Difference between revisions of "Threaded fasteners FAQ"

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(This FAQ compiled by Andy Pugh. Contributions from 'Ric Davis, Alan W Frame, Tim Naylor and Trevor Dennis)
(This FAQ compiled by Andy Pugh. Contributions from 'Ric Davis, Alan W Frame, Tim Naylor and Trevor Dennis)

Latest revision as of 02:19, 6 September 2017

Threaded Fasteners FAQ version 7.2

Written by Andy Pugh

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 parentheses.

Bolts and Screws - an Introduction.

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 including:-

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 (q.v.) 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 universally Phillips.

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 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.

Over-tight or 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 pre-emptively 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 unfamiliar 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 achieve 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. Specialized 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.

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). Alternatively, one way to align a drill with the centre of a bolt head is to use a tightly fitted socket and a nut of the right size. Place the nut in the socket, place the socket on the bolt head and drill through the square drive hole and nut into the offending fastener. 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. 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 suitable piece of tubing) are available the hole sleeved down guide 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.

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 (Heli-coil, Time-Sert etc).

The stripped hole is bored out oversize and tapped with a bastard 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 in my humble opinion for very little else)

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 counter-bored.

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 whole 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 uses the manufacturers recommended torques one should never strip 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 specified in Newton meters onto a foot-pounds scale will cause over-tightening by a factor of almost 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 compounds.

Bear in mind though that torques are specified for clean dry threads, by lubricating the threads you increase axial force a given torque, paradoxically making it more likely thread will strip.

(This FAQ compiled by Andy Pugh. Contributions from 'Ric Davis, Alan W Frame, Tim Naylor and Trevor Dennis)