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This article was first published in 2006.
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Button head, grade 5, pan head, 8 x 1.25 – what’s
it all mean? If you’re buying nuts and bolts, this is all vital information.
Identifying Fasteners
If you don’t know how to describe what you want,
chances are that you won’t be able to get it! Threaded fasteners are identified
by these attributes: type, material, diameter, length, and thread pitch or
count.
Fasteners are divided into
categories based on their function or design eg wood screw, sheet metal screw,
hex bolt, lag bolt, etc. Fasteners in some categories are available with
different drive types such as Philips or Slotted. In other categories - such as
lag bolts - there is either only one drive type or the drive type is implied to
be of a standard type.
|
Fastener Type |
Standard or Implied Drive |
|
Bolt |
Hex Head |
|
Lag Bolt/Screw |
Hex Head |
|
Carriage Bolt/Screw |
No Drive |
|
Socket Bolt/Screw |
Allen Drive |
|
Button Head |
Allen Drive |
Many categories are also available with
different head shapes or styles. eg flat head, pan head, truss head, etc.
Fastener material describes the material from
which the fastener was made as well as any material grade eg stainless steel,
zinc plated steel, silicon bronze, etc. Fastener material is covered in more
detail later in this feature.
Fastener diameter is measured
either as a size number or as a direct measurement. There are several different
locations on a fastener where one can measure the diameter. The most commonly
used diameters are:
In some cases the head size of a hex bolt
(diameter across the flats) is also used. However, due to the fact that
different head sizes may be used with the same diameter bolt, this is not a
reliable way to determine bolt diameter.
The diameter of a hex bolts, machine screws and
socket head screws is the shank diameter, expressed in inches for US bolts and
millimetres for metric bolts. Because this is approximately the same as the
major or thread diameter, the thread diameter measurement can be used for fully
threaded fasteners.
Both nuts and washers are sized by the fastener
they fit. For example a 1/2 inch nut fits a 1/2 inch bolt and a 1/2 inch washer
fits the same bolt.
For fasteners where the head usually sits above
the surface (such as hex bolts and pan head screws) the length measurement is
from directly under the head to the end of the fastener. For fasteners that are
designed to be counter-sunk (such as flat head screws), the fastener is measured
from the point on the head where the surface of the material will be to the end
of the fastener.
TPI stands for Threads Per Inch. This is simply a
count of the number of threads per inch measured along the length of a fastener.
TPI is used only with American fasteners. Metric fasteners are specified with a
thread pitch instead of a thread count. The thread pitch is the distance between
threads expressed in millimetres (measured along the length of the fastener).
For example a thread pitch of 1.5 means that the distance between one thread and
the next is 1.5mm. In general, smaller fasteners have finer thread so they have
lower thread pitch.
US
Bolt Size |
Threads Per Inch (TPI) |
|
|
Coarse Thread
UNC
|
Fine Thread
UNF
|
|
#2 |
56 |
- |
|
#3 |
48 |
- |
|
#4 |
40 |
- |
|
#5 |
40 |
- |
|
#6 |
32 |
- |
|
#8 |
32 |
- |
|
#10 |
24 |
32 |
|
#12 |
24 |
- |
|
1/4 |
20 |
28 |
|
5/16 |
18 |
24 |
|
3/8 |
16 |
24 |
|
7/16 |
14 |
20 |
|
1/2 |
13 |
20 |
|
9/16 |
12 |
18 |
|
5/8 |
11 |
18 |
|
3/4 |
10 |
16 |
|
7/8 |
9 |
14 |
|
1 |
8 |
14 |
|
1-1/8 |
7 |
12 |
|
1-1/4 |
7 |
12 |
|
1-1/2 |
6 |
12 |
Metric
Bolt Diameter |
Thread Pitch |
|
|
Standard
|
Fine
|
Extra or Super Fine
|
JIS
|
|
2 |
.4 |
- |
- |
.4 |
|
2.5 |
.45 |
- |
- |
.45 |
|
3 |
.5 |
- |
- |
.5 |
|
4 |
.7 |
- |
- |
.7 |
|
5 |
.8 |
- |
- |
.8 |
|
6 |
1.0 |
- |
- |
1.0 |
|
7 |
1.0 |
- |
- |
1.0 |
|
8 |
1.25 |
1 |
- |
1.0 |
|
10 |
1.5 |
1.25 |
1 |
1.25 |
|
12 |
1.75 |
1.5 |
1.25 |
1.25 |
|
14 |
2 |
1.5 |
- |
1.5 |
|
16 |
2 |
- |
- |
1.5 |
|
18 |
2.5 |
- |
- |
1.5 |
|
20 |
2.5 |
- |
- |
1.5 |
Downloadable Thread Identification
Tools
If you have a bolt, machine screw or washer and
you’re not sure what it is, use these downloadable pdf files to print out
full-size templates for easy identification.
Fastener Notation
US fasteners are described by material, head
style, and type followed by or preceded by the diameter dash thread count
(machine threads only) X length. The diameter and length are in inches with the
exception of small screws whose diameter may be a size number (ex. 12). In this
case the number will often have a # sign before it (ex. #12)
Example: Brass Philips Flat Head Machine Screw 1/4
– 20 x 6. This is a brass counter sunk (flat head) machine screw with a Phillips
drive. It has a diameter of 1/4 inch, 20 threads per inch (coarse thread), a
length of 6 inches.
There are some variations on this using
abbreviations and sometimes moving the material and or head style and type to
after the size.
