jack nut
A type of nut with a sheet metal body that mushrooms outward, on the far side of the hole through the substrate, when the screw is screwed in, thus providing anchoring with only one-sided access to the hole. It is nearly equivalent to the female portion of a molly bolt.
jack nut: one-sided anchor fastener for thin material
A jack nut is a self-anchoring fastener designed to grip sheet material or thin panels from a single accessible side. When a screw is driven through a pre-drilled hole, the nut's sheet metal body expands or mushrooms outward on the opposite face, creating a mechanical grip without needing access to the far side. This makes it invaluable in confined spaces, sealed cavities, and assembly work where back-side access is impossible or impractical.
The typical jack nut consists of a metal cup or sleeve with a threaded or split design. As the screw tightens, the expanding mandrel or screw threads force the metal walls outward, spreading the nut's body against the substrate. Common materials include mild steel (often zinc-plated), stainless steel, and aluminum. Hole diameters typically range from 1/8 inch to 1/2 inch, though larger variants exist for heavier applications. The grip strength depends on sheet thickness; thicker material distributes load better, but jack nuts perform adequately in aluminum, sheet steel, and even plastic up to roughly 1/8 inch thickness.
Variants and applications
Several designs exist. The most common uses a split collet that spreads when the screw advances, similar to a molly bolt's female end. Others employ a conical mandrel that pushes metal fins or flanges outward. Ribbed or knurled outer surfaces improve grip on smooth materials. In aerospace and automotive assembly, jack nuts reduce weight compared to through-bolting and eliminate the need for backing plates or nutplates on inaccessible panels. They are standard in avionics enclosures, ductwork, and panel fastening where single-sided installation is mandatory.
Failures occur when screw tension is excessive, tearing the sheet metal body, or when material is too thin to resist the outward expansion force. Over-torquing strips the internal threads. Corrosion between the nut body and substrate can cause loss of clamp load over time. Vibration and cyclic loading may cause gradual loosening if the screw lacks a locking feature, though many jack nuts are paired with nylon-insert or deformed-thread locking screws to mitigate this.
The jack nut occupies a middle ground between simple speed nuts (which offer minimal holding power) and threaded inserts (which require access or pre-installation). It is faster to install than a rivet and cheaper than tapping threads into every hole. The name reflects the sudden expansion action, though the mechanism is mechanical spreading rather than hydraulic jacking. For production work involving large panels or assemblies with density constraints, jack nuts are often the economical choice over heavier alternatives.