Metric Fasteners; Fastener Strength Identification - GMC 1986 P-Truck Service Manual

OA-8 GENERAL INFORMATION
1. Grade 2 (GM 200-M)
2. Grade 5 (GM 280-M)
3. Grade 7 (GM 290-M)
4. Grade 8 (GM 300-M)
5. Manufacturer's Identification
6. Nut Strength Identification
7. Identification Marks (Posidriv Screw Head)
Figure 10—Bolt and Nut Identification
Models are primarily dimensioned in the metric sys­
tem. Many fasteners are metric and are very close in
dimension to well-known customary fasteners in the
inch system. It is most important that replacement fas­
teners be of the correct nominal diameter, thread pitch
and strength.
O rig in a l equ ip m e nt m etric fasteners (except
" beauty" bolts, such as exposed bumper bolts, and
cross recess head screws) are identified by a number
marking indicating the strength of the material in the
fastener as outlined later. Metric cross recess screws
are identified by a Posidriv or Type 1A shown in figure
10. Either a Phillips head or Type 1A cross recess
screwdriver can be used in Posidriv recess screw
heads, but Type 1A cross recess screwdrivers will per­
form better.
NOTICE: Most metric fasteners have a blue
color coating. However, this should not be
used as positive identification as some fasten­
ers are not color coated.
General Motors Engineering Standards, along with
other North American Industries, have adopted a por­
tion of the standard metric fastener sizes defined by

METRIC FASTENERS

F-00695
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 example, the customary 1 /4-20 and 1 /4-28
screws are replaced by the metric M6.0X1 screw which
has nearly the same diameter and has 25.4 threads per
inch. The thread pitch is in between the customary
coarse and fine thread pitches.
The difference is shown in figure 11.
property classes are 9.8 and 10.9 with the class
identification embossed on the head of each bolt.
Customary (inch) strength classes range from grade 2
to 8 with radial line identification embossed on each
bolt head. Markings correspond to two lines less than
the actual grade (i.e. grade 7 bolt will exhibit 5
embossed radial lines on the bolt head). Some metric
1. Customary B olt— 1 /4-20
2. Metric B olt— M6.0x1
A. 1 /4-inch
B. 6 mm
C. 20 Threads Per Inch
D. 1 Thread Per M illim eter
(25.4 Threads Per Inch)
Figure 11—Thread Notation
Metric and customary thread notation differ slightly.
FASTENER STRENGTH
IDENTIFICATION
Most commonly used metric fastener strength
F-00705