Hanger bolt

Overview

A hanger bolt is a threaded rod with no head. One end has a coarse wood-screw thread and a sharp point; the other end has a fine machine thread. You screw the wood end into a pilot hole in a leg or post, leaving the machine end sticking out. A nut, a plate or a threaded insert in the other part pulls the leg up tight. Because nothing shows, hanger bolts are the standard way to fit removable legs to tables, stools and benches.

Types and sizes

• Thread pair: M6 on small furniture, M8 on tables, M10 on heavy work (imperial equivalents 1/4-20, 5/16-18, 3/8-16 are different threads, not interchangeable with these).
• Overall length: 50 to 150 mm (2 to 6 in); about half is wood thread.
• Material: zinc-plated steel is usual, stainless for outdoors.
• Mounting plate: a leg plate or an angled block, drilled for the bolt, lets the leg splay by 10 to 20 degrees.
• Drive: with a hex key in the machine end, or two nuts locked together.

Where it is used

Mid-century American legs: splayed, turned and tapered legs fixed through an angled block with a hanger bolt, then finished with a brass ferrule. The type also holds table legs to a top plate, stools together, and legs to a bench frame, all of which can be taken apart.

Choosing and sizing

The bolt diameter should be about one-quarter of the leg's end diameter: M8 in a 40 mm (1-9/16 in) leg, M10 in 50 mm (2 in). The wood thread should go at least 25 mm (1 in) into the leg, ideally 35 to 50 mm (1-3/8 to 2 in). In end grain, use the longest thread you can; hold is weak in a short thread.

Installation

1. Bore a pilot hole in the end of the leg, straight and square: 5 mm (3/16 in) for an M6 hanger bolt in hardwood, 6.5 mm (1/4 in) for M8.
2. Lock two nuts on the machine end and drive the bolt into the hole with a spanner, until the wood thread is fully in.
3. Release the nuts. Leave the machine thread standing proud by the thickness of the plate plus the nut.
4. Fit the leg to the plate or block and tighten the nut or screw it into the threaded insert.
5. Re-tighten after the first heating season.

Wood movement and species

A hanger bolt in the end grain of a leg holds in the weakest direction of the wood. A leg that dries and shrinks across its diameter will loosen the grip a little; hardwood with fine grain such as Hard maple or Black walnut holds it best. In Eastern white pine or Yellow poplar, use a longer bolt or add epoxy to the thread. The leg-to-plate joint wants to rack with the wood's movement: a stiff leg plate or a brace takes the load.

Common mistakes

• Pilot too large: the bolt turns freely in the wood.
• Pilot drilled off-square, so the leg sits crooked.
• Over-tightening, which strips the wood thread.
• No lock washer or thread-lock, so the nut loosens over time.

Related

• Threaded insert: the matching nut, set in a plate or block.
• Machine screw and T-nut: a face-fixed threaded connection.
• Cross dowel and bolt: a barrel-nut alternative.
• Carriage bolt: a through-bolt with a head.