Water pump
Datsun 1200 used a traditional vane-type water pump, driven by the fan belt. The same pump was used from 1966 Datsun B10 to some 1982 Datsun B310 up through 1994 Nissan 1200 Sunny Truck. However, air conditioning-equipped models used a different hi-performance pump.
See also: Heater Motor Fan
Overview
Datsun 1200 style long-nose pump:
Heavy-duty (short-nose) H72 pump. Can be interchanged with long pump if used with matching short-nose spacer, pulley and fan.
If you use regular water in the engine, it will look like this:
Use coolant/antifreeze to to prevent the rust. NOTE: This is mild oxidation, and has not damaged the pump.
Paraut-brand pump
Comparison
There are two basic water pumps for A-series engines, each of which uses a different pulley and fan. 
NOTE: Both setups are exactly the same length block-to-fan, so they interchangeable as a set along with the pump.
Long-nose VS short-nose pumps
http://i297.photobucket.com/albums/mm219/ddgonzal/Datsun%201200/wiki/5681a.jpg
1. Short pump with short pulley, spacer and high offset fan
2. Long pump with long pulley and low-offset fan
You can't mix and match these parts, as the short pump uses a different bolt pattern (the spacer is an adapter allowing the fan to bolt up).
- Datsun 1200 is long shaft, large pulley (spins slower)
- Newer is short shaft with spacer, small pully (spins faster)
- Note that the pulley bolts are wider apart in the newer pump
- Newer pump has studs for the spacer
- Some aftermarkets pumps have a sheet-metal impeller, smaller but more agressive
- Genuine Hitachi and Atsugi pumps (both A12 and A15) have a cast iron impeller
Cast iron vs. stamped steel impellers. Cast iron shaped impeller is superior at high RPM.
Datsun 210
The heavy duty H72 water pump was installed on all 210s (North American B310) starting with 1980 model year vehicles (late September 1979 production).
from engine serial number A14-433246-B * HLB310-693967 * KHLB310-563501 * WHLB310-047416
from engine serial number A15-028246 * KPLB310-201907 * WPLB310-021027
from engine serial number A12A-112069 * LB310-0211047 * LB310-044592
Factory Air Conditioning
NOTE: Dealer-installed Aircon may have any sort of fan, pump and pulley. For example, 1200s may be fitted with flex fan or metal fan (neither of which are Nissan parts).
Main: Air conditioning
Factory A/C used the H72 style (short-nose) water pump, with either a fixed-blade fan or clutch-fan.
B120
Datsun 1200 pickup (B120) used the H72 aircon water pump & pulley, but with a fixed-blade fan:
OP.COOLER PART NUMBERS 21060-N0110 FAN 7804-8109 <0,0> 21060-G1210 [330 mm ?] 21060-G1210 FAN 8110- 21064-G1660 SPACER-FAN 7804-7907 <0,0> 21064-G1615 21064-G1615 SPACER-FAN 7908- 21051-H7200 PULLEY 21010-H7225 WATER PUMP 7804-8109 <0,0> 21010-H7227 21010-H7227 WATER PUMP 8110- 11720-H7200 BELT 7804-8109 L=970 11720-H7201 BELT 8110- L=990
B210-B310
B310 Air Conditioning (also 1977-1978 B210.A14)
Matched set: Water pump, pulley, fan and clutch
Part Numbers
* Standard Duty (type 180) long nose * Heavy Duty (type H72) short nose used on most factory A/C models (starting 1977) used on all 210s (A/C and non-AC starting 1980)
B110 Water Pump Part Numbers 21010-18003 (BA010-M6626) ASSY-PUMP WATER B110 $45.67 21010-18026 ASSY-PUMP WATER WITH GASKET -0770 rebuildable 21010-18027 ASSY-PUMP WATER WITH GASKET 0870- > 21010-18026 21010-H1025 ASSY-PUMP WATER WITH GASKET 0971-1072 (1972 model year) * interchages with <> 21010-H18027 * 08120-62010 BOLT-HEX FIXING FAN (4) * 21069-21000 WASHER-COCK FIXING FAN [bend-tab lock strip]
Gasket (new and old style use same gasket) * 21014-H1001 GASKET-WATER PUMP USD $4.13 * fits A10, A12, A13, A14, A15, A12A
http://www.rockauto.com * Beck/Arnley 1310366 $34 Short type
Others 21010-H5025 (BA010-M6626) ASSY-PUMP WATER WITH GASKET 1172- $45.67 USD * Used through 1982 model years B110, B210, B310
* Bosch 96005 with 1.970" fan pulley bolt pattern.