Sometimes a machine thread is specified as coarse
thread or fine thread, rather than giving the actual thread count. This is
usually done with an abbreviation at the end or occasionally in front of the
diameter and length. Example: Brass Philips Flat Head Machine Screw 1/4 x 6
CT
Metric fasteners material, head style, and type
are described in a similar manner to US fasteners. However, instead of threads
per inch, the metric system uses thread pitch (the length of each thread in
millimetres measured along the shaft of the bolt). The thread pitch is written
between the diameter and length (both measured in millimetres) and is separated
on each side by an X. Typically the diameter of a metric bolt is preceded by a
capital M to designate metric. Example: Class 8.8 Hex Bolt M10 x 1.5 x 20 This
is a hex head bolt with a diameter of 10mm, a 1.5 thread pitch, measuring 20mm
long and made of Class (grade) 8.8 steel.
Note: Metric fasteners may also be specified by
DIN number. The DIN number specifies a particular set of standards that the bolt
adheres to.
Fastener Materials
Fasteners are manufactured in a wide range of
materials from common steel to titanium, plastic and other exotic materials.
Many materials are further separated into different grades to describe specific
alloy mixtures, hardening processes, etc. In addition, some materials are
available with a variety of coatings or plating to enhance the corrosion
resistance, or appearance of the fastener.
Fastener material can be important when choosing a
fastener due to differences between materials in strength, brittleness,
corrosion resistance, galvanic corrosion properties, and of course cost.
When replacing fasteners, it is generally best to
match what you are replacing. Replacing a bolt with a stronger one is not always
safe. Harder bolts tend to be more brittle and may fail in specific
applications. Also some equipment is designed so that the bolts will fail before
more expensive or critical items are damaged. In some environments such as salt
water galvanic corrosion must also be considered if changing fastener materials.
Stainless steel is an alloy of low carbon steel
and chromium for enhanced corrosion characteristics. Stainless steel is highly
corrosion resistant for the price and because the anti-corrosive properties are
inherent to the metal, it will not lose this resistance if scratched during
installation or use.
It is a common misconception that stainless steel
is stronger than regular steel. In fact, due to the low carbon content,
stainless steel cannot be hardened. Therefore when compared with regular steel,
it is slightly stronger than an un-hardened (grade 2) steel fastener but
significantly weaker than hardened steel fasteners.
Stainless steel is also much less magnetic than
regular steel fasteners though some grades will be slightly magnetic. 18-8 is a
specific grade of stainless steel and is the most common grade used in
fasteners. For those familiar with the 300 series of stainless steel, 18-8 is
similar to 304 stainless.
Steel is the most common fastener material. Steel
fasteners are available plain as well as with various surface treatments such as
zinc plating, galvanization and chrome plating.
Steel fasteners are commonly available in four
grades. Many other grades exist but are used far less often. The most common
grades are Grade 2, Grade 5, Grade 8, and Alloy Steel. Grade 2, 5, and 8 are
usually plated with a silver or yellow zinc coating or galvanized to resist
corrosion.
Alloy steel bolts are made from a high strength
steel alloy and are further heat treated. Alloy steel bolts are typically not
plated resulting in a dull black finish. Alloy steel bolts are extremely strong
but very brittle.
Silicon bronze, often referred to simply as
bronze, is an alloy made mostly of copper and tin with a small amount of
silicon. Bronze is used primarily in marine environments. It is preferred over
stainless in wooden boat construction and re-fastening due to its superior
corrosion resistance, and over brass due to its higher strength. Bronze is
similar to copper in colour and is also sometimes seen in fine woodworking where
it is used for its appearance. The main drawback of bronze is its high cost.
Brass is an alloy of primarily copper and zinc.
Brass is highly corrosion resistant and electrically conductive. However, its
use as a fastener is somewhat limited due to its relative softness. It is used
primarily for its appearance.
Determining Bolt Grade
Bolts of different grades are marked on the head
to show what grade bolt they are.
Grade 2 is a standard hardware grade steel.
This is the most common grade of steel fastener and is the least expensive.
Grade 2 bolts have no head marking (sometimes a manufacturer mark is
present).
Grade 5 bolts are case hardened. This means
that the outside part of the bolt has been hardened but that the bolt was not
heated enough to harden the inside portion. This creates a bolt that is fairly
hard but not as brittle as a fully hardened bolt. Grade 5 bolts are the most
common bolts found in automotive applications. Grade 5 bolts have 3 evenly
spaced radial lines on the head.
Grade 8 bolts are fully hardened. This means
the bolt has been hardened all the way through. This creates a bolt that is very
hard but somewhat brittle. Grade 8 bolts are more likely to snap off than bend
under extreme loads. Grade 8 bolts are often found in demanding applications
such as automotive suspensions. Grade 8 bolts have 6 evenly spaced radial lines
on the head.
Coatings
Many steel fasteners are electro-plated with
zinc for better corrosion resistance. Fasteners that have been zinc plated have
a shiny silver or golden appearance referred to as clear or yellow zinc
respectively. They are fairly corrosion resistant but will rust if the coating
is destroyed or if exposed to a marine environment.
Galvanizing is another coating involving the
application of a layer of zinc. Hot dipped galvanizing puts the thickest
possible coating on the metal resulting in superior corrosion resistance. Due to
the thickness of the coating, hot dipped galvanized bolts are not compatible
with other nuts. Galvanized nuts are tapped slightly larger than other nuts to
accommodate this coating.
Hot dipped galvanized fasteners are frequently
seen in coastal environments and in pressure treated lumber where the chemicals
in the lumber may corrode other fasteners.
Chrome is used in plating fasteners for its
appearance. It provides similar corrosion resistance to zinc plating. The main
drawback of chrome is the extremely high cost. If more corrosion resistance is
required, stainless steel may be chrome plated, preventing any corrosion should
the chrome be penetrated.
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Material
in this feature provided courtesy of Bolt Depot
www.bolt.depot.com. Used
with permission.
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