Fasteners; Metric Fasteners; Fastener Strength Identification - Chevrolet Corvette 2005 Owner's Manual

Chevrolet Restoration Kit
Appendix C
Fasteners
Metric Fasteners
This vehicle provides fastener dimensions using the metric system. Most metric fasteners are
approximate in diameter to equivalent English fasteners. Make replacements using fasteners of the same
nominal diameter, thread pitch, and strength.
A number marking identifies the OE metric fasteners except cross-recess head screws. The number also
indicates the strength of the fastener material. A Posidrive® or Type 1A cross-recess identifies a metric
cross-recess screw. For best results, use a Type 1 A cross-recess screwdriver, or equivalent, in
Posidrive® recess head screws.
GM Engineering Standards and North American Industries have adopted a portion of the ISO-defined
standard metric fastener sizes. The purpose was to reduce the number offastener sizes used while
retaining the best thread qualities in each thread size. For example, the metric M6.0 X 1 screw, with
nearly the same diameter and 25.4 threads per inch replaced the English 1/4-20 and 1/4-28 screws. The
thread pitch is midway between the English coarse and fine thread pitches.
Fastener Strength Identification
2
3
4
5
@
6
1. English Bolt, Grade
2
(Strength Class)
2.
English Bolt, Grade
5
(Strength Class)
3. English Bolt, Grade 7 (Strength Class)
4. English Bolt, Grade 8 (Strength Class)
5.
Metric Nut, Strength Class 9
6. Metric Bolts, Strength Class Increases as Numbers Increase
The most commonly used metric fastener strength property classes are 9.8 and 10.9. The class
identification is embossed on the head of each bolt. The English, inch strength classes range from grade
2 to grade 8. Radial lines are embossed on the head of each bolt in order to identify the strength class.
The number of lines on the head of the bolt is 2 lines less than the actual grade. For example, a grade 8
bolt will have 6 radial lines on the bolt head. Some metric nuts are marked with a single digit strength
identification number on the nut face.
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