Monday, March 10, 2008

Repair Guide: Fasteners (Part 1 of 2)

See Figure 1

In most applications, fasteners on vehicles may be reused providing they have not been damaged during a repair. However, in certain special applications where stretch bolts or torque prevailing nuts are used these fasteners must be replaced.

Threaded fasteners are the basic couplers holding your vehicle together. There are many different kinds, but they all fall into three basic types:

Bolts - Bolts go through holes in parts that are attached together and require a nut that is turned onto the other end. A lock - washer of some sort is usually used under the nut.
Studs - Studs are similar to bolts, except that they are threaded at both ends (they have no heads). One end is screwed into a threaded hole and a nut is turned onto the other end. Lock - washers are usually used under the nuts.
Screws - Screws are turned into drilled or threaded holes in metal or other materials.

There are a great variety of screws and bolts, but most are hex headed or slot headed for tightening. Because the fastener is the weakest link in an assembly, it is useful to know the relative strength of the fastener, determined by the size and type of material. It is also important to understand the sizes of bolts, to avoid the expense and work of re - threading stripped holes. Figure 1 Keep an assortment of fasteners and hardware neatly sorted in tackle boxes


See Figures 2 and 3
Screws are supplied with slotted, Phillips, Torx® or Allen heads for screwdrivers or with hex heads for wrenches. Most of the screws used on cars and trucks are sheet metal, hexagon or pan type. Occasionally, you'll find a self - tapping sheet metal screw, with slots in the end to form a cutting edge. These types cut their own threads when turned into a hole.

The size of a screw is designated as 8-32, 10-32 or ¼-32. The first number indicates the size of the thread at the root or minor diameter (the diameter of the screw measured from the bottom of the threads on each side) and the second number indicates the number of threads per inch. Figure 2 Common screw and bolt head types

Figure 3 Screw and bolt measurement terms

Torx® Fasteners

See Figure 4
Torx® fasteners have a star shaped head of either an internal or an external design.

These fasteners come in three different types. The most common being internal, these fasteners require a star shaped driver and are frequently found on headlight retainers and adjusters. The second type is external, these fasteners require a star shaped socket and may be found in odd locations such as the wheel cylinder retaining bolts. The third type is tamper resistant, which are used in places that manufacturer's are very serious about avoiding a Do - It - Yourselfer (DIYer) from touching. These look similar to the internal type however, they have a pin in the center of the fastener preventing the use of the standard Torx® driver. They may be found on components that are meant to be serviced only by authorized repair centers. Figure 4 Two different types of Torx® fasteners.

SAE Bolts

See Figures 5, 6 and 7
Many bolts that were once used on domestic cars and trucks maybe measured in inches, and standards for these bolts were established by the Society of Automotive Engineers (SAE). Special markings on the head of the bolt indicate its tensile strength (resistance to breaking). The SAE grade number, corresponding to the special markings, is an indication of the relative strength of the bolt. Grade 0 bolts (no markings) are usually made of a mild steel and are much weaker than a grade 8, usually made from a mild carbon steel alloy, though a grade 0 or 2 bolt is sufficient for most fasteners.

SAE fasteners are also identified by size. As an example, a 3/8-24 bolt means that the major (greatest) thread diameter is 3/8 inch and that there are 24 threads per inch. The head diameter is always 316inch larger than the bolt diameter. A ½16 bolt would be ½ inch in diameter and have 16 threads per inch. More threads per inch are called "fine" threads and less threads per inch are "coarse" threads. Generally, the larger the bolt diameter, the coarser the threads. There are actually six different classes of threads, but most bolts are Unified National Coarse (UNC) or Unified National Fine (UNF). The term "Unified" refers to a thread pattern to which US, British and Canadian machine screw threads conform. Figure 5 Fasteners commonly found on automobiles

Figure 6 SAE bolt head markings indicate their relative strength

See Figure 7 SAE standard torque specification chart.

Metric Bolts

See Figures 8 thru 14
The International Standards Organization (ISO) has designated the metric system as the world standard of measurement.

