Automotive mechanics (volume i)(part 1, chapter6) seals, gaskets and sealants

Seals, gaskets and sealants 85 Purpose of oil seals 86 Types of oil seals 86 Servicing oil seals 88 Gaskets 91 Gasket materials 92 Handling and storing gaskets 93 Sealants and sealing compounds 93 Installing gaskets 93 How to make a gasket 95 Pipes, hoses and connections 95 Technical terms 96 Review questions 96

Seals, gaskets and sealants

Chapter 6

Purpose of oil seals

Types of oil seals

Servicing oil seals

Gaskets

Gasket materials

Handling and storing gaskets

Sealants and sealing compounds

Installing gaskets

How to make a gasket

Pipes, hoses and connections

Technical terms

Review questions

Seals and gaskets of various kinds are used in many

parts of a motor vehicle They are used to prevent loss

of oil, grease, fuel, coolant, air or gas In some

instances, they are also used to exclude dirt and water.

Several different types of sealing arrangements are

used; these include oil seals, gaskets, O-rings, washers,

boots and sealing compounds.

A number of seals and gaskets are shown in

Figure 6.1 These are part of a repair kit for a manual

transaxle and are typical of the seals and gaskets used

in motor vehicles.

same time, they are operating at high piston speeds and

in conditions of extremely high temperature.

Types of oil seals Cased oil seals

This type of oil seal consists of a moulded synthetic rubber seal enclosed in a metal case The seal is made with a lip which forms a sealing edge against the shaft with which it is used These seals are often referred to

as lip-type seals (Figure 6.2).

Some seals have a circular spring, sometimes called

a garter spring, fitted behind the lip to hold it against the shaft and increase its sealing ability Other seals do not have a spring, but depend on the shape and resilience of the synthetic rubber material to maintain contact between the lip of the seal and the shaft In some applications, a double-lip seal is used.

The metal casing of some seals fits directly against the bore of the housing in which it is mounted Other seals have a rubber coating over the metal which seals against the bore of the housing.

Most lip-type seals act in one direction only, and when installed, the lip of the seal must face towards the inside of the housing When installed this way, any internal pressure will tend to force the lip of the seal against the shaft to help it seal.

■ If a seal is installed incorrectly, its lip will lift off the shaft and the seal will leak.

Multiple-lip oil seal

Figure 6.3 shows the construction of a multiple-lip oil seal, and how the lips fit on a shaft There are three sealing lips, moulded around a metal case Each lip is a different shape and the centre lip is made of Teflon.

figure 6.1 Seals and gasket kit for a manual transaxle

1, 2, 3 gaskets, 4, 5, 6 O-rings, 7, 8, 9 oil seals

HOLDEN LTD

Purpose of oil seals

Most oil seals are used between a shaft and a housing

to prevent oil or grease from leaking past the shaft,

although in some locations, the seal also prevents dirt

from entering In wheel hubs, for example, the seal not

only retains the grease in the hub and bearings, but it

also keeps out dust and water that would contaminate

the grease and ruin the bearings.

In transmissions and rear axles, seals are used to

retain oil, so both the seals and the sealing surfaces of

the shafts must be in good condition to obtain a seal

and prevent oil leaks.

In hydraulic systems, such as those of hydraulic

brakes, power steering and automatic transmissions,

seals have to retain fluid under pressure This places an

even greater load on the seals to prevent fluid loss.

Within the engine cylinders, a different type of seal

is required The piston rings, which are metal sealing

rings, have to seal against air and gas pressures At the

figure 6.2 Metal-cased oil seals – the sectional views

show the shapes of the seals

This seal has been designed for the front and rear of

the crankshaft, which are critical sealing locations in

an engine Teflon has a long life and low friction

against the shaft.

O-rings

O-rings are synthetic rubber rings that fit into grooves

that have been machined in shafts and other parts.

When the O-ring is installed in its groove, it

protrudes above the shaft When the shaft and its

mating part are assembled, the O-ring is compressed

slightly so that it forms a seal between the two parts

(Figure 6.4) O-rings are a simple type of seal, but they

are very efficient They depend on the resilience of the

synthetic rubber to maintain a seal.

O-rings are used as seals in a variety of places In

hydraulic systems they provide a seal between a sliding shaft and a housing, or between a piston and a cylinder They can also be used instead of a gasket to seal between two mating faces The O-ring fits into a groove in one face and, when the parts are assembled,

it is compressed against the mating face to form a seal between the two surfaces.

Figure 6.5 shows a drive unit used for speedometers and some sensors This has two O-rings, and each is used for a different purpose The larger O-ring is used

to seal the housing in its mounting on the transmission, while the smaller one is used as a seal between the drive shaft and the housing.

