Automotive fasteners and measurements

The metric system European and Asian [with the exception of Sweden and the UK] manufactures have built their automobiles to metric specifications since the invention of the automobile In the mid 1970s the United States, Sweden and the UK began the switch to the metric system

The switch to metric Beginning in 1977 when any new component was introduced on an American made vehicle where factory retooling was needed, the threads for all the fasteners were machined to metric specifications Nearly all fasteners on modern domestic cars are metric but you may still find some standard threads on components that have not significantly changed in over 35 years [often on commercial trucks]

Metric is simpler The metric system does not use fractional measurements If your trying to find a wrench where a 13mm is too small and a 15mm is too big – the answer is simple – 14mm Wrench sizes in the US Standard system are fractional If your trying to find a wrench where a ½” is too small and a 5/8” is too big – the answer requires a good grasp of fractions and often requires you find the dreaded ‘lowest common denominator’

Meters Meter Stick Yard Stick A meter is a little bit longer than a yard – 39.37 inches to be exact

Meters 1 meter can be divided into: 10 decameters [rarely used] 100 centimeters {cm} 1000 millimeters {mm} Millimeters are the most common unit used in automotive specifications but centimeters are occasional used 10 millimeters = 1 centimeter

Abbreviations When working with dimensions that are smaller than the base unit lower case letters are used Upper case letters are used in abbreviations of units larger than the base unit 5 mm represents 5 millimeters [5/1000 of a meter] 5 Mm represents 5 megameters [5 million meters]

Fractions are not used in metric dimensions Fractions should never be used with metric dimensions When you measure something with a metric ruler that is halfway between 23 and 24 cm you should record the dimension as 23.5 cm

Hex head bolt dimensions There are four primary dimensions of a hex head bolt: [A] Thread diameter [B] Thread length [C] Thread pitch [D] Hex size [distance between flats]

Distance between flats The hex size determines the size of the wrench needed to loosen and tighten the bolt The hex size is not standardized A bolt with a diameter of 8mm and a thread pitch of 1.25mm will have a hex size of 12mm on an Asian car An 8mm bolt with a 1.25mm pitch will have a hex size of 13mm on a European car 8 x 1.25mm 12mm Asian 8 x 1.25mm 13mm European

Thread pitch 1.5mm In the metric system the thread pitch is the distance in millimeters between each thread Threads that are spacedd close together are called ‘fine’ whereas threads that are spaced far apart all called ‘coarse’ Coarse 1.25mm Fine

Metric standard and fine thread pitch Bolt Dia. Standard Fine Super Fine 6mm 1.0 7mm 1.0 8mm 1.25 1.0 10mm 1.5 1.25 1.0 12mm 1.75 1.5 1.25 14mm 2.0 1.5 16mm 2.0 18mm 2.5 20mm 2.5

US standard coarse and fine thread pitch Bolt Dia. UNC Coarse UNF Fine 1/4” 20 28 5/16” 18 24 3/8” 16 24 7/16” 14 20 1/2” 13 20 9/16” 12 18 5/8” 11 18 3/4” 10 16

USS and SAE In the US system coarse thread bolts may be called UNC or USS bolts [Unified National Coarse or United States Standard] Fine thread bolts may be called UNF or SAE [Unified National Fine or Society of Automotive Engineers]

Coarse thread vs. fine thread Fine thread bolts have superior holding ability but they can only be used with hard metals [steel] Soft metals such as aluminum, cast iron and brass will not tolerate the stress that can be applied to a fine thread bolt

Determining thread diameter Thread diameter is measured in the threaded end of the bolt between the high points of the threads [peak to peak] The shank [unthreaded section is normally slightly smaller than the threaded section Shank

Determining bolt length Bolt length is the distance between the bottom of the bolt head and the end of the threads Universal replacement bolts sold in auto supply stores normally come in 5mm increments 40mm

Determining thread pitch Thread pitch is measured with a thread pitch gauge Place the teeth of one of the gage blades lengthwise along the threads of the bolt If all of the teeth fall into the thread valleys the thread pitch is equal to the number stamped on the blade

Internal thread pitch The thread pitch gauge can also measure internal threads It is not easy to see the gauge teeth when measuring internal threads Identifying the correct pitch on an internal thread is done more by feel than by eye

