Yanmar 1GM User Manual page 12

GENERA L INFORMATI O N 5
1
tener design determin es the type of tool required to work
the fastener. Fastener material is care fully selected to de-
crease the poss ibility offailurc.
M achine Scre ws
metric fasteners. The threads are cut differently than those
of American fasteners ( Figu re 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 counterc lockwise to be tightened. Left-hand
threads are used in locat ions where nonnal rotation of the
equ ipment would tend to loo sen a right-hand threaded fas-
tener.
ISO metric thread s come in three standard thread sizes:
coarse, fine and constant pitch. The ISO coarse piteh is
used for mos t 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 cap ital 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 -
1.25
bolt has a
diameter of8 millimeters with a distance of 1.25 millime-
ters between eac h thread. The measurement across two
flats on the head ofthe bolt indicates the wrench size to be
used. Figure 5 shows ho w to determine bolt d iamete r.
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
of
the holt (Fig ure 8).
Always meas ure holt length in this manner
to avoid p urchasing or ins/ailing bolts that
are too long.
Bolls
ISO
I\letric Screw T h rea ds
Co mmonly called bolts. the technical name for these
fasteners
is cap screw. Metric bo
lts are described by the
d iameter and pitch (or the distance between each thread).
There are many differen t types of machine screws. f-"i2-
ure 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
meta l (flat). See Fi2u re 10.
(2)
Metric
~ ~
60'
60 '
~
-:
Nuts, bolts and screws arc manufactured in a wide
nmgc of threa d patterns. To jo in a nut and bolt. the diamc-
ter of the
bolt
and the d iameter of the hole in the nul must
be the 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 scale r residue from threads if
present. If excessive force is required.
check the thread
condition on each fastene r. If the thread condition
is good
but the fasteners jam, the threads are not compatible. A
thread pitch ga uge (F fgu re 6) can also
be used to de
ter-
mine pitch . Yanmar marine engi nes arc manu factured
with ISO (International Organization for Standard ization)
8