Forum Moderators: open
That's a tough one. So here's both sides:
If you know 100% that >> nuts and bolts << is your top potential keyphrase - use it.
If however you have lots of different nuts / bolts then I would suggest >> nuts << and then for the next link exchange >> bolts << and keep repeating the process, at least until one reaches top ranked position then just keep adding the other anchor.
wing nuts
anti-locking nuts
taper nuts
dead bolts, etc.
as well as >> nuts and bolts
...will all benefit - thus you are diversifying along with rank improvement