Fastener Set

What a fastener set is

Almost every assembly on BOMwiki ends, somewhere near its leaves, in a fastener set, the bolts, screws, nuts, washers and rivets that hold the real parts together. They are the most numerous components in manufactured goods (a car has thousands; an airliner, millions) yet the cheapest, often a fraction of a cent each. A fastener set is not one part number but a small kit chosen together so that thread, grade, length and coating all match the joint. This page explains how those choices are made; the linked catalogue pages list real, in-stock parts in every size.

The families below each have a faceted catalogue: Hex Head Bolts, Socket Head Cap Screws, Hex Nuts, Washers, Threaded Rod, Threaded Inserts and Blind Rivets.

Threads: the language of fasteners

A thread is defined by its diameter and pitch. Most of the world uses the ISO metric system, written M followed by the nominal diameter and, for fine threads, the pitch: M8 is 8 mm with the standard 1.25 mm coarse pitch, while M8x1 is the fine version. North America still uses the Unified system (UNC coarse, UNF fine), sized by a number or fractional inch and a threads-per-inch count, such as 1/4 inch-20. Coarse threads are faster to run down, more tolerant of damage and plating, and the default for general work; fine threads carry slightly more load, resist loosening a little better and suit thin-walled parts. The two are not interchangeable, and mixing them is a classic way to strip a thread.

Strength: property classes and grades

A fastener is only as good as the joint it can clamp, so steel fasteners are sold by strength, not just size. The metric system stamps a property class on the head: 8.8, 10.9 or 12.9 for high-strength work. The first number is roughly the tensile strength in units of 100 MPa (an 8.8 bolt is about 800 MPa), and the second encodes the yield-to-tensile ratio, so a 10.9 bolt yields at about 90% of its 1000 MPa tensile strength. The old US SAE grades 2, 5 and 8 are the rough equivalents of low-carbon, medium-carbon quenched-and-tempered, and alloy steel. Stainless fasteners are marked A2 (304-type) or A4 (316, with added molybdenum for marine and chemical use) followed by a strength such as -70 or -80; stainless trades some strength and galling resistance for corrosion resistance, so it is rarely run above A4-80.

Heads and drives

The head sets how the fastener bears on the surface and how much clamp area it has. A hex head is driven by a wrench from the side and dominates structural work; a socket (cap) head takes an Allen key from above and suits tight spaces and tapped holes; button and pan heads are low and rounded for light cosmetic work; a countersunk (flat) head sits flush in a chamfered hole. The drive is how torque gets in: internal hex/Allen and Torx (six-lobe) resist cam-out and allow high torque, while Phillips and slotted are cheap and self-centering but cam out under load.

Bolts vs. screws, and the rest of the kit

Strictly, a bolt passes through clearance holes and is tightened against a nut, while a screw threads into a tapped hole or the work itself, but usage is loose and the line blurs. The other members of the set each do one job. Nuts provide the mating thread: a plain hex nut for general use, a nylon-insert (Nyloc) or all-metal lock nut where vibration would otherwise back it off, a flange nut to spread load without a washer. Washers sit under the head or nut: a plain (flat) washer spreads load and protects the surface, a split-lock or tooth washer adds spring tension against loosening, a fender washer spans an oversized hole. Where a joint is permanent and accessed from one side, a blind (pop) rivet replaces the bolt-and-nut entirely, setting its own back-side head as the mandrel is pulled.

Materials, coatings and corrosion

Most fasteners are carbon or alloy steel, which rusts, so it is almost always coated. Zinc plating (clear or yellow) is the cheap default for indoor use; hot-dip galvanising lays on a thicker, rougher zinc layer for outdoor and structural steel; black oxide is a thin, mildly protective conversion coating favoured on socket screws and tooling; zinc-flake systems (Geomet, Delta-Protekt) give high salt-spray life without the hydrogen-embrittlement risk that electroplating poses to high-strength parts. Where coating is not enough, the fastener is made from stainless (A2/A4), brass for electrical and decorative work, or titanium and exotic alloys in aerospace.

Preload, torque and keeping it tight

A bolted joint works by preload, the tension stretched into the bolt as it is tightened, which clamps the parts and is what actually carries the load. Assemblers usually set preload indirectly with a torque wrench, relying on a nut factor (the "K" value, about 0.2 for plain zinc) to relate torque to tension; the relationship is approximate because most of the torque is lost to friction under the head and in the threads. Critical joints instead measure angle (turn-of-nut) or bolt stretch directly. Once tight, vibration is the enemy: it is resisted by thread-locking adhesive (anaerobic compounds such as Loctite), by prevailing-torque lock nuts, by tooth or wedge-lock washers, or simply by enough preload that the joint never loosens. Under-tightening, not over-tightening, is the more common cause of failure.