Sealing strips and blocks

Strips of synthetic rubber or composition are used as seals in certain locations An example of this is the crankshaft bearing cap in Figure 6.6 This has a sealing strip on each side, which seals against the web of the crankcase, and a seal in a groove in the bore of the cap, which seals against the crankshaft journal.

figure 6.3 A section through a multi-lip seal HOLDEN LTD

seal

shaft

teflon

figure 6.4 O-ring

(a) installed on a shaft (b) forming a seal

figure 6.5 Speedometer or sensor drive with O-rings

and an oil scroll MITSUBISHI

figure 6.6 Seals in a main bearing cap at the rear of a

crankshaft

Oil scroll

An oil scroll is a spiral groove that is machined in a

shaft The scroll is not the main seal, but it is used to

reduce the amount of oil that reaches the main seal (see

Figure 6.5).

The scroll winds in a direction opposite to the shaft

rotation Oil that is carried along the shaft to the scroll

tends to be wound back along the shaft and so retained

in the housing.

Oil slinger

This is a dished washer that is sometimes used on a

shaft where it fits through a housing Oil reaching the

slinger tends to work outward to the edge of the slinger

and is then thrown back away from the shaft.

This is also a supplementary seal and not a main

seal It reduces the quantity of oil that reaches the main

seal.

Cup-type seals

Cup-type seals are used in the cylinders of hydraulic

brake systems Two of these seals are shown in

Figure 6.7 The primary cup fits against the head of the

piston, and its lips form a seal against the cylinder

walls When the brakes are applied, pressure in the

cylinder forces the lips of the cups against the cylinder

walls to form an even better seal.

The secondary cup is made in the form of a ring, so

that it can fit into a groove provided in the piston The

cup has lips on both its inner and outer edges so that it

seals against both the piston and the walls of the

cylinder.

Hydraulic brake cups and seals are usually made

from natural rubber Hoses and other fittings are made

of synthetic rubber Hydraulic brake fluid is a special

fluid that is used only in braking systems and in

hydraulic clutch controls.

■ Rubber brake parts will be ruined if they become

contaminated by mineral (engine) oil, grease or

solvents.

Boots

Boots are fitted to steering and suspension parts, drive shafts and hydraulic components Some are used to retain lubricant and exclude dirt and water, others are used only to protect the part.

Two different types of boots are shown on the steering tie rod in Figure 6.8 The telescopic boot is fitted between the end of the steering box and the tie rod, and is able to expand and contract to accom-modate large movements of the tie rod as the vehicle is being steered The smaller boot on the ball joint retains grease and excludes dirt and water, while still permitting a limited amount of joint movement.

Servicing oil seals

Seals are removed either because they are leaking and need replacing, or because they have to be removed as part of the procedure when dismantling a component.

If a seal is removed for any reason, then a new seal should be installed.

When installing a seal, make sure that it is not being damaged or distorted by the method being used Check that it is being installed in the correct way Lip-type seals must have the lips pointing in the right direction, otherwise they will leak.

Removing and replacing seals

Some seals, particularly those with a rubber coating on the outside, can be removed by levering them from the housing with a screwdriver or similar lever, provided that care is taken to protect the housing from damage Figure 6.9 shows a small seal being removed and replaced The screwdriver is moved around the seal levering it from the housing.

The new seal is fitted into its bore in the housing and then tapped home, using a socket spanner of the same diameter as the seal as a driving tool.

Larger oil seals

Larger oil seals are a light press fit or a light drive fit

in the housing and usually require special tools.

figure 6.7 Seals used with a hydraulic brake piston

figure 6.8 Boots on a steering tie rod TOYOTA

Figure 6.10(a) shows one type of tool being used to

extract an oil seal from the extension housing of a

transmission The tubular tool has a coarse tapered

thread which is screwed into the metal casing of the

seal The centre bolt, or forcing screw, is then

tight-ened against the end of the transmission mainshaft, and

this forces the seal from the housing.

Other removing tools are levers and tools used with

a slide hammer.

Figure 6.10(b) shows a seal being refitted to a

transmission extension housing The installing tool is

hollow so that it will fit over the transmission shaft,

and the end of the tool is recessed so that only the

outer edge of the tool bears against the outer edge of

the seal.

Installing metal-cased seals

When fitting an oil seal to a housing, force must be

applied to the outside edge If force is applied

anywhere else, it will distort the case and damage the

seal Wherever available, a special tool should be used

so that force is applied in the right place.

