Armstrong Musics unique one rod, double action trussrod, light weight, featuring a slim profile which allows installation into even the slimmest necks. - 18" long. Please note that the 4mm allen wrench is not included but can be ordered separately here.Installation tips: Install the rod assembly with the flat bar up against the fingerboard. This trussrod requires a straight slot, unlike a conventional single tension compression rod that normally require an arched slot. The slot should be cut to a depth that situates the assembly to sit slightly below the top of the neck. 010 to .015" should work well. Be sure that the rods are straight, with no tension applied before installing. A good tip is to wrap the trussrod loosely with teflon plumber's tape to prevent any glue getting on it when installing the fingerboard. It is not advised to use any "stops". This allows the rod to be removed if breakage or other problems should arise. After final assembly and adjusting, be sure that there is slight tension applied to keep the rod from moving in the slot.