The Perils of Bad Neck Shims

Yes, yes, Gerry's on the shimming thing again. It's important, though. I'm on a quest. A quest to stamp out bad neck shims. 

What's the problem with neck shims?

Nothing. If they're done properly. 

They almost never are, though. Usually, this is how a guitar or bass neck will be shimmed:

Don't shim like this. Just don't. 

No. No. No. 

This is how you shouldn't do it. I wrote a little about this before so check out how to shim a bolt-on neck for a bit more background if you like. Long story, short though: shimming like this can actually deform the neck

Poor Shimming can deform your guitar or bass neck

Yes, I said 'deform the neck'.

Over time (and not always a lot of time), that shim can push up the end of your guitar or bass neck, causing a ramp or 'ski-slope' at the end of the fretboard. 

Don't believe me. Take a look at the heel of the bass neck installed over the shim above. 

Example of the 'ramp' problem caused by bad choice of guitar or bass neck shim

The heel of the neck should be flat. It should contact the ruler edge all the way to the end. That harmless looking shim in the image above has pulled a hefty lump of maple out of shape and caused a 'kick-up' at the end of the fingerboard. 

This particular deformation meant a lot of buzzing and choking on the bass (because the last fret or two had been pushed higher than all the others). In this case, the ramp was enough that the guitar had to be re-fretted — the frets were removed and the fingerboard re-levelled before new frets were installed. With a smaller ramp, you might get away with a fret level but poor shimming choices will cost you money to remedy in the future. 

How should I shim my guitar neck properly?

A full-pocket, wedge-shaped shim is the safer way to shim your guitar or bass neck

You need a 'full-pocket' shim. Your shim needs to be slightly wedge shaped and fill the entire neck pocket, so there are no air-gaps. The thick end of the wedge sits nearest the bridge and the wedge tapers and feathers to almost nothing at the other edge of the pocket. 

I've talked before about making your own full-pocket shims and the newer, easier way to safely shim your guitar or bass. Well, things have moved on a little more since then and it's worth revisiting. 

It used to be that you had to (painstakingly) make your own shims if you wanted to do it properly. It was a pain. I disliked it immensely. 

Then, the boffins at Stew Mac came up with a pre-fabricated shim to fit most guitar neck pockets. Brilliant. Non-standard guitars and all basses weren't covered though. But not any more. 

0.5° neck shim for Fender guitars

Wedge shape of full-pocket shim

Now, you can buy shims for standard guitars and basses AND a more 'generic' blank shim that you can cut to fit many other instruments. 

This is fantastic. There's no longer an excuse for poor shimming. Buy a precision-cut, full-pocket shim and save yourself a lot of hassle and probably a lot of money. These shims may seem expensive for a sliver of wood but they can potentially save you the cost of a refret. That seems like a good deal (and to be honest, if you factor in the tooling to make something like this, the cost is pretty reasonable). 

I'm not on commission or anything here but I heartily recommend you check out the Stew Mac guitar and bass neck shims. The standard ones should fit most Fender-type instruments and the blanks will cover lots of others. 

The bottom line

Just to sum up: 

  • Do not shim the 'traditional' way you've seen others do it.
  • If your tech or repair-person shims your neck, check they've done it properly. 
  • Don't use the Fender Micro-Tilt if you can help it. Shim instead.
  • Full-pocket, wedge shims are the only way to go.

Now, wood is a fickle material to build a musical instrument from. It can settle and move and do all sorts of unexpected things. If you never shim your guitar, it might still develop a ski-slope ramp that needs to be addressed — it happens. However, if you shim badly, you are much, much, much more likely to need to visit for fret-work. Take another look at the photo with the ruler if you doubt me.