If you liked that old one you'll like the up to date one even better.
Threaded Fasteners FAQ version 7.4
The aim of this FAQ is to give some guidance to various tactics for
dealing with seized and/or damaged threaded fasteners. In places where
American and British usage differ the American usage follows in
1. An introduction to threaded fasteners
2. Over-tight or Seized fasteners
3. Damaged fasteners
4. Stripped threads
5. Broken off drills taps or screw-extractors
1. Bolts and screws.
Bolts and screws are in essence a wedge wrapped around a cylinder. A
screw is a fastener where the thread covers the entire shank of the
fastener, a bolt has a threaded portion only at the end (generally the
threaded length is 2.5 diameters). Bolts are used where some level of
location perpendicular to the bolt axis is required. Fasteners are
specified by diameter and pitch, where the pitch is the distance from
the peak of one thread to the next measured in either mm or threads per
inch. Note that many threads on Japanese motorcycles are finer than
standard. For a given tightening torque a fine pitch will clamp parts
more tightly than a coarse one, but will also strip more readily because
of this increased force. Fine threads are also more susceptible to
damage by foreign objects or corrosion. There are numerous different
standards for fasteners, and many have different thread forms. This
means that even fasteners with the same diameter and thread pitch may
not fit in the same hole for example Whitworth and BSF have a different
thread form than UNC and UNF, attempts to use one in the hole of another
will lead to unsatisfactory contact between the thread mating surfaces,
and as the core sizes are different may not even fit in at all.
Occasionally one comes across left handed threads. Generally these are
only found on rotating parts, non-breathable gases and turnbuckles Often
the left-handedness of the thread is indicated by little nicks in the
centres of the corners of the nut or the bolt head periphery.
It is important always to use the most nearly correct tool available for
dealing with fasteners. A wide variety of head forms may be encountered
Hex head. The normal heads found on the majority of bolts and nearly all
nuts. The best tool is a 6 sided wall drive socket. There seems little
point in 12 point sockets when most people use ratchets with their
socket sets. Wall drive sockets have curved sides and bear on the flanks
of the bolt heads rather than the corners. They are especially good on
rounded, mangled or corroded heads. There may also be a tendency for
such drivers to spread the nut, rather than compress it as would appear
to be the case with a 12 point socket. The next best tool is a ring
spanner (wrench) followed by a good quality open ended spanner. When
buying spanners check that openings where the bolts fit for a smooth
machined finish, if the manufacturer has relied on the casting process
to get the size right the opening size is likely to not be as good a fit
on the bolt.. If you must use an adjustable spanner (crescent wrench)
make sure that the movable jaw is in the side of the spanner being
pushed, this places the contact point of the head nearer the base of the
sliding jaw, which is the stiffer arrangement.
Star head. These heads are found only very rarely, mainly on aerospace
fittings or big end bolts. They may be either an inverted Torx (qv) or
designed to fit a standard 12-point socket.
Cross head screws. These come in two varieties, Phillips and Posidrive.
The differences lie in the way that the drivers are machined. The
Phillips driver has 4 simple slots cut out of it, whereas in the case of
the Posidrive each slot is the result of two machining processes at
right angles. The result of this is that the arms of the cross are
parallel sided in the case of Posidrive, and tapered in the case of
Phillips. Posidrive is less likely to slip out because of this, but as
there are small protrusions left in the bottoms of the grooves a
posidrive driver will not fit a Phillips screw correctly. Both varieties
come in a range of sizes. The case screws on Japanese bikes are
Hex socket head. Also known as 'Allen Screws' these are generally
manufactured of high tensile steel and are much superior to the original
screws used on Japanese bikes. It is recommended that they are fitted in
place of any cross-head or slot-head screws which need replacing.
Torx. Torx heads exist in both internal and external variants. They look
like a hex with hollowed out flanks. In extremis it is often possible to
loosen them using a tool designed for a plain hex head, though the
correct tool is obviously preferred.
