It's not really hard to change the timing belt on a 300ZX, but make sure you get it right. You also should consider replacing the water pump at the same time because if it goes bad, you'll have to do the job all over again. Same for the tensioner, if it breaks you're out an engine. I'd reccomend changing it every other belt change. Also get the tensioner stud if you're changing the tensioner, it'll probably have to be ordered at most dealers. There's not much sense in putting a new tensioner on with an old heat stressed stud is there? The timing belt dosen't turn the water pump, but if the pump leaks, the water will leak into the timing belt cover and rapidly degrade the belt. Trust me, it's cheap insurance. You also should replace the front main oil seal at the same time, and both cam seals. If they starts leaking, it won't hurt anything, it's just a pain to disassemble everything just to change the oil seal later. The passenger side cam seal for some reason has a tendancy to go bad before the other one. You will need this stuff: Timing belt(~$30), and water pump($40 NEW at auto zone), Oil seal(~$5-$10), and the crank timing spricket key($0.50), also consider changing the thermostat($8 for a Nissan one, which is the only one you should use). Also make sure you have all appropriate gaskets, etc. The water pump should come with a gasket, the thermostat gasket is a dollar or so.
This is all that is needed to change the timing belt/water pump on an early 300ZX:

1. Remove fan, radiator shroud, all engine belts, the crank pulley, the A/C idler pulley, the timing belt cover, the spark plugs, and anything else in the way.
   
2. remove water pump and replace with a new one, do this first so you don't spill coolant all over the new belt.
3. Turn the engine over by hand until the marks on all three pulleys are close to the marks on the rear timing belt covers. May take a few minutes. The location of the marks on the old belt(If you can still see them) is totally unimportant.
   
   This is important: The purpose of setting the engine in this position prior to removing the old belt is so that the engine will be in the "alignment" position to accept the new belt. Once the old belt is off, you can't rotate the engine. If you remove the belt without putting the engine in this position you'll have no way to verify that the timing is correct once the new belt is on.
4. Loosen the tensioner and remove the belt.
5. If replacing oil seals:
   1. Hold the cam sprockets still and unbolt them, remove the sprockets and replace the cam seals. natrally, replace the sprockets and torque to specs. It is easiest to place the car in 5th gear, with the old belt on and simply unbolt the cam pulleys. Only break the bolts lose, then remove the pulleys and belt at the same time. When torqueing the pulley bolts, put the old belt on to avoid stressing the new one. If you have an automatic or if the engine is out of the car, you'd better have air tools, or do it this way: Cut the old belt into one long piece. Wrap it around the cam pulley and hold the pulley with a chain wrench, removing the bolt with a wrench. Do the same when reinstalling the cam pulleys.
      There are no lips behind the cam seals, so if you tap them in too far, they will just pop inside the heads, requiring lots of disassembly to remove. Tap the right(passenger)) side seal until it is flush with the metal surface or slightly below(no more than a couple mm), and the left(driver) side seal until it is flush with the smallest diameter of the taper, you'll know what I mean when you take it apart and look at it.
   2. Pry the crank sprocket off the crank, it will be very tight, you will probably have to destroy the thin metal guide plate behind the sprocket, but you will replace it with the new one you special ordered from Nissan. Replace the seal, and install the new metal plate, the NEW sprocket key, and the sprocket with anti-sieze compound on the inside of it. Do not, under any circumstances, hammer a prying device behind the sprocket or you may break the oil pump housing, which is bad.
6. Installing the new belt with the marks on the belt directly adjacent to the marks on the pulleys, noting that new belt has has a front and rear facing edge.
   
7. Double check your installation. Now triple check it, and check a fourth time. You must be absolutely sure you put the belt on right or your engine will be destroyed when you start it, no second chances, get it right!!
8. Check installation of belt again.
9. Turn the engine over by hand a few times to ensure that nothing interferes. (Note: After you turn the engine over, it takes a few thousand revolutions until the marks on belt and pulleys line up again, do not worry if you are absolutely sure of the timing. If you have any doubts at all, go back to step 3.)
   
10. Adjust the belt tension according to the shop manual. This may take a few tries, but take your time. The tension has a large impact on belt life.
11. Replace all removed Items, and start engine.(replace the 2 inch water hose between the thermostat housing and metal coolant pipe right behind the fan now, they have a very, very bad habit of leaking)

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