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Hanger bolts are studs used to suspend objects from or attach items to wood. Like a typical stud, they are headless, double-ended bolts that have threads at each end—lag threads to screw into wood but, at the other end, machine screw threads to accept a nut or thread into an internally threaded (tapped) hole. Coupling nuts and threaded rod are often used with hanger bolts to suspend electrical conduits, junction/pull boxes, fixtures and sheet metal. The furniture industry uses them with corner brackets, clinch nut plates, tee nuts and whenever wood and machine screw threads are needed in a single fastener. Hanger bolts are also known as:
couch bolts, headless hanger bolts, hanger screws.
Being headless, a little ingenuity is needed to drive them into wood—see below for a few installation tips.
Threads are standard right-hand at both ends. The machine screw threads are Unified inch coarse series (UNC, Unified National Coarse) and the lag threaded portion has coarse, widely spaced threads that end in a tapered blunt point designed to start threading into a properly sized pilot hole. Plain center style has an unthreaded body section; fully threaded types, also called closed center, do not have an unthreaded section.
Common sizes are 8-32, 10-24, 1/4-20, 5/16-18 and 3/8-16 and lengths range from 3/4" to 6". The number of threads per inch (32, 24, 20, etc.) refers to the machine screw threaded end. Measure length from end to end.
Since hanger bolts are headless, use a driver tool to install. If a tool isn't available, you have several options…
Method 1: Double nut—thread two nuts onto the hanger bolt and tighten (jam) them together, then drive the outer hex nut. Although this technique works with all sizes, use double nuts and a socket wrench to drive 1/2-13 hanger bolts, which require much higher torque to install than smaller sizes. Method 2: You can drive small sizes using a cap (acorn) nut. Be sure to use a solid metal nut rather than a hollow Palnut type cap nut. Method 3: On plain center style, use locking pliers. Pliers can also be used on fully threaded hanger bolts but be certain to grip the threads in an area that won't be used because thread damage is likely to occur. Method 4: Make a driver tool by cutting the head off a hex socket head shoulder screw that has, say, a 1" long shoulder, and thread it into a coupling nut. The shoulder of the shoulder screw is then inserted into a drill/driver. (Naturally, the threads of the driver item(s) you choose to use must match the machine screw threads of the hanger bolt.)
Steel is a common material. Plain finish, which lacks a surface treatment to inhibit rust, and zinc plated are the usual finishes.
If you need a double-ended screw that has lag threads on both ends, choose a dowel screw.
Hanger bolts require predrilled, full-depth pilot holes to ensure maximum withdrawal resistance; hole size is based on bolt size and wood density, as suggested in Table 1.
Hanger Bolt Pilot Hole Sizes
|Bolt Size||Pilot Hole Size|
|Soft Wood||Hard Wood|