GMC 1990 Light Truck Repair Manual page 17

GENERAL INFORMATION OA-11
1. Customary B o lt— 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)
_________________________________ F0705
Figure 21—Thread Notation
E - External Drive
B2221
Figure 22—Six Lobed Socket Head Fasteners