Can I add a 24 fret neck on a bass which originally had 21 fret neck with no problems?

Unless the extra frets are on some sort of fingerboard extension that extends past the neck joint, the bass will no longer be the same scale length and it will not intonate properly.

 

The distance between the nut and bridge should be 34” ( if the new neck is also 34” scale) and the 12th fret should be 17” from the bridge. You will most likely have to shift the bridge position (forward or back depending on where the neck attaches) to be halfway from the 12 th fret if you have any chance of proper intonation. The pickups will not correspond to their original placement of the 21 fret neck.

 

Sure, if the original scale was (hypothetically) 8” then yes the distance between the nut and 12 th fret, and the 12th fret and the bridge would be equal.

 

I don't think it's going to work. If you try to move the bridge toward the neck, then won't it collide with the bridge pickup? Not to mention installing a longer neck will change the balance of the bass, possibly making it neck heavy. Also it is not necessarily a safe assumption that the neck will fit the pocket without any routing. Since you are "new to this" I recommend you get the proper replacement neck.

 

The TLDR version of all of this: if after the new neck is installed, the measured distance from the nut to the bridge matches the scale length of the new neck, then it will work.

So if the new neck has 34" scale length fret spacing, and once screwed into the neck pocket the distance from the nut to the bridge is also 34", then you're golden.

 

this may or may not apply to your particular situation, but it is a quick (& incomplete) primer on neck-swappping.

Warmoth Custom Guitar Parts - Bass Necks FAQ2

 

Even shorter version:
After installing, the 12th fret of the new neck should split the string in half.

 

If the 24 frets are only an extension of the fretboard and the heel is the same as the 21 fret,it should fit,unless there is a pickup in the way...

 

There really should be a sticky for this, this question seems to come around every couple months.

 

The length of the 21 fret and 24 fret necks are different (the 24 fret neck is longer). The frets on a 24 fret neck are not smaller to compensate. The gaps between frets 20 and 21 are exactly the same on both necks - the 24 fret neck just has three additional frets, which have to be smaller than the frets before them for intonation purposes.

As mentioned by others, this will not work unless you either make the neck pocket deeper so that the scale length is still 34”, or you move the bridge forward so the scale length is 34”. Personally, I wouldn’t bother.

 

It will not work without modifying the neck pocket or relocating the bridge. As other people have said here, the distance from the bridge saddles to the nut must be roughly 34" (give or take for intonation).

 

Just an example: Danelectro used to make guitars and basses with strange neck designs. There were the "dolphin nose" models where the bodies of both had the bridge in the same location so the guitar necks had 21 frets and the bass necks had 24 frets and extended further out because of the longer scale length. On the "double cut" models, like their doublenecks, the headstocks would line up so the bass bridge had to be much further down the body (about 5") and the neck had only 15 frets.

 

I think what most of us are suggesting is that while you could do this, it’s far too much work to do all that to compensate for changing the length of the neck. You’d be routing new pickup routes, drilling wire tunnels, probably reshaping the neck pocket...it goes on. If there’s something irreparably wrong with the neck, then you’d be better off getting a replacement neck that’s the same length as the old one. If you just really want a 24 fret neck, I would suggest that there are plenty of good, inexpensive Ibanez Soundgears on the used market that would work much better for this purpose.

 

^ This.

Also, if you really need those higher notes and what to stay true to a Fender Jazz, you could always get a 5 string and string it to EADGC, or get a low D with D standard tuning (DGCFBb).

I think there also a couple of odd-ball 4 string Fender Jazz’s with 24 frets, though they’re mostly Japanese Fenders aimed at the Ibanez SR crowd.

 

Find yourself a used one of these:

Problem solved. I love mine.