement is the biggest killer of plated high-strength parts. ISO 4042 defines it clearly:

• Risk threshold: fasteners made from material at R_m ≥ 1,000 MPa, roughly 320 HV or above, carrying tensile stress and electroplated are at risk of hydrogen embrittlement failure.
• Risk reduction: a heat treatment, a baking step, after pickling or plating reduces fracture risk but cannot guarantee elimination. For class 10.9 and above, at 320 HV and above, the standard therefore recommends inorganic zinc coatings or mechanical zinc plating, processes that generate no hydrogen.
• If plating is unavoidable: bake as soon as possible after plating, normally within four hours, at 200–230 °C and below the tempering temperature, typically for 4–24 hours.

In other words: choose a hydrogen-free process for high-strength parts rather than plating and then trying to fix it. That is a design-stage decision, not something production can rescue.

13. Unit conversions

14. The five traps in this handbook

15. Sources and standards

• ISO 898-1:2013, Mechanical properties of fasteners made of carbon steel and alloy steel, Part 1: Bolts, screws and studs with specified property classes — coarse thread and fine pitch thread: mechanical and physical properties of the nine classes, table 3; material and chemical composition, table 2; stress area and minimum tensile load; proof load; marking system.
• ISO 898-2, Mechanical properties of fasteners, Part 2: Nuts with specified property classes: nut proof load, style 1 and style 2 nuts, proof stress for thin nuts.
• ISO 3506-1 and -2, Mechanical properties of corrosion-resistant stainless steel fasteners: groups A1 to A5, property classes 50, 70 and 80, minimum breaking torque.
• ISO 965-1, -2 and -4, ISO general purpose metric screw threads — Tolerances: 6g and 6H tolerance classes and limits; 6az for hot-dip galvanised parts.
• ISO 4042, Fasteners — Electroplated coating systems: coating thickness ceilings against thread tolerance, hydrogen embrittlement risk threshold and baking requirements.
• ISO 18265, Metallic materials — Conversion of hardness values: Vickers, Brinell and Rockwell comparisons, with the statement of applicable materials.
• ISO 273, Fasteners — Clearance holes for bolts and screws: fine H12, medium H13 and coarse H14 clearance diameters.
• ISO 10684, Fasteners — Hot dip galvanized coatings: correction of minimum tensile load for hot-dip galvanised parts.
• ISO 898-5, Set screws and similar threaded fasteners: the threaded fasteners excluded from ISO 898-1.
• ISO 724 and ISO 262, ISO general purpose metric screw threads — Basic dimensions and Selected sizes for screws, bolts and nuts: basic thread dimensions and the coarse and fine selection series.