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Holding the block, turn on the belt sander while keeping pressure on the block. Apply more pressure to the end of the shim that will face the headstock. This will create the subtle wedge shape you need to correct a neck angle that’s too low.
Fig. 10. Checking the bottom of the shim to confirm that the end facing the headstock is sanded a bit thinner than the end facing the bridge. The pencil marks tell the story.
Frequently check the bottom of the shim to make sure it’s getting sanded evenly from side to side, but developing a slight front-to-back angle (Fig. 10). This is where the pencil marks come in handy.
Don’t sand away too much wood—you’ll have a chance to check your shim depth and angle in a few minutes when you string up for the first time. If need be, you can return to the sander to remove a little more wood from the bottom of the shim. By working incrementally, you won’t have to start from scratch again.
Once you’ve sanded the shim into a subtle wedge and smoothed the top, it’s time to punch holes to accommodate the four neck screws. Place the shim into the neck pocket with the shaded side down. Be sure it’s oriented so the thicker end of the wedge faces the bridge. Insert the neck bolts and gently turn them until they each make a shallow impression in the shim.
Remove the shim and place a few drops of superglue around each screw-tip impression to reinforce the wood where you’ll be punching holes for the neck screws.
Fig. 11. Punching holes for the neck screws after reinforcing the screw-tip impressions with superglue.
Using the screw impressions to center your punch tool, make four holes in the shim. Work slowly and carefully to avoid cracking the shim as you punch holes in it (Fig. 11).
Following the same orientation described above, reinsert the shim into the neck pocket. Bolt the neck onto the body, string up, and tune the guitar to pitch. From there, measure the action and test playability.
Remember: The problem you’re correcting is a low neck angle that prevented you from adjusting the saddles down far enough to create comfortable action. The shim is working properly if you can use the saddle screws to set the action the way you like it. The saddle screws should offer sufficient adjustment range to lower or raise the action.
If the action is too low with the shim in place—even with the saddles raised—you need to remove the shim and sand it thinner. It may take several attempts to achieve the correct thickness and angle. Carving a shim takes time and patience, but the reward is well worth the effort.
Fig. 12. The micro-tilt neck adjustment system uses an Allen screw to
press against a metal disc embedded in the neck heel.
Some electric guitars have a “micro-tilt” assembly in the neck pocket (Fig. 12). By inserting a hex wrench through a hole in the neck plate, you can push an Allen screw against a metal disc embedded in the neck heel and adjust the neck angle without using a shim. This invention, in my opinion, is a great way to force the end of the neck to warp or bow. If your guitar has micro-tilt hardware, I’d remove the Allen adjustment screw from the body and simply bolt the neck flat against the pocket. If the neck angle is too low, use a full-pocket shim to correct the problem.