Quote Originally Posted by JimC View Post
There are Fender and 3rd party JB necks with the top access truss rod so its not impossible - indeed the more moderately priced 3rd party JB necks I found last time I looked were pretty much all top access. I suspect it would be more likely to break if you dropped it rather than just in use. Having said that I wouldn't be comfortable with a top access JB neck, but I have zero evidence that its unsound.
FWIW I have one of the PB Jazz profile necks. It has quickly become one of my favorites. It feels a lot like my here-to-fore favorite neck, the #8 "C" type neck that is 1.5" at the nut on my G&L ASAT, which also adjusts at the top and has a 12" radius, like the PB. It's a '94 that I have had since 2002, and I have not noticed any issues, but I haven't dropped it on it's head, either ;-)

At one point I looked to see if there were some sort of advantage to having a top or bottom adjustable neck. I haven't found a trustworthy source to say whether one is better than the other. I think the reason that most current mfgs put them at the top is to make t hem easier to adjust. There are some mfgs that put a traditional fender adjustment at the bottom, I am guessing to make them more vintage-like. There are adaptations (holes in the pickguard, wheel type nutes, etc.) to make the bottom loading type easier to adjust without taking the neck off. Warmoth has even invented a side adjust truss rod. But as far as I can tell all the adaptations and changes from the traditional appear to be for the purpose of making the truss rod easier to adjust.

So, there may be an advantage that I am not aware of, but as JimC notes, Fender puts a lot of them at the top now (including on signature basses like the US made Geddy Lee). My '75 Fender bass has it at the bottom, but Leo Fender's other companies seem to have put them at the top. As far as I can tell, all the Music Man basses and all the G&Ls had top adjust, including the ones made with a jazz profile, like my ASAT.