http://yfrog.com/9e91nissan240sxalternatorj
I don't think there a alternator diagram on S14 SR20DET manual.
But S13 240SX manual should be very similar.
If we look at the top left of the "Checking Drive Belts" picture, that is the alternator pictures. I tightened the 12mm "adjusting bolt". Since there is a flat washer there, I cannot feel if the bolt was tighten enough. But the belt has less slacks after I tightened the 12mm bolt.
The 14mm bolt that you were talking is at the right side of the "adjusting bolt", right?
I didn't touch that bolt, and it seems like it is very tight already. But I will try to tighten the 14mm bolt as well.