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Fastener Basics
Common Fastener Types
Hex
bolts
,
or
hex
cap
screws,
are
used
in
machinery
and construction. Can be used
with
a
nut,
or
in
a
tapped
hole.
Fully
threaded
hex
bolts are also known as tap bolts.
Wood
screws
have
large
threads
and
a
smooth
shank f or pulling two pieces of material
together. They can be used in wood and other
soft materi-als.
Sheet
metal
screws
have
sharp
points
and
threads, and are designed to be driven directly
into
sheet
metal.
They
can
also
be
used
in
softer materials like plastic, fiberglass, or wood.
Machine screws
are fully threaded for use with
a
nut
or
in
a
tapped
hole.
Certain
types
are
some-times referred to as stove bolts.
Socket
screws
are
machine
screws
with
an
internal hex socket (Allen) drive. Longer lengths
may have a smooth shank.
Lag
bolts
,
or
lag
screws,
are
large
wood
screws
with hex heads. Typically used for wood
construction and landscaping.
Carriage bolts
have smooth, domed heads with
a
square
section
underneath
that
pulls
into
the
material to prevent spinning during installation.
Nuts
are
used
to
fasten
machine
threaded
fasteners
in
through-hole
applications.
Lock
nuts help prevent loosening.
Washers
spread
the
load
over
a
greater
surface
area
when
tightening
a
bolt,
screw
or
nut. Lock washers help preventing loosening.
Tip:
Find a more comprehensive fastener type chart
Grade/Class & Fastener Strength
Fastener
Grade
(US)
or
Class
(metric)
refers
to
the
mechanical
proper-ties
of
the
fastener
material.
Generally,
a
higher
number
indicates a stronger, more hardened (but also more brittle) fastener.
For a chart of fastener grades, head markings and mechanical properties, see
Bolt Depot’s Grade markings and Strength Chart
US bolt head markings
Metric bolt head markings
8.8
10.9
12.9
Low Carbon
Grade 5
Grade 8
Class 8.8
Class 10.9
Class 12.9
Note:
In addition to these markings, the head will often have a manufacturer stamp.
Page 1
Fastener Materials
Note:
Do
not
rely
on
this
guide
for
color-matching.
The
appearance of these materials sometimes di ers
signifi-cantly from the photos below.
Zinc-plated steel
is a
low carbon steel for
general use. Relatively
inexpensive, with the
zinc plating providing
moderate corrosion
resistance suitable for
indoors or otherwise
dry conditions. Color is
either a blue-ish tint or
yellow depending on the exact process.
Hot-dipped galvanized
steel
has a thicker zinc
coating for better
corrosion resistance,
making it suitable for
outdoor use. Because
of the thick plating,
only galvanized nuts and washers will fit
galvanized bolts. The coating typically has
a rough, dull grey finish.
Stainless steel
o
ers
good corrosion
resistance, making it
suitable for outdoor
use and marine
applications, but is
more expensive than zinc plated.
Chrome
and
nickel
plated steel
are
smooth and polished
for appearance. The
plating o ers moderate corrosion resistance.
Brass
and
bronze
are
copper alloys with good
corrosion resistance.
More expensive than
steel, these materials
are typically used for
decorative applications.
Colors can vary
significantly.
Alloy steel
is highly
hardened and usually
black oxide and/or oil
coated, o ering little
corrosion resistance.
How Fasteners are Notated: An Example
Machine screws, Phillips pan head, Stainless steel 18-8, #12-24 x 1
Fastener
type
Material
Drive Types
Phillips
Head Styles
and
Hex heads
are typically
Slotted
drives are
used with larger bolts and
Phillips Slotted
common in screws,
screws, and tightened with
but prone to cam-
a wrench.
out (stripping).
Pan heads
have a slightly
Combo
drives, that
domed head that sits
can be used with
above the surface.
Combo
either driver, are
Flat heads
are installed in
available for many
a
countersunk hole for a
fastener types.
flat surface.
Frearson
and
Round heads
are tall domed
Pozidriv
are similar
heads, used primarily for
Frearson Pozidriv
to Phillips, but less
decorative purposes.
prone to cam- out.
Oval heads
are a low
Hex socket (Allen)
domed and countersunk
drives are compact
heads, used primarily
Hex socket
and easy to drive, but
for decorative purposes.
(Allen)
prone to cam-out.
Star
Truss heads
are slightly
and
Square
domed, with a wide head for
drive
are resistant to
an extra large surface area.
Star
Square
cam-out and can be
Socket heads
are narrow
(Robertson)
installed single-
with a socket drive, and
handed.
knurled or smooth sides.
Button heads
feature
Note:
Most
drive
types
(Frearson
and
a
medium dome.
Slotted
being
notable
exceptions)
require
Typically used with a
the correct driver size for proper installation.
hex socket drive.
Nut & Washer Sizes
Nut and washer sizes indicate
the screw or bolt they fit.
For example:
3/8”
-16 Hex Bolt
3
/8” Washer
3/8”
-16 Nut
Di erent washer patterns have di erent
outside
diameters.
For example, hardened US washers
are
available in
USS
(wider) and
SAE
(narrower)
patterns. Fender washers have large outside
diameters.
1/4”
1/4”
1/4” x 1”
SAE
USS
Fender
Page 2
Diameter
Length
Thread Count (TPI)
Measuring Diameter
For most
types of
fasteners,
the diameter
is measured on
the
outside
of
the threads.
Note:
US
diameters
under
1/4”
are
given
as
numbers
(e.g.
#12) instead of inches, in order of increasing size. If you need
to find the actual diameter, use a table corresponding to your
fastener type
Thread Count & Thread Pitch
Machine threaded fasteners specify a thread
density in
Threads Per Inch
(US) or as a
Thread
Pitch
in mm (Metric).
For a given diameter, a fastener may be
available in
coarse
(standard),
fine
and
sometimes
super
fine
thread.
Measuring Length
Fastener length is
usually measured from
where the material is
assumed to be to the
end of the fastener.
Thus, countersunk
fasteners are measured
overall and non-counter-
sunk fasteners are
measured from under
the head.
.
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