Guitar necks can become twisted and warped if they are not properly looked after. Exposure to heat extremes is the main cause of these problems. This is why you should never stand your guitar next to a radiator. The severity of the problem ranges, depending on the amount of twisting that has occurred. A mildly twisted or warped neck may have some intonation problems, while a severely twisted neck could be unplayable. A combination of heat and pressure is often the only way to correct severe twists.
Set up your two clamps to the correct height. The bottom clamp should be roughly the same height above the work bench as the depth of the guitar’s body. This way the guitar will lay flat while the clamps are tightened.
Remove the strings from the guitar. Use a string winder to save time. You need to apply heat directly to the neck. Leaving the strings on will diminish the corrective effects of the procedure and will ruin the strings.
Loosen the truss rod with a hex key. You are going to be adjusting the position and relief of the neck. If the truss rod is fully tightened, it could snap inside the neck. It could also pull against the nut and cause damage to the body of the guitar.
Turn on your clothes iron and set it to a medium-high heat. Guitar neck bending machines are designed to apply heat and pressure simultaneously to correct neck bends, but they are expensive. Similar results can be achieved with a clothes iron and a clamp.
Lay your guitar flat, face up on your work table. Place an old T-shirt or sheet over the fretboard so that the heat doesn’t damage the finish. The sheet should be thin enough to allow sufficient heat through, but not too thin that it doesn’t protect the wood.
Run the iron over the neck of the guitar slowly and evenly for approximately 10 minutes. Don’t use steam or water. Feel along the length of the neck with your hands. Only when there is an even distribution of warmth along the entire neck is it ready to be corrected. If there are any cold spots, continue to run the iron over the neck. Turn off the iron when you’re happy that the neck is ready.
Place the neck of the guitar in the two clamps. The first clamp should be fitted just below the nut and the second clamp just above the heel of the neck, where it meets the body. Very gradually tighten the clamps. Check that the guitar is sitting straight in the clamps. Leave the guitar overnight. As the wood cools and contracts back to its natural size, the clamps will adjust the position of the neck.
If you don't have height adjustable fixed clamps, place a cushion under the guitar so that the neck sits at the same level as the clamp.
If you hear high pitched squeaking when tightening the clamp, stop adjusting it.