Threaded Fasteners, Locking Methods and Serviceability in Tack

Why threaded fasteners matter in tack

Threaded fasteners are used wherever equestrian equipment needs a joint that is strong, compact and capable of being dismantled. Examples include stirrup-bar and stirrup-iron assemblies, saddle fittings, breastplate and martingale hardware, harness components, carriage fittings, mounting brackets, buckles with replaceable screws, stable doors, tack-room fixtures and some safety stirrup mechanisms.

In many items of tack the fastener is not the main visible component, yet its condition can determine whether a load is transferred safely. A loose screw may allow a fitting to rotate; corrosion may reduce the effective cross-section; damaged threads may prevent adequate tightening; and an incorrect replacement may fit dimensionally while having unsuitable strength, corrosion resistance or locking behaviour.

GB road-safety guidance states that riders should ensure tack fits well and is in good condition before taking a horse on the road. That broad requirement does not prescribe a particular bolt, nut or locking method, so the practical responsibility remains with the owner, saddler, manufacturer or business operator to understand the joint and inspect it appropriately. The Highway Code, Rules 52–58, is therefore a useful safety context, but it is not a fastener specification.

Basic terminology

A thread is a helical ridge formed on the outside of a bolt, screw or stud, or inside a nut or tapped hole. The most common general-purpose form in modern equipment is the ISO metric V-thread. Thread size is normally expressed as nominal diameter followed by pitch, for example M6 × 1.0. “M6” identifies the approximate nominal diameter; the 1.0 mm figure is the distance from one thread crest to the next. When the pitch is omitted, the usual coarse pitch is generally implied, but it should not be assumed where interchangeability matters.

A bolt normally passes through clearance holes and is secured by a nut. A screw may engage directly with a tapped hole or form its own thread in a softer material. A stud has external thread at both ends or along part of its length and can remain installed in one component while a nut is removed. A tapped hole is an internal thread cut or formed in the parent material.

The engagement length is the length over which male and female threads contact. Too little engagement can strip the internal thread; excessive engagement does not automatically increase strength and may create assembly or clearance problems. In thin sheet, leather-backed fittings or soft alloys, the parent material may be weaker than the steel fastener, making thread design and bearing area more important than nominal bolt strength.

How a bolted joint carries load

Tightening a threaded fastener stretches the bolt slightly and compresses the parts being joined. This creates preload, also called clamping force. In a well-designed joint, service loads are largely carried by friction between the clamped parts while the bolt remains in tension. If preload is inadequate, the joint may separate, fret or slip. If tightening is excessive, the bolt can yield, the thread can strip, a thin fitting can distort or the parent material can crack.

Torque is only an indirect way of producing preload. Much of the applied torque is consumed by friction in the threads and under the nut or head; only a smaller portion becomes useful bolt tension. Consequently, the same torque can produce substantially different preload when surfaces are dry, lubricated, plated, contaminated or fitted with a threadlocker. Bossard’s technical guidance describes torque and preload values as reference information and warns that they do not replace an engineering calculation for a particular assembly. Bossard, Preload and Tightening Torques.

For tack, this has several consequences:

• “Tighten it as hard as possible” is not a reliable assembly method.
• A torque value borrowed from a similar-looking fastener may be wrong if the material, coating, lubrication or joint geometry differs.
• Torque wrenches are useful only when the value, tool range and assembly condition are known.
• Low-load decorative screws should not be treated as structural fasteners merely because they have a machine thread.

Fastener materials and property classes

Carbon-steel and alloy-steel fasteners are commonly identified by property class, such as 8.8 or 10.9. The marking is not a simple quality grade or a guarantee that the fastener is suitable for every application. It relates to specified mechanical properties under the relevant fastener standard. BS EN ISO 898-1 covers mechanical properties of carbon- and alloy-steel bolts, screws and studs with specified property classes; the corresponding nut requirements are addressed by ISO 898-2:2022. The latter specifies classes including 04, 05, 5, 6, 8, 10 and 12, and states that its requirements do not themselves specify corrosion resistance or prevailing-torque behaviour.

Related industry reference: For businesses moving from technical research into trade sourcing, explore equestrian suppliers and manufacturers on EquiGuild.

Stainless-steel fasteners are not automatically stronger than ordinary plated steel. Stainless grades differ in strength, galling tendency, magnetic response and corrosion performance. The ISO 3506 series covers corrosion-resistant stainless-steel fasteners and assigns property classes to austenitic, martensitic, ferritic and duplex families. For equestrian use, stainless steel may be preferable where sweat, washing, damp storage or repeated exposure to manure and cleaning chemicals makes corrosion a major concern, but the grade and mechanical class still matter.

Plated or coated carbon-steel fasteners can offer useful corrosion protection at lower cost. However, coating damage, cut threads, crevices and trapped moisture can initiate corrosion. Zinc-coated fasteners should not be casually mixed with stainless components in wet, salty or chemically contaminated environments because galvanic corrosion may occur. The visible finish is not enough evidence of service life.