If a special tool is not available, then a suitable piece of tubing could be used If a drift has to be used,

it should have a flat end and should only be used against the outer edge of the seal The seal should be tapped alternately on opposite sides, at the same time making sure that the seal is entering squarely.

Another method is to use the old seal as a pressing

or fitting tool Being the same size as the new seal, it will apply force to the correct place at the outer edge

of the seal.

Protecting seals

Before installing a seal over a shaft, check any keyways, threads or splines for sharp edges or burrs that could damage the seal during installation.

Where the seal could be damaged, a tapered sleeve can sometimes be used on the end of the shaft This protects the seal while it is being fitted into place.

If a tool of this type is not available, then a strip of masking tape can be wrapped around the shaft as

a protection Lubricate the shaft and the seal so that the seal will slide easily, and use a rotary motion as the seal is being moved into place.

Lubricating seals

Synthetic rubber seals, and the shafts on which they operate, should be coated with lubricant before assembling A dry seal operating on a dry shaft will damage the sealing lips With a rotating shaft, a dry seal can produce a loud whining noise, indicating that

figure 6.9 A small seal in a housing

(a) removing (b) replacing HOLDEN LTD

screwdriver

housing

socket spanner

seal

(b)

(a)

figure 6.10 Oil seal

(a) removing from a transmission housing (b) installing

housing tool

spanner

housing seal tool hammer

(a)

(b)

the seal and the shaft surface are running dry and that

the seal is being damaged.

With cased seals, grease should be placed behind

the rubber seal as well as on the lips to provide initial

lubrication.

■ Teflon seals are an exception, as they are usually

installed dry.

Checking sealing surfaces

Before installing a seal, the surface of the shaft against

which the seal fits should be checked for scores and

burrs, and polished lightly if necessary Fitting a new

seal will not prevent a leak past a damaged surface.

A sealing surface that is badly scored or worn may

be restorable by building it up with welded metal and

then machining it to size.

Removing and installing O-rings

An O-ring can usually be raised in its groove by

squeezing it between a finger and thumb and stretching

it above the groove It can then be eased away from the

groove and completely removed.

If the O-ring cannot be removed in this way, then a

small screwdriver or similar tool might have to be used

under it This must be done carefully so that the groove

is not damaged.

When installing an O-ring, the shaft and the groove

should be lubricated and the O-ring rolled along the

shaft and into the groove When installed, check that it

is straight in the groove and not twisted.

Removing and installing boots

Figure 6.11 shows a boot from a rack-and-pinion

steering box.

The steering tie rod has to be disconnected and the

ball joint removed before the old boot can be taken off.

The new boot is fitted over the shaft and over the

end of the steering box It is secured by a clip at each

end The boot must sit straight when fitted The steer-ing box has a similar boot at each end and contains lubricant This is retained by the boots, which have to provide a seal at both ends.

■ Long corrugated boots, like those of a steering box, are often referred to as bellows.

Checking boots

The condition of a boot can be checked by squeezing it

to see if it is torn or perished (Figure 6.12) Boots that are split or perished must be renewed.

The lubricant retained by the boot should be checked, and if it appears to be contaminated, then it must be renewed This will probably mean that the part will have to be dismantled, cleaned, and relubricated before a new boot is fitted.

figure 6.11 A long rubber boot or bellows used on a

steering box and tie rod HOLDEN LTD strap bellows clip tie-rod end

figure 6.12 Boots are checked to make sure that they are

not split or perished

Hydraulic brake seals

Cups and seals in hydraulic brake systems are renewed during a major brake service When being installed, they are coated or dipped in hydraulic brake fluid Other hydraulic brake parts are lubricated with a special rubber grease These are the only lubricants that can be used Any other oil or grease will make the rubber parts swell and cause severe brake problems.

Oil filter seals

The oil filter has a synthetic rubber ring which seals against the flat surface of the filter mounting on the engine block (Figure 6.13) This is an important seal that has to withstand engine oil pressure An undetected leak at the oil filter could empty the oil pan

and run the engine short of oil, causing extensive

engine damage.

When installing an oil filter, the seal is given a light

coat of oil or grease and the filter is tightened firmly,

but not overtightened After running the engine, the

filter should be checked to make sure that it has been

tightened correctly and that there is no oil leaking past

its seal.

■ The procedure for changing an oil filter is covered

in Chapter 11: Engine-lubricating systems.

Gaskets

The engine and other major mechanical components

are made up of a number of parts of different shape

and size which are bolted together to make up the

complete assembly Although the surfaces of the parts

are machined flat during manufacture, gaskets are

needed to seal the joints and prevent leaks.