B210, B310 OPTIONAL Aircon A14 From 0876 * 21010-H7225 (BA010-H7289) ASSY-PUMP WATER WITH GASKET FOR AIR COND From 0876 Optional A14 $57.17 * Bosch 96024 with 2.362" pulley bolt pattern
Specifications
Casting marks * Hitachi 13 long (smooth shaft): TO * Atsugi 13 short (ribbed shaft): X11
Datsun 1200 Pump vs Short Pump * pulley flange bolt centers: 1.970" vs 2.370" * pulley flange diameter: 2.42" vs 2.95" * pulley flange center locating hub: 0.85" vs 1.24" * Height of flange outer face from block: 2.38" vs 1.98"
Fan
Datsun 1200 uses a tough plastic 4-blade fan of 310 mm diameter (12.2 inch). 330mm fan was Datsun 1200 OPTION FOR TROPICAL USE.
Main: Fan
USA dealer option flex-fan
Pulleys
These are the common pulleys:
- Datsun B110: 129 mm (~5.05") diameter, 2.33" high, locating hub 0.85"
- Datsun B310: 107 mm (~4.21") diameter, 1.74" high, locating hub 1.98"
Use a matched set of pulley and water pump. Both sets end up the same distance from the radiator.
1. Long pump with long pulley and low-offset fan
2. Short pump with short pulley, spacer and high offset fan
These are not mix-and-match parts, as the short pump uses a different bolt pattern (the spacer is an adapter allowing the fan to bolt up).
The long pump (B110) uses a fan with positive offset. The short pump (B310) uses a fan with negative offset, but with a spacer. The overall height of fan from block face is the same for both models.
21051-18000 PULLEY-FAN & WATER PUMP * Original Style: 1.970 inch bolt centers * A10, A12, A13, A14, A12A * B10, B110 * B210 (with spacer) * B310 to 0779 (with spacer) 11720-18002 BELT-FAN L=838 [For B110]
B310 North America pulley (for short nose pump, use with spacer):
21051-H9900 PULLEY-FAN & WATER PUMP * New style: 2.362 inch bolt centers. * Spacer has 1.970 holes on the fan end * B310 from 0879 * A12A, A14, A15 11720-H9910 BELT-FAN(987) A12A Cal From 0879 [For B310] * replaced 11720-77701 BELT-FAN(980) 
The two combos of pump/fan/spacer interchange as a whole combo. The newer type allows the fan to be unbolted while the radiator is still in the car.
Presumably the A-series moved to a small diameter pulley to slow down pumping flow, perhaps to prevent cavitation at high speeds.
A third pulley is the B310 Aircon setup:
21051-H7200 PULLEY-FAN & WATER PUMP (D=103) * B310 * OPTION A14,A15, JAP.A12,A13 * No spacer, but instead a clutch * 7-blade fan (350 mm) 11721-H7210 BELT-FAN(970)(FOR AIR-CON)
A fourth pulley is:
21051-H8500 (appears to fit long-nose pump, with no spacer) smaller diameter than the 180 type pulley? * B120 8910- * B310 JAPAN.(A14S.S,K+A14E) -7909 * B310 JAPAN.A14S.S,K.MT 7910-
Billet Pulley
Custom made
Belts
Main: Fan belt
Replacement
Leaking? Time to replace the pump
After unbolting
Clean the gasket surfaces carefully. Remove all traces of prior gasket
Use the thin paper gasket, and torque the bolts to spec. It should not be very tight, these are small bolts.
Repair
1200 water pumps made after August 1970 are non-serviceable.
Up to July 1970, a kit was made to repair the pump
Early Datsun B10 pumps were rebuildable. Seal and seat with gasket.
Electric Water Pump
Water Pump delete cover plate
http://i297.photobucket.com/albums/mm219/ddgonzal/Datsun%201200/album/23394.jpg 
Datsun 1200 spec is 12 liter/minute @ 5 meters. Outlet is 1 inch inside diameter. It requires the maximum flow at peak HP, so it depends on how much HP you will build in, and the radiator size/efficiency.
For drag racing, you can turn off the pump while accelerating. For street use, an electric water pump decreases the reliability (because the standard mechanical pump is virtually 100% reliable).
lamb_daiquiri: I've got an 80lpm Davies Craig pump on my A15 with a pump controller. Most of the time it just pulses (when I can hear it). The controller replaces the thermostat for regulating coolant flow by adjusting pump speed based on a temperature sensor. Once up to temperature I don't see the needle move at all. Don't ever run the pump dry, they have ceramic seals in them that don't like to be run dry.
I've also got a electric fan switched off a 70°C thermo switch mounted on the radiator outlet. Comes on when it needs to.
http://i297.photobucket.com/albums/mm219/ddgonzal/wiki/cp4678800517011537614.jpg 
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