Question: What is the most important yet typically taken for granted
component of an internal combustion engine?
Answer: Fasteners If one little rod bolt fails you can general kiss the
bottom end goodbye. If a cylinder head does not get the proper
clamping force from a fastener, the head gasket is sure to blow.
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Fasteners are devices that hold the parts of a car
together. There are thousands of fasteners that hold
a vehicle together.
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Bolts: A bolt is a metal rod with external threads on
one end and a head on the other. When a bolt is
threaded into a part other than a nut it can also be
called a cap screw.
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Every bolt behaves like a spring. If we apply a
tensile (a metal's resistance to being pulled apart)
pre-load to the bolt it will elongate (stretch). Also
present within the bolt is a force waiting to return
the bolt to its original length.
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It is this property of the bolt trying to return to its
original length that gives the fastener its holding
ability.
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As long as the bolt's pre-load is within the elastic
range of the stress/strain, permanent deformation of
the bolt will not result.
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Bolt and Nut terminology:
Bolts and nuts come in various sizes, grades,
strengths, and thread types. It is important to be
familiar with these different bolt dimensions.
Bolt size – the measurement of the outside diameter
of the bolt threads.
Bolt head size- the distance across the flats or the
outer sides of the bolt head (same as wrench size)
Bolt length – measured from the bottom of the bolt
head to the thread end of the bolt.
Thread pitch- Is the number of threads per inch.
With metric fastener it’s the distance between each
thread in millimeters.
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Thread types:
Course threads UNC- unified national course
Fine threads UNF- unified national fine
Metric threads SI (international standard)
Thread pitch gauge
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.
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Metric fasteners are specified with a thread pitch
instead of a thread count. The thread pitch is the
distance between threads expressed in millimeters
(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.
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Tensile strength or grade, refers to the amount of pull a
fastener can withstand before breaking.
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Bolts are made of different metals, some stronger than
others. Tensile strength can vary from bolt to bolt.
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US customary bolts are marked with slash marks on the
head of the bolt .
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Metric bolts are given a number for example 8.8 or 10.9
the larger the number the stronger the bolt.
¾ - 10 UNC- 2A X 1 3/4
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¾ - Thread diameter or bolt size
10 – Threads per inch
UNC – Unified coarse
2A – Fit symbol
1 ¾ - Bolt Length
M – 20 X 2.00 X 50 – 8.8
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M-20 – Metric thread/ diameter of
threads in mm (bolt size)
2.00 - Distance between threads (pitch)
50- Length in mm
8.8 – Strength (grade)
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There are many types of nuts used in a vehicle. These are
some of the most common ones used in the automotive
manufacturing sector.
A cotter pin slides through a castle nut and a hole in the
bolt. This makes sure the nut cannot turn and come off.
Nylon nut uses a plastic ring placed at the end of the nut to
act like a stop. These two styles of nuts are often used in
the automotive industry.
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Washers are used under bolt heads and nuts. The two basic
types are flat washers and lock washers.
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Washer must be correctly sized to fit the shank of the bolt
to prevent damage to the bolt's shank and be concave
(dish) resistant. If the washer is too soft, the washer will
concave and fail to provide a uniformed load transfer
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Washers that are too hard could fracture and fall out of
position, releasing the bolt's pre-load.
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Increases the clamping surface under the fastener
and it prevents the bolt or nut from digging into a
part.
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Prevents the bolt or nut from becoming loose under
stress and vibration.
Lock washers are not to be used were high heat is
present or they loose their locking memory. These
are split lock washer and a toothed lock washer
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Are similar to bolts, but they normally have screw
type heads. They are threaded along their full length
and are relatively small.
Machine screws are used to secure parts when
clamping loads are light. They come in various head
shapes.
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A snap ring fits into a groove in a part and commonly
holds shafts, bearings, gears, pins, and other similar
components in place. Snap ring pliers are needed to
remove and install snap rings.
The snap rings shown are internal type, external type, and
external “E”-type
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A metal key fits into a key seat, or slot, cut into a shaft and
a key way cut into the mating part, such as a gear, pulley,
or collar. The key prevents the part from turning on its
shaft.
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Are normally used to lock a part onto a shaft. They
can be used with or without keyways.