Common threaded fastener arrangements in tack

Through-bolt and nut

A bolt passing through two or more components and secured by a nut is usually the most inspectable arrangement. It permits replacement of the nut and allows the joint to be dismantled without damaging a tapped hole. It also requires enough clearance, correct washer arrangement and protection against protruding thread that could catch clothing, reins or the horse.

Machine screw into a tapped fitting

This is compact and visually neat, but the internal thread is part of the component. Repeated removal, overtightening, cross-threading or corrosion can damage it. When the tapped part is aluminium, brass or a thin steel plate, thread strength may be limited. A longer screw is not necessarily a safe repair: it may bottom out, foul a moving part or damage a hidden surface.

Countersunk and low-profile screws

Countersunk screws sit flush in a conical recess. They are useful where a projecting head could interfere with movement or contact the horse, but the countersink removes material from the component and introduces a wedge effect. The joint must be designed for the screw head and included angle. Substituting a flat-head screw of a different angle can produce poor seating and unreliable clamping.

Shoulder screws and pivot fasteners

A shoulder screw has a smooth cylindrical section that acts as a controlled bearing or pivot surface. It should not be replaced by an ordinary fully threaded screw simply because the diameter appears similar. Threads in a pivot can increase wear, create a changing fit and damage the mating hole. Pivot joints also need inspection for ovalisation, looseness and cracking around the hole.

Locking methods

Thread locking prevents unwanted rotation or loss of clamping force. It does not repair a poorly designed joint, compensate for insufficient thread engagement or make a damaged bolt safe.

Prevailing-torque nuts

A prevailing-torque nut resists rotation because of a designed interference feature. Types include all-metal distorted-thread nuts and nuts with a non-metallic insert. ISO 2320:2015 specifies functional properties for prevailing-torque steel nuts and covers both all-metal and non-metallic-insert types. The standard notes that prevailing torque measured in laboratory conditions can vary in practical application, and that temperature affects performance.

In tack, a nylon-insert nut can be convenient where temperatures are moderate and the nut will not be repeatedly removed. It should not be assumed that a used nyloc nut retains its original locking performance. The insert can wear, become contaminated or be damaged by heat and solvents. An all-metal prevailing-torque nut may be preferable where repeated heating, vibration or chemical exposure makes a polymer insert unsuitable, although its thread friction can be higher and more variable.

Threadlocking adhesives

Liquid anaerobic threadlockers cure in the close-fitting space between metal threads when air is excluded. They are available in grades intended for low, medium or high resistance to removal. Selection should consider thread size, material, service temperature, chemical exposure, desired dismantling force and whether the fastener must be adjusted frequently.

Threadlocker should be applied sparingly to clean, dry threads unless the product data sheet specifically permits an alternative preparation. Excess compound can migrate into moving parts, contaminate leather or make later servicing unnecessarily difficult. Some formulations are designed to tolerate light oil contamination, but this should not be generalised to every product. The manufacturer’s technical data sheet is the controlling source for cure time, temperature range, chemical resistance and removal method.

Threadlocker is not a substitute for a nut where a joint requires a positive mechanical stop, nor is it suitable for every plastic, painted or porous substrate. On small tack screws, the amount of product can be difficult to control; a drop on the male thread is usually more appropriate than flooding the hole.

Slotted nuts, split pins and wire

A castellated or slotted nut secured with a split pin provides a positive retention method. The pin prevents the nut from backing off but does not establish correct preload by itself. The nut must be tightened to the correct position or within the permitted adjustment range before the pin is fitted. Reusing a bent or previously installed split pin is poor practice because its deformation and fatigue history are unknown.

Lockwire is common in specialist mechanical applications, but it requires suitable drilled heads, correct routing and a disciplined installation method. Improvised wire wrapped around a tack fitting is not equivalent. Wire ends must not create sharp projections or contact the horse, rider or reins.

Washers and mechanical anti-rotation devices

Plain washers spread bearing load, protect softer surfaces and reduce damage during tightening. They are not, by themselves, reliable vibration locks. Spring washers, tooth washers and serrated washers may have a role in particular designs, but their performance depends on material, hardness, surface condition and joint geometry. A washer added during repair can change stack height, thread engagement, bolt grip length or the position of a moving component.

Where a joint relies on a tab washer, lock plate or formed retaining feature, the part should be replaced if it has been bent, cracked or flattened beyond its intended service condition. Re-bending repeatedly can cause fatigue and loss of retention.

Corrosion, wear and contamination

Equestrian equipment operates in an unusually challenging environment. Sweat contains salts; leather conditioners and oils can migrate into joints; washing introduces water; manure and bedding retain moisture; and tack may be stored in unheated rooms. Corrosion is therefore not merely cosmetic.

Inspect for red rust, white corrosion products on zinc, pitting, dark staining around crevices, lifting plating, seized threads and rust lines radiating from a joint. Pitting is especially important because it removes material and creates stress concentrations. A fastener that is heavily pitted, bent, necked, cracked or damaged at the head should be replaced rather than cleaned and returned to service.