Most gaskets are made from materials which are

slightly compressible This takes care of the small

irregularities in the surfaces and provides a seal to

prevent leaks Gaskets are made in a variety of shapes

and sizes and from a number of different materials

(Figure 6.14).

The material from which the gasket is made is

selected to suit the particular component and will

depend on:

1 the type of surfaces being sealed

2 the pressure of the liquid or gas

3 the temperature that the gasket must withstand Compounds and sealers are also used to seal joints While it is accepted practice to use sealers with certain gaskets, sealers should not be applied to all gaskets as

a matter of course.

■ Some compounds and sealers are designed to be used between parts without a gasket.

Gasket construction

Figure 6.15 shows the basic construction of gaskets and the materials from which they are made:

1 Plain gaskets These gaskets are cut to shape from sheet materials, which include special jointing papers, cork, cork composition, and synthetic rubber.

2 Layered gaskets These have a layer of composition material sandwiched between two thin sheets of steel The metal is rolled over the holes for reinforcement.

3 Bonded gaskets These are made of a composition material that is bonded to a perforated metal core The holes in the gasket are often reinforced with metal ferrules.

4 Embossed steel gaskets These are made from a single sheet of hard steel Corrugations or beads are pressed into the steel around the holes to assist sealing.

figure 6.13 Oil filter seal

(a) oil filter mounted on the engine block (b) bottom of filter with its sealing ring

seal

figure 6.14 Sealing between flanges

(a) gasket compressed (b) beads of sealing

compound (c) sealant spread to seal the joint

(a)

(b)

Gasket materials

Following are brief descriptions of some materials

which are used for gaskets, together with the locations

where they are generally used.

Cork and cork composition

Cork gaskets are cut from granulated cork sheets, or

sheets of cork and rubber composition They are very

compressible and are often used on pressed-metal parts

such as timing covers and oil pans These parts do not

have rigid surfaces, and a compressible material is

therefore needed.

Special jointing materials

Gaskets of special papers and jointing materials are

used in many places to seal against water, fuel or oil

leakage Locations are water pumps, some fuel pumps,

transmissions and rear axles These jointing materials

are made in various thicknesses and some are more

compressible than others.

Fibre and nylon

These materials are used mainly as washers under

plugs, bolts or nuts to prevent oil leaks The material

can be slightly compressed, but it is strong enough to

allow the plug to be tightened without extruding the

washer.

Synthetic rubbers

There are a number of flexible materials with different properties that can be classed as synthetic rubber These are very compressible and resilient materials They are made in sheets, from which gaskets are cut Synthetic rubber is also moulded to form O-rings, circular seals and gaskets of various shapes Most synthetic rubbers are very durable and resistant to both oil and water.

The valve cover shown in Figure 6.16 has a moulded synthetic rubber gasket This fits into a recess in the face

of the valve cover It is an example of a synthetic rubber gasket used with an aluminium casting.

figure 6.15 Types of gasket construction

figure 6.16 Valve cover with a moulded synthetic rubber

gasket

valve cover

moulded rubber gasket

Steel core and composition

These consist of a perforated steel core, with a special composition material bonded to each side They are often used for cylinder heads and manifolds The steel core provides reinforcement and holds the composition material in place under pressure Material of this type

is impervious to oil and water, and is also resistant to heat Because the composition is not a metal, it will not corrode.

■ The composition materials for gaskets are made from manufactured fibres, graphite and bonding materials.

Composition gaskets

Composition gaskets are used in locations which are subject to high temperatures, such as cylinder heads, manifolds and exhaust systems They are usually made

by cutting from sheet material Some gaskets have a layer of composition sandwiched between two outer layers of steel.

Metal gaskets

Embossed metal gaskets are used for some cylinder

heads and exhaust manifolds The embossing consists

of beads, or ridges, stamped in the metal around the

cylinder bores and coolant holes This gives the gasket

a form of compressibility and so provides a better seal

in these areas.

Metal gaskets are very resistant to heat and

pressure They can be a single layer, or made of two or

more layers For example, an exhaust manifold gasket

could have two layers of stainless steel and one thin

layer of aluminium.

The gasket is fitted with the steel layer against the

cast-iron manifold, and the softer aluminium layer

against the aluminium cylinder head.

Handling and storing gaskets

Gaskets should be handled and stored carefully to

prevent damage which could render the gasket useless.

Some points to be observed are as follows:

1 Storing Store gaskets flat to prevent them from

becoming twisted or damaged.

2 Gasket kits Gaskets are packaged with a cardboard

backing for protection and to keep them flat The

package should remain closed until the gaskets are

about to be used.

