Fastener Strength Identification - Chevrolet 1988 Light Duty Truck Repair Manual

NOTICE: M o st m e tric fa s te n e rs have a b lu e
c o lo r c o a tin g . H ow ever, th is s h o u ld n o t be u sed
as p o s itiv e id e n tific a tio n as som e fa ste n e rs are
n o t c o lo r coated.
G eneral Motors E ngineering Standards, along with
other North American Industries, have adopted a portion
of the standard metric fastener sizes defined by ISO
(International Standards Organization). This was done to
reduce the number of fastener sizes used and yet retain
the best strength qualities in each thread size. For
GENERAL INFORMATION OA-3
example, the custom ary 1 /4-20 and 1 /4-28 screws are
replaced by the metric M 6.0x1 screw which has nearly
the same diam eter and has 25.4 threads per inch. The
thread pitch is in between the custom ary coarse and fine
thread pitches.
Metric and custom ary thread notation differ slightly. The
difference is shown in figure 14.
FASTENER STRENGTH
IDENTIFICATION
Most com m only used metric fastener strength properly
classes are 9.8 and 10.9 with the class identification
em bossed on the head of each bolt. Custom ary (inch)
strength classes range from grade 2 to 8 with radial line
identification embossed on each bolt head. M arkings
correspond to two lines less than the actual grade (i.e.
grade 7 bolt will exhibit 5 em bossed radial lines on the
bolt head). Some metric nuts will be marked with single
digit strength identification numbers on the nut face.
Figure 6 shows the different strength m arkings.
W hen replacing metric fasteners, be careful to use bolts
and nuts of the same strength or greater than the original
fasteners (the same num ber marking or higher). It is
likewise im portant to select replacem ent fasteners of the
correct size. Correct replacem ent bolts metric fasteners
available in the afterm arket parts channels were designed
to metric standards of countries other than the United
States, and may be of a lower strength, may not have the
n um bered head m arking system , and may be of a
different thread pitch. The metric fasteners used on GM