grades of inch-sized bolts

SAE grades

The best-known grades for inch-sized steel bolts are those defined by the SAE, a sequence of grades from 0 to 8, on the basis of the metal from which the bolt is made and the manner of manufacture. Available grades run from 2 to 8, with 8 the strongest. Higher grade numbers almost always mean increased strength (an exception is that some grade 6 bolts are stronger than grade 7).

Grade	What kind of bolts	Proof load, ksi	Yield strength, minimum, ksi	Tensile strength, minimum, ksi	Marking
1
2
3
4	studs
5
5.1
5.2
6
7
8
8.1

The heads of steel bolts are marked to identify their grade.

drawing of markings on bolt heads

J429

ASTM International grades

Specification		Mark
A307	grade A
	grade B
A449
A325	type 1
	type 2
	type 3
A354
A490	type 1	A490
	type 2	A490
	type 3	A490

Is a higher grade always better?

Replacing a bolt with a stronger one can be a bad idea, cost aside.

Some bolts were deliberately chosen so that they are weak enough to fail before the stress or strain damages some more expensive or critical part of the equipment. For the same reason, in making furniture cabinetmakers use glues that are weaker than wood. That way, if the furniture is overloaded, the joints break. It is much easier to reglue a broken joint than to replace a piece of broken wood.

Perhaps a more important reason is that the same processes that make a fastener harder and stronger are more subject to fatigue and corrosion To quote Alexander Blake, "The impression that we get a better product for the money because of the increased strength can eventually hurt us since higher strength means greater susceptibility to stress corrosion and fatigue failure."

In the 1980s, large numbers of counterfeit bolts appeared in the United States, almost all imports. For this reason, the SAE grade markings can no longer be trusted unless one knows exactly who made and graded the bolt. Aerospace-grade bolts are also being counterfeited (even NASA has been duped, to the tune of one million dollars to disassemble the Astro 1 space lab to remove counterfeit and defective fasteners).

Alexander Blake.
What Every Engineer Should Know about Threaded Fasteners.
New York: Marcel Dekker, Inc., 1986.

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