Insert nut

An insert nut, or knock in nut, provides a threaded socket for a wooden work piece, similar to a wall anchor. Insert nuts are inserted into a pre-drilled hole by one of two means: screw in and hammer in. Screw-in insert nuts have an external thread and hex head and are screwed in with an Allen wrench. Hammer-in insert nuts are lined with barbes and are hammered in. In both cases, the external protrusions bite into the wood, preventing the nut from either turning or pulling out. Insert nuts should not be confused with barrel nuts or T-nuts, which require that the working piece be drilled through and are fastened behind the work piece.

The screw-in "insert nuts" come in various sizes, and take different screw/bolt sizes. They also require an Allen wrench (metric) to screw them into the work piece. For example, a "1/4 - 20" insert nut will take a 1/4 inch bolt. It requires a drilled hole 3/8" diameter, and a 6mm Allen wrench to screw it into the work piece. The pre-drilled hole must be as deep as the length of the insert nut plus any portion of the bolt that may be screwed past the end of the nut in the work piece. Sales personnel may or may not reveal the above information, so it's best to ask.