At first glance, the reason this neck was sent to us was unclear. After thoroughly inspecting it and adjusting the truss rod nut to a neutral, loose position, the neck had a back bow of .024" from level. My choice of tools for measuring the back bow was a set of feeler gauges my brother Bob bought for me in the late ‘80s and a 25.5"-scale notched straightedge. Once the straightedge was resting firmly against the fingerboard, I measured the gap between the playing surface of the fingerboard and the leveled surface of the straightedge by using my feeler gauges at the first position.
Correcting this neck for .008" to .010" of relief under string pressure, while still having a half to a full turn of truss rod nut adjustment, was not going to be achieved in one session. Much like going in for some chiropractic adjustments for neck or back pain, one session is rarely enough to get the adjustment to take hold. Guitar neck problems like this are usually caused by alternating extremes of dampness and dryness, or the expansion and contraction of the neck and fingerboard woods. There are also a variety of different warps going on from neck to neck. The neck we restored here had a gradual yet overall extreme back bow. I used a neck heater and a variety of shims and clamps to manipulate everything back into place. There are a number of ways to heat a neck for correcting its posture, using such products as a Halogen lamp, a heating blanket or an iron bar neck heater. For this project I used a discontinued product by Stew Mac that I purchased from a repairman twenty some years ago. I know it as the S.M. Neck Heater.
I first attached the neck with four screws to a raw, leftover body blank that I had around for just these kinds of jobs. Then I placed it into my string tension simulator/Erlewine neck jig and adjusted it to create excess forward relief in the neck. I shaped two shims out of mahogany with a 7.25" bottom radius to sit on each end between the fingerboard and the neck heater, elevating the iron bar heater about 3/8” from the fingerboard surface. Using these shims helped protect against any damage to the fingerboard or clay dots during this long, drawn-out process.
I then used a camless clamp on each end to secure everything in place, with a surface thermometer sitting top and center on the neck heater. The heater is designed to regulate at 250 degrees, but I always like to monitor it to ensure the temperature is stable and not going into a danger zone. Considering the value of this neck, it’s an important safety precaution.
By the end of the day of the first session, the bottom of the neck felt comfortably warm. I shut off the neck heater overnight, allowing for a cooling-down period so the soft glue joint between the fingerboard and neck could harden and reset. I used the neck relief gauge between sessions to measure each result. It took a total of four sessions to get the result I was looking for.
Keep in mind, it’s always a good idea to go by the 15–20-minute rule when first applying heat to see the results achieved before a marathon session—and perhaps going too far and having to reverse the neck bow you just put in.
The following tools from Stew Mac were used for this project. Though there may be other tools and repair procedures to achieve similar results, this is what worked for me when restoring this particular neck:
#3814 Notched Straightedge
#1811 Feeler Gauges
#4603 Surface Thermometer
#5252 Erlewine Neck Jig
#3708 Small Camless Clamps
#3709 Large Camless Clamps
#2004 Neck Relief Gauge S.M. Neck Heater (discontinued)
Thanks to you, PG readers, “Restoring an Original” is off to another fantastic year! See you next month.
John Brown, of Brown's Guitar Factory, is the inventor of the Fretted/Less bass. He owns and operates a full guitar manufacturing and repair/restoration facility, which is staffed by a team of talented luthiers. He is also the designer of guitar making/repair tools and accessories that are used today by instrument builders throughout the world.