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Understanding Fastener Grades and Classes

Fastener Tech Data  |  Understanding Series Introduction

Many fasteners (screws, etc.), especially smaller sizes, are typically not graded—their strength isn't specified. Larger sizes and those for specialized purposes, though, are made to meet certain strength requirements. Inch fasteners—those commonly used in North America—will have a grade or ASTM rating. Property class (often just "class") specifies metric fasteners. Special markings on screw heads and nuts identify the fastener's grade. Replace a graded fastener with same or higher grade (don't replace a Grade 8 fastener with a Grade 5 or 2). If in doubt, ask a professional for assistance. The chart below summarizes some of the more common grades and classes available here at Fastener Mart.

Table 1. Common fastener grades and property classes.
Grade/Class Strength
Grade A Nuts Strength exceeds Grade 2.
ASTM A325 Bolts meet ASTM A325 Type 1 standards for structural steel joints.
Grade B Nuts Strength is similar to Grade 5.
Grade B7 Threaded
Stud & Rod
Same as Grade 5. Use with Grade 2H and Grade C nuts.
Grade C Nuts Strength exceeds Grade 5. Use with heat-treated medium-strength steel fasteners like ASTM A325 structural bolts.
Grade G Nuts Strength is similar to Grade 8. Use with ASTM A325 structural bolts.
Grade 2 Low strength.
Grade 2H Nuts Strength is similar to Grade 5. Use with ASTM A325 structural bolts.
Grade 5 Medium strength.
Grade 8 High strength.
Metric Class 4 Similar to Grade 2.
Metric Class 8.8 Similar to Grade 5.
Metric Class 10.9 Similar to Grade 8.
Metric Class 12.9 The highest metric class for strength, it exceeds Grade 8.

Because there are so many different grades and fasteners all look about the same, medium carbon and alloy steel strength grades are marked for identification. Bolts and screws also bear a mark to identify the manufacturer. If strength is important, ensure that both the grade and manufacturer identification markings are present.

Figure 1. SAE grade head marks.
SAE Grade 2
SAE Grade 2 Head Mark
SAE Grade 5
SAE Grade 5 Head Mark
SAE Grade 8
SAE Grade 8 Head Mark

Screws are marked on the top of their heads, the exceptions being small (under about 1/4"), slotted and recessed head fasteners where there is insufficient room for head markings. A head with no markings is Grade 2, one with 3 lines spaced 120 degrees is Grade 5, and 6 lines spaced at 60 degree intervals is Grade 8.

Nuts are marked in several different ways—see Identification Marks for Hex and Heavy Hex Nuts below.

For other grade and property class specifics, look at these reference charts…

Be sure to visit our Fastener Tech Data section for even more information.

Fastener Tech Data  |  Understanding Series Introduction