3 Spare gaskets Some gasket kits are made to cover

more than one model of a vehicle These kits can

contain more gaskets than are actually needed for

the particular job Check to see which are to be

used and which are spare.

4 Check old and new gaskets Compare replacement

gaskets with the originals to make sure that they are

both the same shape and size.

Sealants and sealing compounds

There are a number of different grades of sealants or

sealing compounds that cover a wide range of

applications, from sealing joints between components

to sealing threads.

Some threads are coated with sealant to prevent

leaks, while others have a small quantity of a different

sealant applied to prevent the bolt from becoming

loose (Figure 6.17).

Other grades of sealants are more adhesive and are

used as retainers for parts such as bearings, pulleys and

gears Some grades of sealants are used instead of

gaskets to produce ‘formed-in-place’ gaskets (see Figure 6.21).

Sealants and sealing compounds are supplied as a liquid or paste, which automatically cures from the inside out by a chemical action known as an anaerobic cure.

A film of sealant applied to the part will remain liquid as long as it is exposed to air, but curing begins once the parts are assembled and the air is excluded This is assisted by the catalytic effect of the metal surfaces of the parts Curing sets the liquid, although some grades remain flexible after curing.

■ Parts that were assembled with these compounds can be dismantled with normal tools, provided the correct grade of sealant or adhesive was used during assembly.

Installing gaskets

Some important points for the installation of gaskets are as follows:

1 Treat gaskets with care to avoid damage; careless-ness can ruin a new gasket even before it is used.

2 The surfaces of the parts, against which the gasket fits, should be clean, with all traces of the old gasket removed Cast iron and aluminium alloy surfaces can be checked for flatness, and burrs removed by lightly draw-filing with a smooth flat file.

3 During installation, small gaskets can be held in place by a light coating of grease or oil Both sides

of the gasket are sometimes coated to assist with initial sealing.

4 Where a gasket is fitted between two parts, the bolts which hold the parts should be tightened gradually and evenly A torque wrench should be used to tighten the bolts where torque specifications are provided.

figure 6.17 Sealant being applied to a thread

Cylinder-head gaskets

A cylinder-head gasket is illustrated in Figure 6.18.

This is the most important gasket in the motor vehicle –

it is subjected to the greatest heat and pressure It must

provide a good seal against hot coolant and oil, as well

as sealing the gas pressure in the cylinders Even a

small leak of any of these will cause serious problems.

Replacing the cylinder-head gasket requires

attention to detail, and the following are points that

should be observed.

Cleaning

Before installing the gasket, it is essential that the

cylinder-head and cylinder-block faces are clean Carbon

and residue from the gasket can be scraped off, but

sharp-edged scrapers, which could scratch the surfaces, should

not be used Final cleaning of the combustion chambers

can be done with a wire brush in a portable electric drill.

Small burrs on the surfaces of cylinder blocks and

heads can be carefully removed with a fine flat file.

A light draw-file will remove burrs and still preserve a

good surface finish.

Cylinder-head bolts or studs should be cleaned,

particularly the threads The bolt holes in the cylinder

block should also be cleaned Dirty threads will cause

friction and give false torque settings.

■ Residue from cleaning must be completely removed

before parts are assembled, as foreign matter

entering the engine will cause damage.

Tightening

Vehicle manufacturers provide torque specifications

for cylinder-head bolts These should be used in

conjunction with an accurate torque wrench so that all

bolts are evenly tightened.

The sequence of tightening is also specified by the

maker As a general rule, this should commence at the

centre of the head and work diagonally outwards as

shown in Figure 6.19 The bolts are tightened

pro-gressively until the specified torque is reached.

Bolts should be tightened in two steps, with the first step half of the final torque Many manufacturers require the head bolts to be re-tightened after an initial run-in period The increased use of monotorque head gaskets that only require tightening once has reduced this service requirement.

Housing gaskets

Where the machined surfaces of housings are flat, thin paper gaskets, which are slightly compressible, are often used Rubber moulded gaskets are also used These fit into a groove in one surface and are compressed against the other.

In some locations, housing gaskets are not used, but

a coating of sealing compound is applied directly to the surfaces.

■ To prevent distortion and damage, the bolts securing a housing should be tightened progres-sively in a diagonal pattern (Figure 6.20).

figure 6.18 Cylinder-head gasket for a four-cylinder

engine is reinforced around the cylinder bores

figure 6.19 Tightening sequence for cylinder-head bolts

figure 6.20 Tightening bolts which secure a retainer to a

housing – the bolts should be tightened progressively MAZDA