Hardware: Threads and Types

It seems that everyone is looking for a host of technical articles to help them perform some very complex repairs on today's sophistocated, electronically controlled, transmissions. While it is true that there is a lack of information, and understanding concerning the "hi-tech" concepts, I have seen similar lack of understanding of the basics; things that I tend to take for granted, and assume to be "common knowledge". As I have said before, we need to crawl, before we can walk, and, eventually, run, and, without a complete understanding of the basics, we cannot begin to tackle the common, "hi-tech" problems that "go with the territory" when you attempt to fix late-model, computer controlled transmissions.

First, let's explore some of the many different types of hardware commonly used in today's automobile. Obviously, we have screws, bolts, nuts and washers, but there are actually many types of each of these catagories.

Screws

Screws have male threads, and are basically, like a bolt, but unlike a bolt, the shank is threaded all the way up to the head.

Bolts

Bolts also have male threads, but, unlike a screw, have a threaded portion, on the shank, as well as a non-threaded portion, towards the head.
Screws, and Bolts, can have all different types of heads; some of which are:

Flat head
Round head
Hex head capscrew
Flanged hex head capscrew
Allen head capscrew
Torx head capscrew

Nuts

Nuts have, as most of you already know, female threads and thread onto screws or bolts.
Nuts can be of many different types; some of which are:

"regular" Hex nut
Flanged hex nut
Crimped hex nut
Nyloc hex nut

Washers

Washers also come in different varietys and are used to distribute load, as well as prevent a bolt, or nut, from coming loose.
They also come in different varieties; some of which are:

Flat SAE washers
Flat USS washers
Crimped lockwashers
Warped lockwashers

Grades of Hardware

There are many different grades of "Inch-style" hardware; grade 1, grade 2(commonly referred to as "butter bolts"), grade5(commonly referred to as "automotive grade"), grade 8(commonly referred to as "tractor grade"), to name a few; the most common, for automobiles, is "grade 5". As the number rises, so does the "hardness" and "toughness" values that the bolt must meet, so, a grade 8 bolt, can be torqued much more, and hold much more, than a similar grade 2 bolt, and, similarly, using a grade 2 bolt, where a grade 8 bolt is required, can, and will, cause bolt failure, as the grade 2 bolt, quite simply, will not "hold up" under the harsh demands intended for the Grade 8 hardware.
When working on automobiles, always use Grade 5 hardware, unless otherwise specified; don't substitute a grade 1, or grade 2 bolt. Grade 1 bolts usually have no markings, with the possible exception of a brand marking, on the head, Grade 2 bolts usually have two "slashes" on the bolt head., Grade 5 bolts have 3 "slashes" on the bolt head, and Grade 8 bolts have 6 markings on the bolt head.


A "Grade 2" Bolt Head No "Slashes"	A "Grade 5" Bolt Head 3 "Slashes"	A "Grade 8" Bolt Head 6 "Slashes"

Metric hardware also has different grades, or "classes"; "standard" (nothing on bolt head), which would be like grade 1 or 2 in "inch hardware", "class 8.8" ("8.8" on bolt head), which would relate to grade 5, "class 10.9" ("10.9" on bolt head), which would relate to a grade 8 bolt.

Hardware Threads

In addition to types, and grades of hardware, there are, also, many different thread "aspects" to consider.

Thread direction: Thread direction merely refers to the direction the hardware must turn to "tighten". "Normal" , or the most common direction is "right hand" (clockwise to tighten), although there are some circumstances where "left hand" threads are required. Some wheel studs are left hand thread; it is important to understand the difference, because many wheel studs, as well as other pieces of hardware, have been broken because the mechanic was unable to recognize the hardware was left hand thread, resulting in tightening, when they were attempting to loosen the hardware.
Thread Diameter: Thread Diameter relates to the outer most diameter of the threads, or, to put it another way: Diameter can be "The smallest hole that the threaded portion of the bolt will fit through". For example, a 3/8" bolt can fit through a 3/8" hole, but not a smaller hole. Thread diameter is usually measured in "Inches" for "Inch"-based hardware, and "Millimeters" for "Metric"-based hardware.
Thread Pitch: Thread Pitch relates to the distance between threads, from highpoint to next corresponding highpoint, and, the type of measurement used to determine "pitch", depends upon whether the hardware is "Inch" or "Metric" -based hardware. "Inch"-based thread pitch is measured by "Threads Per Inch" (tpi), which means that the higher the number, or the more "threads per inch" (tpi) relates to a fine thread, while a lower number would relate to a coarse thread (example: 24threads per inch is a "finer" thread than 18threads per inch). "Metric"-based thread pitch is measured, typically, by number of millimeters (mm) per thread, so, the lower the number, or the less millimeters per thread, the finer the thread would be, while a higher number would mean a coarser thread (example: Metric bolts are usually defined by their diameter, first, and then, the pitch, so 8mmX1.25 relates to an 8mm bolt with threads of with 1.25 mm per thread and would be more coarse than an 8mmX1.0 bolt).
Thread Standards: There are many "standards" set, concerning thread pitch. In the "inch" variety, two "standards" are most common, in the automotive field; "UNC" (National Coarse; threads far apart), and "UNF" (National Fine; threads closer together). For example, a 3/8"-16 bolt, refers to a 3/8" diameter, 16threads per inch, bolt, which happens to be a 3/8"UNC bolt, while a 3/8"-24 bolt refers to a 3/8" diameter UNF bolt.
In the "metric" variety, different pitches are also available, but, usually the "default" is the coarse variety, while you must specify pitch if you want threads of a "fine" variety (example: an 8mm bolt is always 1.25 in pitch, unless otherwise specified, so, an "8mm bolt" is the same as an "8mmX1.25 bolt", but, if you want an 8mmX1.0 bolt, you must specify "8mmX1.0").

Common bolt sizes/pitches and their maximum allowable torques
(unless otherwise specified by your repair manual)

"Inch"-based hardware

Diameter	"Standard"	Pitch	Grade5 torque (ft.lb.) Lubricated / Dry threads	Grade8 torque (ft.lb.) Lubricated / Dry	"Common" hex size (wrench size)
1/4"	UNC	20tpi	6.3 / 8	9 / 12	7/16"
1/4"	UNF	28tpi	7.2 / 10	10 / 14	7/16"
5/16"	UNC	18tpi	13 / 17	18 / 24	1/2"
5/16"	UNF	24tpi	14 / 19	20 / 27	1/2"
3/8"	UNC	16tpi	23 / 30	35 / 45	9/16"
3/8"	UNF	24tpi	25 / 35	35 / 50	9/16"
7/16"	UNC	14tpi	35 / 50	50 / 70	5/8"bolt 11/16"nut
7/16"	UNF	20tpi	40 / 50	60 / 80	5/8"bolt 11/16"nut
1/2"	UNC	13tpi	55 / 75	90 / 120	3/4"
1/2"	UNF	20tpi	65 / 85	90 / 120	3/4"

"Metric"-based hardware

Diameter	Pitch	Class 8.8 torque (ft.lb.)	Class 10.9 torque (ft.lb.)	"Common" hex size (wrench size)
6mm	1.0	7	10.5	10mm
8mm	1.25	17.25	25.5	12mm or 13mm
8mm	1.0	18.25		12mm or 13mm
10mm	1.5	34.5	51	14, 15, or 17mm
10mm	1.25	36.75		14, 15, or 17mm
10mm	1.0	39		14, 15, or 17mm
12mm	1.75	59.25	87.75	17, 18, or 19mm
12mm	1.5	62.25		17, 18, or 19mm
12mm	1.25	65.25		17, 18, or 19mm