Two way truss rod for bass. The two way action allows for the correction of upbow and backbow issues, giving total control over the neck relief adjustment. Two way truss rods are easier to install than single action ones, they don’t need a curved bottom cavity, what makes the routing process straightforward.
1. Mark the center line on the neck blank using a marking gauge or ruler, mark also the cavity width lines. The channel can be made using a table saw or a router. Rout the wood in several passes until the final depth is reached. In the case of truss rods with 4 mm. allen nut we must drill an access cavity of 6mm - 1/4" of diameter and a approximate depth of 13mm - 1/2". For acoustic guitars with adjustment at the heel, this should be drilled to allow access to the key.
2. Set the truss rod into the slot. Although the rods are cushioned with PVC tubing, it is advisable to apply a small amount of silicone sealer in the slot bottom to absorb any possible rod rattle. Then press the rod as deeply into the slot as possible. It is very important to use a minimum amount of silicone to avoid squeeze-out.
3. It is very important to avoid the glue getting into the truss rod cavity so, before we glue the fretboard, we should cover the truss rod cavity over the installed truss rod with a strip of masking tape, slightly wider than the channel. Spread the glue over the neck, remove the tape and clamp the fingerboard in place. We'll notice a small area on each side of the truss rod that has no glue, but if done properly, the glue will spread just to the edges of the slot and not beyond when pressure is applied.
To adjust neck relief, turn the nut clockwise to correct an upbow, and counter clockwise to correct backbow. The truss rod should not be forced if you encounter excessive resistance, first loose the truss rod to its neutral position and then ease the neck into a corrective bend with clamps. After this operation the rod can be adjusted.