The aim of this article is to give some guidance to various tactics for dealing with seized and/or damaged threaded fasteners.
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 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 box wrench followed by a good quality open ended wrench.
When buying wrenches 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 crescent wrench make sure that the movable jaw is in the side of the wrench 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 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 four 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.
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.
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 blowtorch 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. Wrenches and socket drivers can be extended by the use of tubes and pipes. A box wrench or socket extension can be pressed into service to extend an Allen key. Application of a hammer (preferably rawhide or lead) or the heel of the hand to a wrench 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 wrench 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 paraphenalia. 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 C-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 belabor 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 faste