2. Over-tight/seized fasteners.
If you find that a fastener is tighter than expected it is wise to pause
and assess the situation before proceeding. Any use of excessive force
is likely to cause damage, and damaging the head is certain to make the
job harder. Penetrating lubricants are very useful, in fact it is a good
idea to preemptively lubricate any bolts you expect to have difficulty
with the night before and leave them to soak. If time is no problem it
does no harm to persist with penetrating oil for a few days, many
seemingly immovable bolts will succumb to this treatment alone. It often
helps initially to turn the fastener in the tightening direction, it
helps free them and should the tool slip any damage will be to the less
important faces of the head. Heat can often help. If there is no danger
of damage to surrounding components a blow-torch can be used on the
component through which the bolt goes, the aim being to expand the
component to enlarge the hole into which the bolt screws. Alternatively
liquid nitrogen or dry ice could be used to shrink the clamped
components and reduce the axial clamping load. In the case of large
bolts the use of brute force may be the correct next step. Spanners and
socket drivers can be extended by the use of tubes and pipes. A ring
spanner or socket extension can be pressed in to service to extend an
Allen key. Application of a hammer (preferably rawhide or lead) or the
heel of the hand to a spanner can often shock things loose. A trip to a
garage to have them apply an air impact wrench to the offending article
can also be useful. Be very cautious extending ratchets, they can be
easily damaged. For high torque applications a simple T bar is much
stronger (and cheaper). In the case of cross head screws the use of an
impact driver is strongly recommended. Make sure you have the correct
head of the correct size fitted, and give it an initial tap in the
tighten direction. If unfamilar with the driver using it on a piece of
scrap wood will indicate which direction it is set to work in. In the
absence of an impact driver it can help to give the offending screw a
sharp tap with a punch to loosen it. A T-handle screwdriver can exert a
usefully greater torque, as can a spanner on the shank of the
screwdriver if it is square or has a hexagonal portion for this purpose.
Cross head bits can also be obtained which can be driven by socket set
paraphernalia. With all these measures the risk of stripping the head is
also increased. Axial force is important. By pushing hard you make the
driver less likely to slip out and unload the threads reducing friction.
In many cases a G clamp may be used to achive usefully higher forces.
Bear in mind though that you will have to slacken off the clamp in order
for the screw to come out. In the case of seized nuts a cold chisel can
be used to split the nut off. An assistant to hold a large inertial mass
on the opposing flat of the nut can be useful. Hold the cold chisel in
the centre of one flat with the blade parallel with the nut axis and
belabour it with a large hammer. If done properly it is possible to
remove a nut this way without damaging the thread of the bolt. A similar
tactic with a blunt drift can sometimes expand the nut enough for it to
be unscrewed. Specialised nut splitters are available, they take the
form of a very sturdy C clamp with sharpened jaws. In extremis the head
or nut can be ground off with a disc grinder or occasionally cut off
with a hacksaw. In many cases once the tension in the fastener is
released it can be removed quite easily.
3. Damaged Fasteners
A common problem in motorcycle contexts is a cross-head screw with a
stripped head. If the head is not too badly stripped it is normally
possible to tap the material back into place with a ball pein hammer or
a punch. The head can then be reformed by tapping an impact screwdriver
bit into the head. An impact screwdriver may well then remove it easily.
In cases where the head is stripped beyond redemption it is often
possible to cut a slot using a hacksaw or Dremel tool and use a flat
bladed screwdriver. My favourite way to deal with badly stripped heads
is (after the usual penetrating oil and sharp tap with a hammer) to use
a dot punch (a slightly more steeply angled version of a centre punch)
to make an indentation near the periphery of the head. If you then angle
the punch so as to impart a tangential force on the screw head you can
usually get it to come free. This technique with a flat bladed cold
chisel can also be effective on Allen screws and hex-head bolts. If you
must resort to this kind of butchery though, please replace the fastener
with a new one:-)
If the above methods fail it may be necessary to drill off the head.
Start by centre punching the centre of the head (this is, of course, not
necessary with cross-head or socket head fasteners). Work through
successively larger drills until you get to one just bigger than the
shank size of the fastener, once this hole gets through the head the
remaining portion of the head will simply pop off. An alternative way to
guide the drill bit is to use a nut of the same size as the head of the
bolt and a socket to align the two (drill through the square drive hole
of the socket). Once the head has been removed it is generally possible
to remove the components,revealing a length of shank which may be gripped
with Mole(Vise) Grips or slotted with a saw to use a flat bladed
screwdriver. You may even be able to use an eccentric cam stud extractor
or Stilsons (pipe wrench).
In cases where the fastener has sheared off in the hole, or there is
insufficient protrusion to work with a number of alternative methods are
available. It may be possible to weld a length of rod to the fastener
(just allowing an arc welding rod to stick may do the trick). The
tangential dot punch mentioned above may also work if the thread is not
too badly seized in the hole.
Screw-extractors (Ez-outs) are sold specifically for this purpose,
though I know of few people for whom they have proved successful. The
idea is to drill a hole in the end of the offending shank and then screw
in a left-hand threaded hardened steel widget. These can work well if
used in the portion of the fastener protruding above the surface, but if
the break is flush with or below the surface they can actually make
things worse as their action tends to expand the shank and make it a
tighter fit. Screw extractors should be used with a tap-wrench and
caution, they snap easily and are hard enough to be practically
impossible to drill. Some people prefer to hammer a sharpened hexagon
key into a hole drilled in the end of the fastener, if ground flat at
the end they have a lesser tendency to spread the head.
If these methods fail there may be no alternative but to drill out the
screw. The method is the same as drilling off a head, centre punch the
exact middle of the fastener and then drill down the screw axis with
successively larger drills culminating with one the same size as the
core size of the thread. In an ideal world this will leave a thin spiral
of thread which may be picked out with a pointed stick. The use of
left-handed drills (available from specialist engineering tool
suppliers) and a reversible drill for this job is highly recommended.
