Quick nut width question

If the string slots are pre-cut, they will be a little far in on 38mm vs 40. I think you will notice the difference.

 

I agree with @Gilmourisgod, the 38.5mm nut will be a bit off. If it's slotted, the string spacing will be too narrow, and there will be sharp edges at the fingerboard at the nut slot. Additionaly, it will likely look out of place on that bass.

I don't know how handy you are, but in this case (and because you posted this in the luthier's corner) I recommend making a nut from scratch. You can buy nut blanks of various sizes. You can cut it to the exact width, shape it to the correct height and then file in the slots. The good thing is that you can't really do any damage here, and if it doesn't turn out as you want it, you can simply start over with a new blank. This is a very useful skill for repairs and setups.

 

No need to use actual nut slot files. You can make bass nuts with a razor saw and a few cheap needle files. Or just wrap sandpaper around appropriately sized drill bits.

 

Yes, exactly. I use a tapered needle file, and depending where on the taper I file I can cover anything from 40-135. Those files are nowhere near as expensive as "proper" nut slotting files.

 

It also helps to run a collar of tape around the file at the point of the taper that matches the diameter you're shooting for, so you don't go too far. I haven't made a nut yet, but I've done that for other things.

 

If you prefer to buy a preslotted nut check with GraphTech (graphtech.com) . . . they have varying 4-string lengths including 39.5mm and 41.0mm.

 

I have used this one on my last build, it's exactly 40mm wide and 5 mm thick. Also, with the individual saddles it allows for different string gauges (EADG vs BEAD) and is easy to adjust.
ABM 6240 Nut

E-G measurements and spacing depends on the model and manufacturer, ABM for example has constant 10mm spacing center-center.

 

+1 on the graph tech recommendation.

If you don't see a good blank to start from, contact support, they sell a lot more than what you see on their page. I once found their dealer listing and special ordered via email some blanks that saved me lots of time.

Another tip, check to see if your nut has a raidiused bottom or flat bottom. If it is raidiused, you can choose to flatten the slot. There's also some that come pre-radiused.

 

I gewgled "40mm" bass nut and came up with a number of readily available...40mm bass nuts.

 

Bass Guitar Bone Nut 40mm 4 String 1 9/16''
https://www.amazon.com/Musiclily-Plastic-Slotted-Bottom-4-string/dp/B08H7ZHPPP
TUSQ NUTS - Bass
Nut Bone 4 String Bass Guitar 40mm 1 9/16 Inch Narrow Slot Slotted RH 109253 for sale online | eBay
Wider than a J, thinner than a P: 1+9/16" necks

Here are a few, including one in bone, and also a link to a TB thread about the same thing that includes a fair bit of information relating to your inquiry.

 

What about #1240?
Is your 40mm actually 40mm? or a bit less?

(see next post, the top is tapered a bit)

 

Careful with that 1242, if you have a bass with a Gibson/Epiphone type nut (not in a fretboard slot, and angled down toward the headstock) then you're fine. If you're trying to put it in a fender type nut slot, (mostly flat front to back) you'll need to get creative to flatten out that angle a bit. Look closely at the 3rd picture.

Also, I found the OEM catalog. It is here https://cdn.shopify.com/s/files/1/2372/8913/files/oem-catalog_2014_2.pdf

 

You always want them to angle a bit, even on a Fender-type nut, you want the witness point to be a point, not the whole nut. You can make a 1240 look like a 1242, but vice versa is really hard.