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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

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the metric system
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
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
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’
  • A meter is a little bit longer than a yard – 39.37 inches to be exact

Meter Stick

Yard Stick

  • 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

1 Meter

  • 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 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
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
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



8 x 1.25mm



thread pitch
Thread pitch
  • 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’





metric standard and fine thread pitch
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
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
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
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


determining bolt length
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


determining thread pitch
Determining thread pitch
  • Thread pitch is measured with a thread pitch gauge
  • If all of the teeth fall into the thread valleys the thread pitch is equal to the number stamped on the blade
  • Place the teeth of one of the gage blades lengthwise along the threads of the bolt
internal thread pitch
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
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
  • 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 gauge
  • Bolt distributers often give away plastic bolt gauges that make it easier to identify bolt diameter and length
grade number
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
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 bolt grade numbers
  • Metric bolts have a number stamped on the head of the bolt that indicates both tensile and yield strength


  • 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
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
Flat washers
  • A flat washer is placed between the head of the bolt and the component for a number of reasons:
  • 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
  • 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
lock washers
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




flange bolts
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
  • 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
all metal lock nuts
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 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 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 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
  • 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
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
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
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
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
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
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
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
  • 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
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
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

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
  • 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
  • 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


hand wrenches
Hand Wrenches
  • Adjustable
  • Open end
  • Box end
  • Combination
  • Flare nut
  • Ratcheting
adjustable wrench
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
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
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
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
box end wrenches1
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 wrenches2
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 wrenches3
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
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 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
  • 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
  • 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
Thin wall and Impact sockets
  • 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

Weak spot

thin wall and impact sockets1
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
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 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
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
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
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
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 nutting1
Double nutting
  • Thread two nuts onto the stud
double nutting2
Double nutting
  • Hold the lower nut with a wrench while tightening the upper nut
double nutting3
Double nutting
  • To gain extra grip arrange the wrenches so they can be squeezed like a pair of pliers
double nutting4
Double nutting
  • Loosen the stud by turning the lower nut with a box end wrench
double nutting5
Double nutting
  • Continue turning until stud is free from casting
double nutting6
Double nutting
  • To remove the nuts from the stud hold one nut with a wrench while turning the other nut counter-clockwise
stud remover
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
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 studs1
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
Install the stud remover
  • Working as fast as possible install the stud remover using a long ½” drive extension
install the stud remover1
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
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
Tap and Die
  • The size and thread pitch of the tap is printed or stamped on the side of the tap
removing broken bolts
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
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
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
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 hole1
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
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
Recommended drill size
  • Printed on the side of the EZ-Out is the recommended drill bit size to enlarge the hole to


drill index
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
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
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
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
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
Visible thread lines
  • When you can just begin to see thread lines stop drilling
apply pressure to the tap
Apply pressure to the tap
  • Slowly rotate the tap while applying firm pressure to the tap handle or ratchet
lubricate the tap
Lubricate the tap
  • Spray a few drops of lubricant on the tap
1 full turn in turn out
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
Restored threads
  • When the tap is removed blow out any metal chips and the threads should be as good as new