As far back as the early 1970's when Ford introduced the 2300cc, 4 - cylinder engine in the Pinto, the use of metric fasteners have become more prevalent in domestic vehicles. Probably the majority of domestic vehicles on the road today have more metric fasteners than the inch - size (SAE) type and almost all the fasteners on vehicles currently being produced are metric.

The mixture of metric and SAE fasteners on the same vehicle means that you have to be very careful when removing bolts to note their locations and to keep metric nuts and bolts together. At first glance, metric fasteners may appear to be the same size as their SAE counterparts, but they're not. While the size may be very close, the pitch of the threads (distance between threads) is different. It is possible to start a metric bolt into a hole with SAE threads and run it down several turns before it binds. Any further tightening will strip the threads. The opposite could occur also; a nut could be run all the way down and be too loose to provide sufficient strength.

Fortunately, metric bolts are marked differently than SAE bolts. An ISO metric bolt larger than 6 mm in diameter has either "ISO M" or "M" embossed on top of the head. In addition, most metric bolts are identified by a number stamped on the bolt head, such as 4.6, 5.8 or 10.9. The number has nothing to do with the size, but does indicate the relative strength of the bolt. The higher the number, the stronger the bolt. Some metric nuts are also marked with a single - digit number to indicate the strength, and some may have the M and strength grade embossed on the flats of the hex.

Metric nuts with an ISO thread are marked on one face of the hex flats with the strength grade (4, 5, 6, 8, 12, and 14). Some nuts with a 4, 5 or 6 strength grade may or may not be marked.

A clock face system is used as an alternate means of strength grade designation. The external chamfers or faces of the nut are marked with a dash at the appropriate hour mark corresponding to the relative strength grade. One dot indicates the 12 o'clock position and, if the grade is above 12, 2 dots identify 12 o'clock.

The size of a metric fastener is also identified differently than an SAE fastener. A metric fastener could be designated M12 x 2, for example. This means that the major diameter of the threads is 12 mm and that the thread pitch is 2 mm (there are 2 mm between threads). Most importantly, metric threads are not classed by number of threads per inch, but by the distance between the threads, and the distance between threads does not exactly correspond to number of threads per inch (2 mm between threads is about 12.7 threads per inch).

The 25 standard metric diameter and pitch combinations are shown here. The first number in each size is the nominal or root (minor) diameter (mm) and the second number is the thread pitch (mm).

Remember that the nominal bolt diameter is the measurement of the bolt diameter as taken from the bottom of the threads NOT the top (which would be major diameter). Figure 8 A thread gauge will instantly identify the thread size

Figure 9 Metric grade to SAE grade comparison
Metric Grade Nominal Diameter (mm) Corresponds to SAE Grade
4.6 M5 thru M36 1
4.8 M1.6 thru M16 -
5.8 M5 thru M24 2
8.8 M16 thru M36 5
9.8 M1.6 thru M16 -
10.9 M5 thru M36 8
12.9 M1.6 thru M36 -

Figure 10 Metric bolts are marked with numbers that indicate the relative strength of the bolt. These numbers have nothing to do with the size of the bolt.

Figure 11 Typical ISO bolt and nut markings

Figure 12. The 25 standard metric diameter and pitch combinations
25 Standard Metric Diameter and Pitch Combinations
M1.6 x 0.35 M20 x 2.5
M2 x 0.4 M24 x 3
M2.5 x 0.45 M30 x 3.5
M3 x 0.5 M36 x 4
M3.5 x 0.6 M42 x 4.5
M4 x 0.7 M48 x 5
M5 x 0.8 M56 x 5.5
M6.3 x 1.0 M64 x 6
M8 x 1.25 M72 x 6
M10 x 1.5 M80 x 6
M12 x 1.75 M90 x 6
M14 x 2 M100 x 6

Figure 13 Thread forms replaced by ISO Metric

Metric Size

- - 10 10 M5
3/16 3/16 - - -
- - 12 12 M6
1/4 1/4 1/4 1/4 M6
5/16 5/16 5/16 5/16 M8
3/8 3/8 3/8 3/8 M10
7/16 7/16 7/16 7/16
1/2 1/2 1/2 1/2 M12

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