Fasteners often come loose during the drilling process and it is much
better for them to come spiralling out than to screw themselves into the
deepest recesses of the hole. The initial hole may be centred by
refitting the component which was previously held on by the fastener and
starting the drilling process with a drill which fits the clearance hole
exactly. The conical indentation made by this drill (don't drill too
far) can be used to start a smaller drill. If machining facilities (or a
suitable piece of tubing) are available the hole can be sleeved down to
guide a core-sized or smaller drill.
Brake cylinder bleed nipples are especially amenable to drilling out as
they have an axial hole pre-drilled. This axial hole is also the reason
they need drilling out so frequently, as water gets down them into the
thread. This problem is much reduced if rubber dust caps are fitted.
The hole is also useful to get penetrating lubricant into the other side
of the threads, though it is likely that the hole will need to be
cleaned out with a drill first.
4. Stripped threads.
There are few things in bike maintenance as demoralising as feeling the
thread in a tapped hole let go. If the thread is a nut or a bolt then
the solution is trivial, replace it. If it is a tapped hole in a casting
then things are less straightforward. If the hole is longer than
required it may be possible to use a longer bolt (perhaps after tapping
to a greater depth) or to fit a nut to the back.
Where these solutions are not practicable the simplest solution is the
use of a thread repair kit (Helicoil, Time-Sert etc). The stripped hole
is bored out oversize and tapped with a ******* tap (same pitch as the
original hole, but larger diameter) and a stainless steel wire spiral is
wound in using a special tool (or needle-nosed pliers). This technique
is mechanically superior to the original tapped hole, and in some
applications is standard for all tapped holes in soft alloy castings. If
the tap and drill are greased this procedure can be carried out in-situ
on such places as sump-drain plugs and spark plug holes. A related
technique is to use a threaded insert, which is similar but thicker and
may well use a standard diameter and pitch for the outer thread. Both
these techniques allow you to reuse the original fastener which may be
an advantage, especially if it is unusual or pretty.
For those with less equipment it is possible to drill and tap the hole
oversize and either use an oversize fastener or screw in a length of
bolt pre-sawn to shear off flush with the surface and then drill and tap
this. Aluminium bolts are especially useful for this (and IMHO for very
Another alternative is to fill the hole with weld (a tame TiG welder is
useful if the component is aluminium) and then drill and tap.
Threads can also cause problems by partially stripping. If the thread is
pulled out such that it stands proud of the mating surface it can
interfere with sealing and after a period of fretting might lead to parts
coming loose. It is wise to examine nuts to see that they are not
suffering from this malaise and replace them if they are. If threaded
holes suffer this problem they should be filed flat or slightly
5. Broken off drills, taps and stud extractors.
All the above are (generally) too hard to drill, though the use of
carbide, cobalt or even diamond tooling may be worth trying. In the case
of taps a three or four fingered extractor may be made or obtained to
fit down the flutes. A two-fingered one may well work for drills. It may
be possible to shatter the offending item with a punch (this certainly
works with taps, which are generally micro- cracked by the shock of the
initial fracture). Again a Dremel tool may allow a slot to be ground in
the end, or with patience and a tiny wee abrasive burr the hole thing
could be ground away. If all else fails it is time to check the yellow
pages for spark eroders (also know as electric discharge machining or
EDM). Spark erosion can cut through any conductive material irrespective
of its hardness and such people specialise in just this kind of problem.
One of the best investments to prevent problems with fasteners is a
torque wrench. If one usues the manufacturers recomended torques one
should never stip a thread again, unless it is already damaged to the
extent of unservicability. Care should be taken, however, that the
correct torque value is used. Dialling in a torque in newton meters onto
a foot-pounds scale will cause overtightening by a factor of two.
Another potential pitfall with 'click' type torque wrenches is to use
them in the wrong direction; most are reversible, but they only click in
one direction. There is generally an arrow somewhere on the body to make
it clear which way it should be used.
When working on vehicles it is good practice, and saves a lot of future
trouble, to lubricate bolts and screws to ease later removal. Copper
grease works well, as do some molybdenum compounds. The use of
lubricants is especially valuable where steel bolts go into aluminium
components. The lubricant helps to prevent galling of the threads on
assembly, helps to prevent galvanic corrosion in service and helps with
removal too. Various thread locking compounds (Loctite make a large
range) can help to prevent corrosion and galling of threads. If you
know that a thread locking compound has been used and the fastener will
not budge heating to above 100 degrees C will denature most loctite
Bear in mind though that torques are specified for clean dry threads,
by lubricating the threads you increase the axial force for a given
torque, paradoxically making it more likely that the thread will strip.
(This FAQ compiled by Andy Pugh. Contributions from 'Ric Davis, Alan W
Frame, Tim Naylor and Trevor Dennis)