A set screw is a headless fastener normally
designed to accept a hex (Allen) wrench or
screwdriver
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It is very important that bolts and nuts are tightened
properly. This is called torquing.
If over tightened, a bolt will stretch and possible break.
The threads could also fail.
If under tightened the bolt may loosen and fall out. Part
movement could also shear the fastener or break a gasket,
causing leakage.
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Tightening values are given by the auto
manufacturer so technicians can tighten the
fasteners to their stretching limits without
compromising the integrity of the fastener. Torque
specifications are given for all precision assemblies,
such as engines, transmissions, and differentials.
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Service manuals sometimes recommend new bolts
because of torque to yield process (Head bolts).
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Bolt tightening methods requires a specific bolt torque,
followed by turning the bolt a specific number of degrees.
After using a torque wrench, a degree wheel adapter is
placed between the wrench and the socket. The fastener is
then turned until the degree wheel reads as specified by the
manufacturer.
This stretches the bolt to its correct yield point and
preloads the fastener for better clamping under varying
conditions.
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Determined by measuring bolt length change while
torquing the bolt.
Example; measuring connecting rod bolts before and after
tightening using a gauge.
Too much stretch indicates bolt weakness. Not enough
stretch may indicate thread problems affecting torque.
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Keep a steady pull on the wrench
Clean the fasteners threads
Avoid using universal joints
Read a torque wrench looking straight down at the tool
Only use a general torque scale when the manufactures
specifications are not available
Pull only on the handle of the wrench
Tighten the bolts in a step torque
Retorque when required by the manufacture. Example
cylinder heads, and exhaust manifolds.
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A technician must be able to remove broken bolts, screws,
studs and fasteners that have rounded-off heads.
Locking pliers - used for badly rusted heads and rounded
off fasteners.
Stud puller-can remove studs and broken bolts that have
been broken off at the surface of the part.
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If fasteners are too short to grasp with a tool there are a variety of
methods to use;
Hammer and punch – Angle the punch on the fastener so the blows
from the hammer will walk out the bolt.
Hacksaw and Flat screwdriver- Cut a channel into the broken
fastener then use the screwdriver to turn the fastener out
Weld a bolt onto the broken fastener
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“Easy out” screw extractor – Drill a hole into the center of
the broken fastener. Then, lightly tap the correct size
extractor into the hole. Then, unscrew the broken bolt by
simple turning the extractor with a wrench.
Note: DON’T BREAK THE EXTRACTOR OFF
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It is a chemical that dissolves rust or corrosion. It is often
applied to rusted fasteners to aid in their removal.
Allow the penetrate to soak for a few minutes. This will
help the fastener to free up without breakage.
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Gaskets and seals are used between parts to prevent
leakage of engine oil, coolant, transmission oil, and
other fluids.
Gasket: Is a soft, flexible material. It can be made of
fiber materials, rubber, neoprene, cork, treated
paper, or thin steel.
When the parts are fastened tightly together the
gasket is compressed and deformed.
This fills in small gaps or imperfections in the
surfaces and a leak proof seal is created.
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Inspect for leaks before disassemble.
Be careful not to nick, gouge, or dent mating surfaces
while removing parts.
Clean off old gaskets carefully.
Wash and dry parts thoroughly using solvent. Then wipe
the surface clean.
Compare the new gasket to the surface of the part to make
sure everything will line up (bolt holes, cut outs).
Use sealer if needed. Use sealer sparingly!
Tighten the fastener in steps. When more then one
fastener is used to retain the part don’t over tighten.
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They help prevent leakage and hold the gasket in place
during assembly.
There are numerous sealers with different uses:
Hardening sealers - used on permanent assemblies, such as
threads and fittings. They are usually heat and chemical
resistant.
Non-hardening sealers - used on assemblies that are to be
removed, such as plates, covers, flanges.
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Are special sealers used instead of conventional
fiber or rubber gaskets. The two types are:
R.T.V (room temperature vulcanizing)- comes in a
tube and is used to make a rubber like gasket that
dries when it comes in contact with air.
Anaerobic sealer - cures to a plastic like substance
in the absence of air. It is used on thick parts and the
surface must be smooth, true and not flexible.
Lets make a gasket