1990 nissan 300zx nonturbo - change alternator belt how?

If you look at the front of the alternator, behind the belt, you'll see a curving grooved metal track - that's the adjuster track for the alternator. There's a bolt going through the groove into the alternator, which holds the alternator in place. You need to loosen that. I believe it's a 14mm bolt. You may also need to loosen a couple of the 12mm bolts that mount either the bracket or the alternator, to allow it to move. You'll probably find that it's easier to cut the old belt off (so it's out of the way) and then do the bolt work with a little more room to spare. Belt changes are always a pain on these cars - stick with it even if it takes awhile. If that doesn't get the alternator loose, loosen or remove other bolts on the alternator mount or on the track mount, and it will move eventually - not much, but it'll move. You may find that you can partially seat the belt on the alternator pulley, but can't get the belt fully onto the pulley. If that's the case, get it as far on as you can, and then put a 27mm socket on a long breaker bar, put the socket on the crank pulley bolt, and with the car in neutral, rotate the crank pulley by hand. This will pull the belt onto the alternator pulley.

The alternator on a 90-96 300zx is held in place by a large 17mm bolt on top ,, you just loosen it a turn or two no more. ON the bottom of the alternator is the adjustment screw and locking nut.. You'll have to back the screw off about and inch, then unlock the locking nut on the back side. both the screw and locking nut are 12 or 13 mm. Once you've done all of the above the belt will be loose enough to remove.