#1
Very frequently here in GB&C someone comes along and asks if such-and-such a neck will fit with such-and-such a body. This thread will attempt to cover everything you need to consider when you're planning a neck swap or parts build. If there's anything you think I've missed or got horribly wrong, then please do let me know.

Firstly, let's assume we're talking about bolt-on necks here. Swapping out a set neck is not beyond the realms of possibility, but it's far more work and far more prone to disaster than a simple bolt-on swap. That's not to say the points covered here would not apply equally well to set necks, but I imagine removing the original neck is going to be difficult enough to dissuade all but the most determined modder. Neck-throughs, of course, you are stuck with - although, again, you could possibly convert it to a bolt-on or set, but why would you want to?


NECK SCALE

This is the single most important consideration. The placement of the bridge on the guitar body is determined by the neck scale; it must be exactly the same distance from the nut to the 12th fret as it is from the 12th fret to the bridge. This distance is half the scale length. So on a 25.5" scale guitar it would be 12.75". It follows then that you can determine the scale of a neck by measuring from the nut to the 12th fret and then doubling it.

When swapping out a neck, your best chances of success come with swapping out for another neck of the same scale. However, it is not a guarantee as there are other factors at work (see below).

It is important to note that it is impossible to determine what scale neck is required if you just have a body. You can measure from the bridge to the edge of the neck pocket, but unless you know the relationship between the end of the neck heel and the 12th fret it won't tell you whether your neck is going to be suitable. Which brings us nicely on to the next point.


NECK HEEL/POCKET

This needs to be a good fit both to hold it firmly and get the best resonance possible. Obviously the width must be a good match, but there are a couple of common variations on the heel shape. It is commonly stated that Strats have a slightly curved heel, whereas Teles are flatter. Of course this is true for Fender parts, but just because you have a Strat- or Tele-style guitar doesn't mean the neck or pocket will follow those rules. And what about all the other styles of guitar out there? You need to check.



You can always reshape a heel to make it fit in a pocket, but be aware that this will affect how deeply into the pocket the neck will sit, and this in turn will affect the distance from the nut to the bridge, which is our critical dimension. Once again, if you're matching up a neck and body, you need to keep this dimension in mind. Even if you're swapping out a neck of the same scale, the way the heel and pocket match up will affect the nut-to-bridge distance, as the following diagram demonstrates.



There are ways to compensate for this. You can either pad or remove material until the correct scale length is achieved, or you may be able to move the bridge; a non-string-through hardtail should be no bother at all to move, although you may have some holes to fill. Any bridge that involves major holes or body routing will be somewhat harder to move about.

Another thing to think about is how deep the pocket is compared to the thickness of the neck heel. This will affect how far above the body the fingerboard will sit. Again, you may need to pad or shave to make the fit correct. But before you do, you also need to think about...


NECK ANGLE/BRIDGE

For a consistent action we want the strings to be a more-or-less constant height above the fingerboard. The string height is governed by the height of the nut, the height of the bridge, the angle of the neck, and, to a certain extent, the neck relief. As we're talking about neck swaps here, my main concern is with the neck angle, because the neck angle is determined by the joint you are going to be making. First, let's have a quick look at how the neck angle, in combination with the bridge height, affects the string height.



As you can see, if we put a higher bridge on, say replacing a vintage Strat style with a T-o-M (I know) then the neck needs to be angled back to keep the action acceptable. Similarly, adding a lower bridge (all those who are asking, "Can I put a Floyd on my Epiphone Les Paul") requires the neck to be angled forward. This is also one of the reasons why a recessed T-o-M is sometimes used. This neck angle may be achieved by angling the bottom of the pocket, or possibly the heel of the neck or maybe just by shims (thin strips of material - often wet & dry paper for some reason) in the joint.

Once again, this is something that needs to be considered. Not so much for straight neck swaps but definitely when you're piecing together a partscaster.


FINGERBOARD OVERHANG

The last thing that might catch you out when fitting a neck is how much the fingerboard juts out beyond the neck heel. This is mainly because it has the potential to interfere with the neck pickup and is particularly likely if you're thinking of fitting a 24 fret neck to a previously 21 or 22 fret guitar. Even so, I have also had issues with a fairly standard 22 fret neck when fitting it to what I thought was a fairly standard Strat-style body. It's probably not going to be a major issue, but still something to be aware of.


ANY OTHER BUSINESS

With all that going on you can see why some people might consider it easier to just fill the existing pocket in completely and start from scratch.

Warmoth have a fairly comprehensive Will it Fit My Guitar? page for those of you considering buying from them.

Warmoth also offer conversion necks. There is nothing particularly magical about these other than they have been designed to fit a standard Strat or Tele and place the nut 24.75" from the bridge. If you think about it, there's no reason you couldn't make, say, an 18" scale neck to go on a standard guitar, but the bridge would be 9" from the 12th fret so you probably wouldn't get too many more frets on it. In fact, this is exactly what you're doing when you use a capo - 18" equates to about capo at the 6th fret on a 25.5" scale neck.

There is a comprehensive list of guitar scale lengths on Wikipedia.
#2
I'm not entirely convinced that it makes a massive difference in terms of resonance to have a pocket that fits perfectly. You've got mechanical fasteners that firmly attach the neck to the heel, and I think that's where most of the mechanical connection comes from.
EH


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#3
Quote by eddiehimself
I'm not entirely convinced that it makes a massive difference in terms of resonance to have a pocket that fits perfectly. You've got mechanical fasteners that firmly attach the neck to the heel, and I think that's where most of the mechanical connection comes from.
I actually agree with you on this one. In theory, maximum surface contact will give maximum transfer of vibrations but I would imagine the effect would be minimal, as you say.
#4
One thing you didn't go over that I never get a solid answer for is if a 22-21 fret neck and 24 neck are interchangeable if the scale is the same
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#5
That all rather depends on the necks in question. However, I would suggest that in general a 24 fret neck will sit deeper in the body (further toward the bridge) than a 21/22 so a straight swap would be unlikely.

Having said that, I believe you can also get special 24 fret necks designed to replace 21/22 ones, that have the extra frets on a fingerboard overhang, in which case you'll run into the problem of it interfering with the neck pickup.