Bolt dimensions When ordering metric add the letter ‘M’ in front of the dimensions The order of dimensions is: Diameter Pitch Length M8 x 1.25 - 40 The bolt listed here: Is 8mm in diameter Has a 1.25mm thread pitch Is 40mm long

US standard bolts US Standard bolts may have a UNC or UNF prefix [Unified National Coarse or Unified National Fine] The bolt dimensions are in the same order [diameter, pitch and length] 1/4 x 20 - 3/4 Thread pitch is expressed in threads per inch in US Standard bolt nomenclature

Bolt gauge Bolt distributers often give away plastic bolt gauges that make it easier to identify bolt diameter and length

Grade number The bolts used to hold automotive parts together must be stronger than general purpose bolts used for household applications Bolts are graded by the tensile strength of the steel the bold is made from Tensile strength is the maximum amount of force [pull] that can be applied to a bolt before the bolt will not return to its original length after the pressure is released The yield strength is the amount of force [pull] it takes to make the bolt break

US standard bolt grade numbers Grade 3 – approx 100,000 psi / sq in Should not be used for automotive applications Grade 5 – 120,000 psi /sq in OK for low stress automotive uses – accessory brackets, exhaust system etc. Grade 8 – 150,00 psi / sq in Needed for high stress components – rod bolts, clutch and flywheel bolts etc.

Metric bolt grade numbers Metric bolts have a number stamped on the head of the bolt that indicates both tensile and yield strength 10.9 The number in front of the decimal point is 1/10th of the tensile strength measured in kg/mm2 In the example here the bolt is made from steel that can withstand 100 kilograms of force over one square millimeter and still return to its original length The number after the decimal point is the 1/10th of the percentage of tensile strength to the yield strength In the example the tensile strength is 90% of the yield strength

Metric grade numbers Unmarked - equivalent of Grade 3 Should not be used for automotive applications 8.8 Equivalent to Grade 5 OK for low stress automotive uses – accessory brackets, exhaust system etc. 10.9 Equivalent to Grade 8 Needed for high stress components – rod bolts, clutch and flywheel bolts etc.

Flat washers A flat washer is placed between the head of the bolt and the component for a number of reasons: The flat washer distributes the clamping load over a greater surface area The use of flat washers helps to achieve a more accurate torque value The flat washer prevents gouging on soft metal or painted/plated surfaces Never use more than one flat washer per bolt Besides the fact that the retaining bolt will loosen over time the use of multiple washers is evidence of shoddy workmanship

Lock washers Split lock washers are made of spring steel and have a sharp edge that bites into the base metal to prevent the bolt from loosening Split lock washers are not very effective and are not normally found on modern cars Star washers have sharp radial spurs that dig into the base metal to prevent loosening Star washers are often used on chassis ground electrical terminals Wave washers and Belleville washers are made of thin spring steel and provide a small amount of spring tension to the clamping load Split Star Wave

Serrated lower surface Flange bolts Most modern automotive threaded fasteners usually have the flat washer integrated into the head of the bolt or nut. Often the lower surface of the integrated washer is serrated to help prevent the nut or bolt from loosening The integrated washer allows the hex to be smaller – which allows the bolt to fit into tight places making installation easier This type of bolt is called a ‘Flange Bolt’ Serrated lower surface

Nylock nuts Nylock nuts are often used in applications where minimal clamping force is needed – valve covers etc. Nylock nuts have a nylon ring embedded in a groove at the top of the nut that prevents the nut loosening Nylon ring Nylock nuts cannot be used in high temperature applications

Top thread is out of round All metal lock nuts This type of nut is similar in appearance to a nylock nut but can withstand high temperatures Instead of nylon the top thread is pinched inward at four points and is slightly out of round Top thread is out of round

Allen and Torx head bolts Allen head and Torx head bolts have recessed caps that require an Allen or Torx sockets Torx head bolts have become popular in recent years because they provide the maximum amount of surface area exposed to torquing force of any common fastener so they are less likely to be deformed during torquing and de-torquing The Torx head bolt allows for more accurate torquing of machine assembled components Allen head Torx head

Torx head screws and bolts Torx head screws are commonly used in the assembly of small lightweight chassis components Headlight assemblies Instrument clusters Heating and air conditioning under-dash components etc. The size of a female Torx screw is expressed as a number after the letter ‘T’ Torx screws normally range from T8 to T65

