Metric Fasteners

Current model GM vehicles are primarily dimensioned in the metric system. Many of the fasteners used on GM vehicles are metric, but are very close in dimension to well-known inch system fasteners. Use replacement fasteners which match the original equipment fasteners in the following ways:

Original equipment metric fasteners (except "beauty" bolts, like exposed bumper bolts and cross-recess head screws) are identified by a marking or a number which indicates the strength of the material in the fastener.

Metric cross-recess screws are identified as either Posidriv or Type 1A. Either Posidriv or Type 1A cross-recess screwdrivers can used in Posidriv recess screw heads. However, Type 1A cross-recess screwdrivers perform better.

Prevailing Torque Fasteners

A prevailing torque nut creates interference between the nut threads and the bolt threads. A nylon patch on the threads in the middle of the hex flat achieves interference. Interference secures the nut against loosening.

A prevailing torque bolt creates interference at any of the following fastening points:

The prevailing torque bolt achieves interference in the following ways:

Interference secures the bolt against loosening

Recommendations For Reuse

Observe the following guidelines for reusing prevailing torque nuts and bolts:

Fastener Strength Identification

Metric nuts and bolts are divided into metric fastener strength property classes. The most common metric strength property classes are 9.8 and 10.9. Class identification is embossed on the head of each bolt. Some metric nuts are marked with single digit strength numbers on the nut face.

Replace a metric fastener with a fastener of equal or greater strength. The strength identification number on a replacement fastener should be the same as, or higher than, the strength identification number on the original fastener.

Replace torque-to-yield fasteners with identical fasteners using GM SPO part numbers.

The metric fasteners used on GM products are designed to international standards. Some non-domestic nut and bolt suppliers may not observe these standards. Non-domestic fasteners may not meet GM requirements for the following reasons:

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 1A 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 of fastener 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.

(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.

The correct fasteners are available through GM SPO. Many metric fasteners available in the aftermarket parts channels are designed to metric standards of countries other than the United States, and may exhibit the following:

•	Lower strength

•	No numbered head marking system

•	Wrong thread pitch

The metric fasteners on GM products are designed to new, international standards. The following are the common sizes and pitches, except for special applications:

•

M6.0 X 1

•

M8 X 1.25

•

M10 X 1.5

•	M12 X 1.75

•	M14 X 2.00

•	M16 X 2.00

Prevailing Torque Fasteners

Prevailing torque fasteners create a thread interface between the fastener and the fastener counterpart in order to prevent the fastener from loosening.

All Metal Prevailing Torque Fasteners

These fasteners accomplish the thread interface by a designed distortion or deformation in the fastener.

Nylon Interface Prevailing Torque Fasteners

These fasteners accomplish the thread interface by the presence of a nylon material on the fastener threads.

Adhesive Coated Fasteners

These fasteners accomplish the thread interface by the presence of a thread-locking compound on the fastener threads. Refer to the appropriate repair procedure in order to determine if the fastener may be reused and the applicable thread-locking compound to apply to the fastener.