Editing Z31/timingbelt php (section)

=== The Tear Down and Installation ===

* Remove lower engine shield (usually missing from most Z31s).
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Next is the cooling fan. Remove the four nuts holding it to the water pump pulley. Be careful not to drop them.

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Remove the fan shroud and all engine belts.

* Remove the crankshaft pulley (23mm bolt). For manuals, put the car in 5th gear and set the parking brake, then break the bolt loose. For automatics, use an air impact gun. Reinstall the bolt (without the pulley) so you can rotate the motor to align timing marks.
* Remove the A/C idler pulley (held on by bolts at each end).
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Remove the timing belt covers. Start with the upper one (many 8mm bolts), then the lower one.

* Optional: Remove the spark plugs to make it easier to turn the motor for aligning timing marks.
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Align the motor and mark the location on the old belt and pulleys.

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Remove the old thermostat and water pump and replace them. Do this with the old belt in place to avoid coolant spilling on the new belt.

* Decide if you will be replacing the oil seals.
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If replacing the oil seals, hold the cam sprockets and unbolt them, remove the sprockets, and replace the cam seals.

* Replace the sprockets and torque to spec. Use the old belt for holding pulleys if needed. For automatics or engines out of the car, use air tools or a chain wrench with the old belt.
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There are no lips behind the cam seals—tap carefully!

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Loosen the crank pulley bolt (1 x 27mm) using a real penetrating lubricant (PB Blaster). Remove the crank sprocket gently; do not pry or you may damage the oil pump housing.

Pro tip: Drill/tap two holes in the crankshaft sprocket to use a wheel puller for easy removal.

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Relieve tension on the timing belt tensioner, then remove the old belt. Replace the tensioner and tensioner stud.

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Install the new belt with the marks lined up on the pulleys. Be absolutely sure it’s correct!

* Turn the engine over by hand a few times to ensure nothing interferes. (Note: The marks on belt/pulleys only line up again after many revolutions; do not worry if you are confident in your timing.)
* Adjust belt tension according to the shop manual. This may take a few tries—correct tension is crucial for belt life. Torque the lock nut to spec.
* Replace all removed items and start the engine. (Tip: Replace the 2-inch water hose between the thermostat housing and metal coolant pipe now, as these often leak.)