External Torx bolt heads External [male] Torx head screws are commonly found in modern automotive engines External Torx head bolts will require special sockets that have the letter ‘E’ preceding the size number External Torx head screws and bolts normally range from E4 to E24 The numbers for external Torx bolts are much smaller than internal A T55 bolt has the same internal dimensions as the external dimensions of an E12 bolt

Machine screws Machine screws are small screws used in subassemblies such as instrument clusters, heater controls etc. US Standard machine screws are numbered When as hash mark ‘#’ is placed in front of a number it denotes a US Standard machine screw Metric machine screws are preceded by the letter M and do not use the hash mark ‘#’ so the size designation is the same as regular metric bolts Unlike conventional bolts, machine screws are normally threaded their full length

US Standard machine screws num pitch dia. #4 40 0.112” #6 32 0.138” #8 32 0.164 #10 24 & 32 0.190 #12 24 0.216

Metric machine screws num pitch dia. M1 .25 1.0 mm M1.22 .25 1.2mm M1.4 .30 1.4mm M1.6 0.35 1.6mm M2 0.40 2.0mm M2.5 0.45 2.5mm M3 0.50 3.0mm M3.5 0.60 3.5mm M4 0.70 4.0mm M5 0.80 5.0mm

Thread engagement Bolts that are threaded into engine blocks, cylinder heads, etc. must be long enough to insure the threads on the bolt have sufficient grip to withstand the stress when the bolt is torqued to specifications.

Bolt to Short If the bolt is too short the threads in the block or the bolt will be stripped If the threads in the block are stripped the hole will need to be drilled oversize and a thread insert [Heli-Coil] installed

Bolt to Long If the bolt is too long it will bottom out and bind in the hold. Do not install extra washers to correct this –additional washers will reduce the clamping force If the bolt snaps when its torqued you will not be able to remove the broken stub with an EZ-out because the threads are bound

Thread engagement – Dia./Length ratio Lightly stressed bolts that thread into cast iron or steel housings should have a minimum threat engagement ratio of 1 to 1 A 1.5 ratio is better Aluminum, brass and other soft metals require more thread engagement – typically 2 to 3 times the bolt diameter 8 mm 8 mm

Thread engagement – visual check 8 mm When installing a bolt check how far the head of the bolt is from the component when the threads first catch. The head of the bolt should extend 1 to 1.5 times its diameter on lightly stressed bolts Heavily stressed bolts [head bolts, main bearing cap bolts etc.] will have threads engaged 3 to 4 times their diameter 8 mm

Shoulder bolts Shoulder bolts have a shank that is much wider than the threads They allow for a fixed gap between the housing/flange surface and the head of the bolt Shoulder bolts are found on flexible exhaust flanges and valve covers where soft silicone gaskets are used The shoulder bolt prevents the soft rubber gasket from being damaged by over tightening the bolts

Left hand threaded fasteners Nearly all threaded fasteners use right hand threads A right hand threaded bolt is tightened when rotated clockwise – to the right as viewed at the top Left hand thread fasteners a threaded in the opposite direction A left hand threaded bolt is tightened when turned counter-clockwise – to the left as viewed at the top Left handed fasteners are often stamped with the letter ‘L’

Applications for left hand threads The most common application for left handed threads are the tie rod ends in the steering linkage Right hand threads Sleeve Left hand threads The outboard tie rod shaft has right hand threads and a right hand locknut The inboard tie rod shaft has left hand threads and a left hand lock nut The center sleeve has left hand threads on the inboard end and right hand threads on the outboard end. When the sleeve is rotated in one direction – the tie rod gets longer. Rotate the sleeve in the opposite direction and the tie rod gets shorter

Studs Intake and exhaust are often retained by studs and nuts instead of bolts Studs are often used on aluminum heads and blocks. They eliminate the potential for damage to the soft threads in the aluminum castings when components are removed and installed

Studs An unthreaded ‘shoulder’ separates the two threaded ends of the stud The shoulder is slightly larger than the threaded sections. This allows the stud to be retained tightly to the casting Most studs for modern engines have a external [male] Torx end that simplifies installation Torx head Shoulder

Hand Wrenches Adjustable Open end Box end Combination Flare nut Ratcheting

Adjustable wrench Only two stress point Adjustable wrenches should never be used to loosen or tighten automotive fasteners They often found in an emergency tool kit but you’d be better off with an inexpensive set of sockets There are many different types, but all of them will damaged the head of the bolt if a large amount of torque is applied

