| It depends on where and possibly when it was made. The '80s MIJ ones use a 5mm one, but the US "bullet nut" used on Jazzes and Telecaster basses in the '70s used a similar but not the same sized SAE wrench.
Smartest way to find out is to buy yourself a set of good ones in both metric and SAE sizes, and try them until you find the right size. They're useful for a lot of stuff so I've never regretted buying any allen keys. Quick way might be to go to FMIC's "Mr. Gearhead" and find the stuff for your particular bass.
jte
__________________
JTE
Spelling, grammar, and punctuation do matter, despite the threats of death by grease fire!
"Without space, music is just noise piling up on itself." TRK
Lakland Owners' Club # 248
| 
