GENE
RAL I
NFORM
ATI ON
5
1
tener design determines the type
of
tool required to work
the fastener. Fastener material
is carefully selected to de-
crease the possibility
offai
lurc.
Ma
chin
e Screws
metric fasteners. The threads are cut differently than those
of
American fasteners (Figure 7).
Most threads are cut so that the fastener must be turned
clockwise to tighten it. These arc called right-hand
threads. Some fasteners have left-hand threads: they must
be turned counterclockwise to be tightened. Left-hand
threads are used in locations
where no
nna
l rotation
of
the
equipment would tend to loosen a right-hand threaded fas-
tener.
ISO metric threads come
in three standard thread sizes:
coarse, fine and constant pitch. The
ISO coarse piteh is
used for most common fastener applications. The fine
pitch thread is used on certa in precision tools and instru-
ments. The constant pitch thread is used mainly on ma-
chine parts and not for fasteners. The constant pitch
thread. however. is used on all metric thread spark plugs.
ISO metric threads are
specified by the capital letter M
followed by the diameter in millimeters and the pitch (or
the distance between each thread) in millimeters sepa-
rated by the sign - . For example. a M8 -
1.25bolt has a
diameter
of8
millimeters with a distance
of
1.25millime-
ters between each thread. The measurement across two
flats on the head
of
the bolt indicates the wrench size to be
used.
Figure
5 shows how to determine bolt diameter.
NOTE
If
purchasing a bolt from a dealer or parts
store, it is important to know how to specify
bolt length.
The correct way to measure holt
length
is to measure from underneath the
holt
head to the end ofthe holt (Figure 8).
Al
ways measure holt length in this manner
to avoid purchasing or ins/ailing bolts that
are too long.
Bolls
ISO
I\le
tric
Scr
ew Threa ds
Commonly called bolts. the technical name for these
fasteners is
cap screw. Metric bolts are described by the
diameter and pitch (or the distance between each thread).
There are many differen t types
of
machine screws.
f-
"i2-
ur
e 9 shows a number
of
screw heads requiring different
typesoftuming
tools. Heads are also designed to protrude
above the metal (round) or to be slightly recessed in the
metal (flat). See
Fi2ure 10.
(2)
Metric
~
~
60'
60'
~
-:
Nuts, bolts and screws arc manufactured in a wide
nmgc
of
thread patterns. To join a nut and bolt. the diamc-
ter
of
the bolt and the diameter
of
the hole in the nul must
bethe same.
It
is also important that the threads on both be
properly matched.
The best way to tell
if
the threads on two fasteners
match is to tum the nut on the
bolt (or the bolt into the
threaded hole) by hand. Make sure both pieces are clean;
remove Loctite or other scaler residue from threads
if
present. If excessive force is required. check the thread
condition on each fastener.
If
the thread condition is good
but the fasteners jam, the threads are not compatible. A
thread pitch gauge (Ffgure 6) can also
be used to deter-
mine pitch. Yanmar marine engi nes arc manufactured
with ISO (International Organization for Standardization)
8
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