Open end wrench Only useful in places where a socket or box end wrench can’t reach – exhaust manifold bolts, valve lash adjuster lock nuts etc. The wrench size of each end of a double open end wrench is a different – 6 wrenches can cover 12 sizes

Open end Only two stress points Like the adjustable wrench the open end wrench only applies pressure on two corners of the bolt head Never apply a large amount of torque to the head of a bolt with an open end wrench unless there is no other alternative Once the bolt has been loosened it is often used to work the bolt out of the threads as it is much faster to reposition than a box end wrench

Box end wrenches Two general types of box end wrenches 6 point 12 point The advantage of a box end wrench is that when the bolt is torqued/de-torqued pressure is applied equally to all six flats

Red indicates high stress area Box end wrenches Pressure is applied equally at six points on the bolt head The design of the box end allows pressure to be applied away from the corners This makes it less prone to round the corners of the bolt head when high torque is applied Red indicates high stress area

Box end wrenches The 6 point box end wrench has twice the surface area in contact with the bolt head so it is less likely to round off the corners on the head of the bolt The 12 point box end wrench is easier to work with as it only needs to be rotated 30 degrees to engage another set of flats

Box end wrenches The 6 point box end wrench applies pressure over a surface area that is twice as large as that of a 12 point box end wrench The pressure is applied away from the corners where the metal is thin The six point open end wrench and the 6 point socket can apply more torque to a bolt head then any other type of wrench or socket without rounding off the corners

Offset box end wrenches Like the double open end wrench the double box wrench has two different sizes Most double box wrenches are slightly offset – typically about ½” Deep offset box wrenches have an offset of 1 to 1 ½” and are designed to get around obstacles. Deep offset box wrenches are often needed to remove carburetors and distributers and are often used in valve adjustment on commercial diesel engines

Flare nut wrench Flare nut wrenches are used to loosen and tighten the nuts on steel brake and hydraulic lines The are open at one end so that they can slip over the steel brake line Unlike conventional open end wrenches they make contact with the nut at 5 points They are also thicker than conventional wrenches

Specialty wrenches Specialty wrenches are wrenches that are bent or curved to service one component on one brand of vehicle New car dealers will have hundreds of specialty tools in their tool room Some specialty tools are available from tool manufactures Often mechanics make their own specialty tools by heating and bending an ordinary wrench into a shape that will fit

Socket Wrenches Socket wrenches are the best type of wrench to remove and install automotive components Like a box end wrench they provide maximum amount surface area for the forces of torqueing and de-torqueing the bolt without damaging the flats on the bolt head Unlike hand wrenches sockets can be attached to a torque wrench so that a measured amount of torque is applied

Thin wall and Impact sockets Weak spot There are two general types of sockets: Thin wall Impact Thin wall sockets are designed for hand tightening They are thinner than impact sockets so they can fit into places that an impact socket cannot

Thin wall and Impact sockets Thin wall sockets should never be used with an impact wrench Serious injury can result when the socket shatters and the pieces fly outward at high speed Thin wall sockets are normally chrome plated but an industrial black anodized finish is also available

Impact sockets Thicker wall Impact sockets have a thicker wall which makes them much stronger than thin wall sockets Impact sockets normally have a dull black oxide finish Black oxide is a type of rust. It forms a bond with the steel that prevents further rusting Impact sockets have a hole drilled through the base of the socket so that the socket can be secured to an impact extension A spring loaded pin holds the socket secured to the extension so that it cannot fly off at high speeds Hole for retaining pin

Shallow and deep sockets Shallow sockets are designed to fit bolt heads and nuts where there are only a few exposed bolt threads Deep sockets are intended for nuts where there are too many exposed bolt threads to allow the use of a shallow socket Always use a shallow socket when ever possible A shallow socket transmits more torque and helps keep the torqueing force centered on the bolt

Broach depth Deep sockets can have a variety of broach depths In a deep broach socket the hex is machined very far down the center of the socket When installing fasteners the deep broach allows the bolt or nut to drop to the bottom of the socket Most deep sockets are shallow broached – the nuts are held close to the open end of the socket where they can easily engage stud threads

Spark plug socket 13/16” hex Special deep broached sockets are made for the removal and installation of spark plugs Spark plug sockets have a recessed area where a soft rubber sleeve is installed The rubber sleeve grabs a hold of the spark plug to prevent it falling out of the socket The rubber sleeve also protects the glass insulator from being damaged during installation Rubber sleeve

