The best option is not to have a stuck fastener in the first place. Anti-seize lubricants are useful, and thread-locking compounds can provide a liquid-proof seal which prevents corrosion occurring in the first place. But if the fastener is stuck, the first step is to use the correct tool: the right-sized ring spanner or socket is more likely to grip a rounded-off nut securely, and a six-sided socket (or a surface-drive socket, which concentrates torque on the flats of the hexagon) is better still.
There are even special sockets designed specifically for damaged fasteners: SATA’s BoltBiter sockets have tapered fluted openings, and are claimed to “eliminate the need for a striking tool and reduce jamming in the socket compared to traditional extraction”.
An impact wrench is useful, and for tight areas the old-fashioned impact driver – with a cam-driven screwdriver bit, struck with a hammer – can give a very good grip on a screw head.
The next step is penetrating oil, but whether your favourite is WD-40, 3-in-1 or an exotic import such as Kroil (“the oil that creeps”), the key to making it work is time. The longer it soaks, the more the low-viscosity oil can wick through gaps in the threads by capillary action, even uphill.
Without the luxury of time, heat is next. More precisely, differential heating: using the fact that different materials expand at different rates, to break the bond between them – particularly if galvanic corrosion has formed between an aluminium component and a steel fastener.
Heating with a blowtorch is hazardous, and can distort or crack castings. For localised heating, an induction heater comes into its own: a relatively small handpiece can heat up a single metal fastener rapidly without any risk from a naked flame or hot air – and even be used on hydraulic fittings without damaging attached rubber hoses.
THE NEXT LEVEL
The next stage is a welder: tack-welding a nut on to a seized stud introduces a thermal shock and improves purchase. But you need to be careful not to distort or further stress components.
If none of these methods has worked, it’s time to get destructive: nut splitters are useful in some applications, although they can be unwieldy and difficult to use on site. Sometimes a cutting wheel on an angle grinder works better.
You can also drill a fastener out, and if it has broken below the surface this may be the only option. A left-handed drill, or a tapered stud extractor, can help to move the fastener, but try to avoid generating swarf and debris – or at least block any holes where it can enter. If swarf does get in, a magnet or a grease-covered stick can be a lifesaver.
If you have drilled out the fastener altogether, the thread will be damaged and should be cleaned out with the appropriate tap, using a lubricant or cutting fluid.
The key to drilling and tapping is to keep the tool perfectly concentric to the existing hole, and square to the face. Ideally, mount the component in a milling machine and dial-in the operations; otherwise, a magnetic drill press is useful. In any case, freehand drilling is not recommended.
INSERTS
If the thread is too damaged for a tap, or has been drilled out completely, you can repair it with a threaded insert. Screwed into a larger tapped hole, this provides a clean threaded hole of the correct size. These are not just used for repairs, but also to give fasteners more strength in softer materials such as aluminium.
The best-known type of insert is the Helicoil – a precision-made diamond-section steel wire coil: the outer profile fits snugly into a threaded hole, while the inner part forms a smaller-diameter threaded hole. It uses minimal space and is relatively inexpensive, but does need a special tool to drive it into place, and a punch to knock off the driving tang from the bottom of the coil. This tang must be removed or recovered.
Other inserts include the Time-Sert: a thin-walled cylinder, threaded inside and out, with a collar on top, requiring the hole to be counter-bored with a special bit. On installation, the bottom of the insert is deformed into the outer thread, locking it in place; in fact, it is claimed to dampen vibrations and to be gas- and liquid-tight, and can be used near an edge.
Keyed inserts are solid-bodied, with pin-like keys which initially protrude from the top. They require a countersink on the threaded hole; once they have been screwed in to just below the surface, a special guide tool is used with a hammer to drive the ‘keys’ down and lock the outer thread in place. These are also available as heavy-duty versions with a larger outer diameter for badly-damaged holes, or as a solid ‘plug’ to be drilled and tapped by the user – ideal for non-standard threads or off-centre fixing. However, some manufacturers do not recommend them for brittle materials such as cast iron.
Some threaded inserts are self-tapping, even in metal components, although these are generally intended for manufacturing rather than repair. Tappex claims that, in light alloy, its self-tapping steel insert resists a pullout force greater than the yield point of an 8.8 grade bolt.
BOX: BREW YOUR OWN PENETRATING OIL
Some technicians swear by home-brewed penetrating oil recipes; the most common is a mixture of automatic transmission fluid (ATF) and acetone. ATF is a low-viscosity mineral oil, usually containing detergents and other additives, while acetone is an organic solvent best known as a nail polish remover and paint thinner. Beware: it is highly flammable, with a flash point of -20°C.
The ATF’s low viscosity allows it to act as a penetrating lubricant on its own, while the acetone thins it further and helps to dissolve contaminants. Some sources claim that the acetone converts rust into ferric carbide (said to be brittle and easily broken), but this seems fanciful.
A 2012 study by students at Drexel University aimed to create a cost-effective alternative to conventional penetrating oils for third world use. It found that a 7:1 mixture of vegetable oil and acetone was cheap and reasonably effective.
BOX: Helicoil, HELICOIL or Heli-Coil?
The original Helicoil was devised in the US in 1938 and is still made there by Stanley Engineered Fastenings. HELICOIL (in capitals) is a trademark rather than a generic name, although the principle has become used by many manufacturers. The official design has been produced in Germany under licence since 1954 by Böllhoff, and it now has worldwide rights to the name (although, confusingly, it is also called Heli-Coil in the US).
Böllhoff now makes numerous variants of the basic Helicoil, including free-running and Screwlock versions, which have a shaped profile to hold the fastener with a specified torque. A tang-free version uses a special tool to engage with a barbed end to the thread, so that no tang debris can fall into the hole. The Smart variant uses the bottom end of the thread coil itself as the tang; this is bent into place during installation, leaving no vulnerable notches in the assembly.
Other Helicoil variants include coated types to minimise the risk of galvanic corrosion in aluminium or magnesium – particularly relevant for aerospace applications.