Leaking Water Pump

SR20 water pumps are not too bad of a job to do. Easily do-able at home with basic tools. 

A quick run down:

1. Drain coolant
2. Remove top hose (to allow shroud to be removed)
3. Remove radiator fan shroud (from the top)
4. Back off the clutch fan bolts (x4) but do not remove them
5. Back off the alternator/waterpump belt
6. Remove clutch fan and pulley
   [Steps 4-6 is done in this order so that you can undo the 4 clutch fan bolts while the pulley is held stationary by the belt/engine]
7. Unbolt the water pump, give it a gentle tap with a rubber mallet or pry with screwdriver making sure not to score the mounting surfaces of the water pump. There will be a lot of coolant in there that comes rushing out so using a big tray underneath while you do this is a good idea.
8. Remove as much coolant from block as you can, just soak it all up with rags.
9. Raise the front of the car a bit more so that no coolant can seep out and interfere with the setting of the sealant
10. Clean the mounting surfaces well, remove all old sealant with a razor, use some scotch brite and brake cleaner/grease wax remover. How well the new sealant sets and the longevity of the seal itself is dependant on how clean these surfaces are, this is not an area to skimp on.
11. Transfer the studs of the old water pump to the new water pump
12. Apply sealant to the mounting surfaces of the water pump and tighten bolts to torque specs. (IIRC I just did it with a 3/8 ratchet by feel, didn't have a torque wrench at the time but it is a good idea to do this and also do the bolts in sequence - from the inside to the outside). I used Permatex Ultra Grey and followed their directions, which was to mount the water pump finger tight and let it set for a while before applying full torque to the water pump bolts.
13. Let the sealant fully set (time depends on sealant) before putting coolant back in.
14. Remember to re-install the drain plug on the radiator.
15. Refit water pump pulley and fan.
16. Refit all belts
17. Refit radiator shroud
18. Refit top hose
19. Undo the top radiator brackets and remove them
20. Slip a large cut out of cardboard (or similar) to block air flow to the radiator while bleeding the cooling system (so that it can reach normal operating temperature thus opening the thermostat and cycling the cooling through for air bleed purposes)
21. Fill coolant up to the top of radiator and run the engine
22. No need to turn the heater on as there is always coolant running through the heater core on S chassis vehicles - don't run the fan as you'll be cooling the coolant
23. Once the lower hose gets hot that means the thermostat has opened and coolant is circulating
24. Undo the bleed bolt at the top hose to head junction until coolant comes out and reinstall
25. Fill the radiator to the brim and close the cap
26. Remember to remove the airflow blocking cardboard and reinstall the top radiator brackets
27. Fill coolant reservior about 1cm above max
28. Check for leaks

Trick here is to make sure the area is bone try and drop the rear end of the vehicle down or raise the front up so no coolant situates itself on the sealant while it sets. There is one little bleed screw on the top hose to the head junction for when you put the coolant in. I use Nissan genuine and wouldn't use anything else, its not expensive, and easy to get.. so why get anything else that may shorten the life of your water pump or offer compromised corrosion protection of your cooling system.

Take note, not all SR water pumps are the same. S13 black/red top run different water pumps to S14/S15!