Removing and installing studs Studs that a relatively new and show very little evidence of corrosion can sometimes be removed using an ‘E’ type [external] Torx socket If the socket does not fit tightly onto the Torx end of the stud use the double nut method or apply heat to the casting around the stud

Double nutting The double nutting technique for stud removal should be used when there is no Torx end on the stud or the Torx end is too corroded to use an ‘E’ type Torx socket Corroded Torx end

Double nutting Thread two nuts onto the stud

Double nutting Hold the lower nut with a wrench while tightening the upper nut

Double nutting To gain extra grip arrange the wrenches so they can be squeezed like a pair of pliers

Double nutting Loosen the stud by turning the lower nut with a box end wrench

Double nutting Continue turning until stud is free from casting

Double nutting To remove the nuts from the stud hold one nut with a wrench while turning the other nut counter-clockwise

Stud remover A stud remover tool damages the threads It should only be used if the stud is going to be replaced The stud remover can also be used to remove broken bolts if there is at least ¾” of bolt exposed The tool connects to a ½” ratchet and extension

Using heat to remove rusted studs Heat is essential to the process of removing corroded studs and bolts An oxy-acetylene torch is needed to provide the level of heat needed to loosen rusted bolts Heat causes the casting to expand – opening up the threaded hole so that the stud or bolt can be turned easily

Using heat to remove rusted studs A rosette or cutting tip is needed to produce the kind of heat necessary to loosen the stud The tip of the torch is moved back and forth over the casting immediately surrounding the stud The flame is not directed onto the stud – we want the stud to remain cooler than the casting

Install the stud remover Working as fast as possible install the stud remover using a long ½” drive extension

Install the stud remover More heat may be needed if the threads begin to bind as the stud is worked out of the threads

Clean up the threads The threads in the casting should be cleaned of any debris using a tap This homemade 8 x 1.25 mm thread chaser was made from an old ¼” socket and a tap

Tap and Die The size and thread pitch of the tap is printed or stamped on the side of the tap

Removing broken bolts When studs or bolts are broken with little or no exposed threads the only way to remove them requires drilling

Center punch A center punch is used to make a dimple in the center of the broken bolt The dimple prevents the drill bit from ‘walking’ when the drill is first started The dimple must be a close to the center of the broken bolt as possible

Make a dimple Locate the point as close to the center of the broken bolt as possible then strike the center punch with a hammer

Drill a pilot hole Start with a small drill bit Make sure that you hold the drill as close as possible to the bolt hole center axis

Drill a pilot hole If the hole is not blind drill the pilot hole all the way through If the hole is blind, check the depth often to make sure that you don’t drill all the way through the manifold or cylinder head

Enlarge the hole Install a slightly larger drill bit and drill through the pilot hole Keep enlarging the hole until the hole is big enough to accept an EZ-out

EZ-Outs EZ-Outs are a brand of screw extractors They utilize a tapered, spiral hardened steel screw that wedges itself tighter as torque is applied in a counter-clockwise direction [loosening]

Recommended drill size 5/32” Printed on the side of the EZ-Out is the recommended drill bit size to enlarge the hole to

Drill Index The case that holds drill bits is called a ‘drill index’ The drill size can be determined by the smallest hole in the drill index that the drill bit will fit

Hammer in the EZ-Out Tap the EZ-Out with a hammer until the flutes are firmly engaged in the drill hole

Install a tap handle to turn the EZ-Out The EZ-Out can be turned using a tap handle

Using a 4 point socket to turn the EZ-Out In tight spaces a 4 or 8 point socket can be used in conjunction with a ratchet and extension to rotate the EZ-Out

Drilling and re-taping Half of the time an EZ-Out will not work The next option is to keep enlarging the hole until the original threads just begin to be seen Then a tap of the original diameter and pitch can be run through the hole to remove the remaining material

Visible thread lines When you can just begin to see thread lines stop drilling

Apply pressure to the tap Slowly rotate the tap while applying firm pressure to the tap handle or ratchet

Lubricate the tap Spray a few drops of lubricant on the tap

1 full turn in – ¼ turn out Rotate 1 full turn then turn the tap backwards ¼ turn This allows the metal filings to drop into the tap flutes Repeat this process – 1 full turn in – ¼ turn out until the tap reaches the end of the threads

Restored threads When the tap is removed blow out any metal chips and the threads should be as good as new