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bolts: metric standards

Standards for metric bolts are promulgated by the ISO, but adopted by individual nations.

Dimensions

A table of major diameters and pitches, up to 30 mm in diameter.

Thread classes of fit

Please go here.

Property classes

In metric bolts, strength is described by “property classes.” A property class designation consists of two numbers separated by a decimal point. There are nine commonly encountered: 4.6, 4.8, 5.8, 8.8, 9.8, 10.9 and 12.9. In the United States, property classes 8.8.3 and 10.9.3 are also defined; the final “3” indicates the fastener is made of weathering steel.

The number before the first decimal point is approximately one-hundredth of the nominal minimum tensile strength of the bolt in newtons per square millimeter (or megapascals, which is the same thing).
The number after the first decimal point is approximately the ratio between the minimum yield stress and the minimum tensile strength, times ten.

markings

Hex head bolts M5 and larger must have the property class marked on the head of the bolt, either in raised or depressed lettering. Fasteners smaller than M5, and those with slotted or recessed heads, whatever their diameter, need not be marked. Radial lines, such those used to mark SAE grades, are never used on metric bolts.

drawing of bolt heads marked for property class 8.8

Property Class	Fastener Sizes	Composition	Marking
4.6	M5-M100		4.6
4.8	M1.6-M16		4.8
5.8	M5-M24; lengths ≤ 150mm	cold worked	5.8
8.8	M16-M72	Q&T	8.8
A325M Type 1	M16-M36	Q&T	A325M 8S
8.8	M16-M36	Q&T*	8.8
A325M Type 2	M16-M36	Q&T*	A325M 8S
A325M Type 3	M16-M36	weathering steel; Q&T	A325M 8S3
9.8	M1.6-M16	Q&T	9.8
9.8	M1.6-M16	Q&T	9.8
10.9	M5-M20	Q&T	10.9
10.9	M5-M100	Q&T	10.9
A490M Type 1	M12-M36	Q&T	A490M 10S
10.9	M5-M36	Q&T	10.9
A490M Type 2	M12-M36	Q&T	A490M 10S
A490M Type 3	M12-M36	weathering steel; Q&T	A490M 10S3
12.9	M1.6-M100	Q&T	12.9

* Q&T= quenched and tempered

Equivalents in other grading systems

Approximately Equivalent Grades
ISO class	Inch-based systems
4.6	SAE J429 grade 1 ASTM A 307, grade A
6.8	SAE grade 2
5.8	SAE J429, grade 2
6.8	SAE grade 2
8.8	SAE J429, grade 5 ASTM A 325, types 1 & 2 (M16 and larger) ASTM A 449
8.8.3	ASTM A325, type 3 (weathering steels)
9.8	9% stronger than SAE J429, grade 5 ASTM A 449
10.9	SAE J429, grade 8 ASTM A 354, grade BD
10.9.3	ASTM A 490
12.9	ASTM A 574

ASTM International.
Standard Specification for Carbon Steel Externally Threaded Metric Fasteners (Metric), F 568M - 07.

ISO R898

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