Part of the guitar setup is truss rod adjustment to make the neck straight. 

Features:
Two Way Dual Brass Head guitar truss rod. 
The total length is 440mm rod for making or repairs.
Made of high-quality steel.
Fast and easy to operate.
For all instrument types.
A must-have tool for pro luthier shops and home shops alike.

Note:
Please allow a 1-2mm error duo to manual measurement.