Shimming a Bolt-On Neck

If you’ve been around guitars/basses and guitarists/bassists for a while, there’s a good chance you’ve heard of ‘shimming’ your neck. Maybe you’ve even done it.

Usually, we shim because the action is too high, even when the saddles are lowered as far as they can go. If your saddles are bottomed out, but you’ve still got a high action, we might want to shim.

Also, shimming a neck is sometimes done to purposely raise the bridge in order to get a better ‘break angle’ (and therefore more ‘downforce’) over it. Some of the offset Fenders (Jags and Jazzmasters), for example, can sometimes benefit from this. Once the bridge is raised, shimming allows us to recover a more reasonable action.

So, What Does Shimming a Neck Actually Do?

Changes the angle.

Essentially, you’re performing a neck reset. Check it out:

Why we might shim a neck

The top guitar has its neck bolted on as normal but, for whatever reason—maybe the neck pocket was routed a little too low or the neck was made a little thinner than normal—we can’t get the action down to where we want it. The saddles won’t go any lower.

The usual (and wrong) sort of neck shim

We remove the neck and add a shim underneath the end. This angles the neck back, allowing a more comfortable action.

Typically, people will make a shim from a piece of cardboard, (business card), plastic (old credit card/guitar pick) or wood (lolly sticks or similar) and will insert it between the back pair of neck-screws and the end of the pocket.

Bad. No. Don’t do it.

How NOT to shim a neck

The problem with short neck shims

When you place a shim at the end of the neck and then screw the neck back in, there’s a small ‘air pocket’ left. Because the neck is tightly bolted to the body, there’s a constant pressure, pulling it to ‘fill’ this gap. The (exagerated) image above shows what I mean.

Over time, the neck distorts and you end up with a hump (some say ‘ski-slope’) at the end of the fingerboard which can cause problems with buzzing and choking notes. Nasty.

How should I shim a neck PROPERLY, then?

You need to eliminate that air gap. To do so means making a wedge-shaped shim that runs the length of the neck pocket. We call this a full-pocket shim.

This longer shim will be the appropriate thickness at the back of the pocket but will taper to pretty much nothing at the front end.

You may be able to buy one

Since I started writing about all this stuff, I've always said, "I wish someone would make these shims and sell them." Well, Stew Mac now does just that. If you're a guitarist, anyway. 

At the time of writing, you can buy tapered neck shims for guitar and for bass. This saves you a lot of hassle and potential swearing. 

But, if you're broke, you could always make one. 

If you want/have to make one

I’ll be honest, making these is a bit fiddly. I don’t even like doing it but it’s the best way to minimise the potential for a neck hump. I know that you might look at it and think, “You’re crazy if you think I’m doing that. It’s the fiddliest job in the world.” However, if you need a shim, a full-pocket shim is the best way to go about it. And, if you don’t want to do it yourself, at least you know the right way it should be done (so you can check if a tech does it for you).

Using a suitable bit of hardwood, you need to cut it to shape. If you lay your guitar neck heel on it, you can draw around the shape with a pencil. Cut this out carefully and finesse the edges until it’s a nice snug fit in the neck pocket.

Next up, make the holes. Place your shim in the neck pocket and poke a screw through each screw hole to mark its position. Then drill each hole carefully. Use a bit that's bigger than the neck screws so they fit through easily. Go gently at this.

From one tricky job to an even-more-tricky job. We need our wedge shape. The pice has to be thicknessed so it’s the right height at the back—maybe only .020”/.5mm to .040”/1mm thick (tricky, like I said)—and so it ‘feathers’ to almost nothing at the front edge. A belt-sander is a big help here but it’s possible to do by hand if you’re massively patient. Some sandpaper double-sticked to a flat surface can be used but I’d hate to be the one doing it.

Speaking of double-stick tape, it can be helpful to tape your shim to a bigger block. This makes it easier to hold and manipulate when you're sanding it (especially if you're using a belt sander). Just be careful when it comes time to 'un-stick' it. Go slowly and be careful or you risk leaving half your shim stuck to the block. Warming it a little can help the tape let go. 

Wedge-shaped shim in progress

Completed, full-length shim

The first image below shows the shim in progress. It’s still much too thick in this image but you can see the wedge shape. By the way, in this image, I’ve thicknessed to a wedge before cutting to shape. I actually prefer doing it this way but you might not agree. Once thicknessed, the piece is very delicate and difficult to cut—it’s probably easier to cut before thicknessing (do as I say, not as I do). ;-)

Last thing is to make the holes. Again, this bit if fraught with danger because of the delicacy of your shim. You can use something like a leather punch—rotate the wood back and forth a little as you go rather than just trying to punch through in one go. Slowly does it. I usually use a sharp drill bit and go gently.

If you’re up for attempting this job, good luck. Take it slow and be prepared for your shim to crumble into a number of parts at any stage. I wish someone would sell pre-made shims like these.

Fender’s Micro-Tilt

Fender have been fitting a tilting system in their neck heels and pockets for a while now. It’s a small hex-head screw recessed in a hole under the neck plate. Turning the screw raises it to push against a small metal plate in the neck heel.

It’s very convenient. I don’t like it.

Actually, I love the convenience—it beats fiddling around with a wooden shim—but I feel it causes the same problems as a partial shim. Again, this is based on my own experience but, if it were my guitar that needed shimming, I definitely wouldn’t use the Micro-Tilt.

These Things I Believe

The last point leads nicely to this:

Now, it’s worth my stating that the shim/hump correlation is based on my own experience. There are other repair people who share this view but I doubt they’ve studied it in controlled, scientific trials any more than I have. Any guitar can end up with a hump. In my experience, though, shimmed guitars are much more likely to end up causing hump hassle.

Also, good coupling is a major factor in good tone. An air gap between neck and body does not achieve good coupling. If nothing else, this is good enough reason to shim properly. 

How To Fix a Neck Hump

If you’ve got a ski-slope at the end of your fingerboard, chances are you’re looking at some fretwork to make things right. A fret level may do the trick for a smaller hump. More pronounced humps might require a refret and some levelling of the fingerboard itself.

Prevention is better than cure.

The Bottom Line

If you need to shim, full-pocket is the way to go so buy one, make one, or make sure your repair-person does it right.

Any guitar or bass can develop a neck hump (and I should mention that NOT having a shimmed neck is no guarantee you’ll never get humpy) but lazy shimming will increase the hump hazard.

I might make a bumper sticker with, “Humps Happen”. ;-)