Thread contamination can produce false tightness. Dirt or corrosion may prevent the nut from reaching the intended seating position, while the torque required to overcome thread friction can be mistaken for useful preload. Threads should be cleaned using a method compatible with the material and finish. Abrasive cleaning that removes protective plating may shorten service life.

Inspection and serviceability

A serviceable joint is not simply one that feels tight. Inspection should consider the complete load path: the fastener, nut, washers, fitting, parent material, adjacent stitching, holes, pivots and any locking feature.

1. Before use: look for missing hardware, obvious looseness, protruding or sharp thread, cracked fittings, distortion, corrosion and displaced washers.
2. During cleaning: expose concealed joints and remove dirt that may hide movement or cracking. Do not saturate incompatible materials or force water into sealed bearings.
3. After adjustment or repair: check that the joint moves as intended, that the fastener does not bottom out, and that sufficient thread engagement remains.
4. At planned intervals: dismantle critical or repeatedly adjusted joints when competent inspection requires it. Examine threads and contact surfaces rather than relying only on an external visual check.
5. After an exceptional event: inspect after a fall, entanglement, vehicle incident, major impact or suspected overload. Hidden bending and thread damage may not be obvious.

The UK Health and Safety Executive’s inspection guidance for work equipment emphasises identifying deterioration before it creates a risk and says inspection frequency should be determined by risk assessment. Although individual items of personal tack are not automatically “work equipment”, the principle is directly relevant to riding schools, livery yards, manufacturers, dealers and carriage businesses. Where safety depends on installation or condition, inspection may require dismantling rather than visual examination alone. See HSE, Inspection of work equipment.

Replacement and repair decisions

The safest replacement is one that matches the original specification, not merely the visible diameter. Record or identify:

• thread system, diameter and pitch;
• length measured under the relevant head convention;
• head style and drive type;
• material, coating and property class where marked;
• nut type and locking method;
• washer dimensions and order;
• required clearance, pivot fit or shoulder length;
• exposure to water, chemicals, heat and repeated adjustment.

Do not replace a metric fastener with an imperial part because it “almost fits”. Similar pitches can engage for a few turns while damaging both threads. Do not substitute a high-strength bolt without considering the rest of the joint: a stronger bolt can overload a softer fitting or encourage excessive tightening. Conversely, replacing a marked structural fastener with an unmarked decorative screw is unacceptable in a load-bearing joint.

For commercially manufactured tack, a change to fastener type, coating, locking method or tightening procedure should be controlled through the manufacturer’s design and quality system. Traceability of supplied fasteners, incoming inspection and documented assembly instructions are quality indicators. A responsible repairer should be willing to explain what was replaced and why, especially where the joint is safety-critical.

Common misconceptions

• “Stainless means indestructible.” Stainless steel can gall, pit, seize or fail through fatigue, and its strength depends on grade and property class.
• “A nyloc nut can be reused indefinitely.” Repeated removal can reduce the insert’s locking effect. Replace it when the locking function is uncertain.
• “Threadlocker makes any screw safe.” Adhesive cannot compensate for stripped threads, inadequate engagement, a wrong fastener or a cracked fitting.
• “A lock washer always prevents loosening.” Washer performance depends on the joint. A plain washer primarily distributes load; it is not a locking device.
• “Tighter is safer.” Over-tightening can strip threads, crush leather or thin metal, distort fittings and reduce fatigue life.
• “If the bolt fits the hole, it is the correct replacement.” Diameter alone says nothing about pitch, grip length, grade, corrosion resistance, head geometry or locking arrangement.

Practical selection guide

Conclusion

Threaded fasteners in tack should be treated as engineered components rather than generic bits of hardware. Sound practice combines correct thread identification, appropriate material and strength, controlled tightening, a locking method suited to the service environment, corrosion management and inspection of the whole joint. Standards such as ISO 898-1, ISO 898-2 and ISO 2320 help define fastener properties, but they do not replace application-specific design or competent inspection.

For owners, the most useful habit is to stop using equipment when a fastener is missing, loose, corroded, damaged or of uncertain specification, and to seek a competent repair or manufacturer-approved part. For businesses and manufacturers, serviceability should be designed in: joints should be inspectable, replacement parts identifiable, locking methods declared and maintenance instructions realistic for wet, dirty and frequently handled equestrian environments.

Sources and further reading

• UK Government, The Highway Code: Rules about animals.
• Health and Safety Executive, Inspection of work equipment.
• ISO 2320:2015, Fasteners — Prevailing torque steel nuts — Functional properties.
• ISO 898-2:2022, Mechanical properties of fasteners — Nuts with specified property classes.
• BS EN ISO 898-1:2013, Mechanical properties of bolts, screws and studs.
• BSI, ISO 3506 series — Stainless-steel fasteners.
• Bossard, Preload and Tightening Torques.

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