Below is a summarization of replacing the water pump on a Spridget Roadster.

Buying parts

The first thing you need to do is buy parts.

We offer a great Spridget Water Pump Kit (part no. MMKT0134)

and Specific Water Pumps HERE

To find all the parts mentioned in this article Look Right --->

There are two kinds of water pumps that go in these motors:

• Early 948cc, 998cc and 1098cc motors/engines came with the short impeller (.325).
• Later 1098cc and 1275cc motors/engines came with the longer impeller (.625).

The longer impeller protrudes further into the engine block for increased flow. The early motors took the short impeller (.325), but the longer impeller (.625) will fit earlier motors. Some of the longer impeller pumps have been known to “rub” the vanes depending on which motor the pump is going into. 

Here is how to check:

1. Fit the pump without gasket prior to installation.
2. Check clearance between the vanes, and the block. Tip: use chalk
3. Grind away slag or casting to fit.
4. Again, test fit the pump without gasket before installation.

Looking around, I also saw some with iron impellers vs steel impellers. 

"What is the benefit of having a longer, high flow water pump with a iron impeller?"

The impeller in these aluminum bodied pumps is long lasting cast iron, not cheaper stamped steel as in many aftermarket pumps. Use of a high capacity water pump helps to reduce cavitation in the water jacket, particularly at high RPM, as well as circulating a greater volume of coolant. (this is a good thing)

Since pulling the radiator is a bit of an ordeal, it makes sense to replace a few other failure-prone parts while you have the thing apart. These include:

1. Top hose (#5) HERE (part no. RH1130) from thermostat to radiator
2. Bottom hose (#7) HERE (part no. RH1131) from the pump to the bottom of the radiator
3. Little, ribbed bypass hose (#11) HERE (part no. 12A1093/MS) that goes from the pump to the engine head Note: The clamps (#11a) that came with this hose from Moss Motors were too small to be fitted. I ended up using screw-type clamps I had lying around.
4. Six-bladed plastic fan (part no. 12G1597) in favour of the stock metal fan (#12). The fan provides significantly more airflow with no issues, but there’s nothing wrong with the stock fan if you are not having cooling issues (in traffic, etc). If you purchase the plastic fan, you will also need to replace the bolts with longer ones. You want 1/4″-28 (fine thread) bolts that are 1/2″ longer than stock. They are available HERE (part no. HZS515)
5. Water pump gasket (#15) sometimes comes with the pump, can be found HERE (part no. 88G215) find the gasket for a Midget 1500 HERE (part no. 138701)
6. Fan belt (#13) Compare to your existing belt. The numbers are dimensions in mm. Earlier cars want a shorter belt. Belts can be found HERE
   
   

   Fan Belts

7. Radiator cap (#3) HERE
8. Radiator tappet (#2) HERE (part no. 660-020) If yours isn’t leaking, don’t bother replacing it. It can be a bear.
9. Coolant. It’s hard to find coolant for brass radiators. Here’s why. I ended up using a universal type which I intend to change out in two years.
10. Thermostat (#9) HERE (part no. GTS102) and gasket (#10) HERE (part no. GTG101) Mine were fine, so I didn’t bother. It’s also not a necessary part of water pump replacement, but if you have the coolant out of the car, it makes sense to change it (they’re cheap, you're already there).

Removal

1. Drain the radiator. Easiest way is to remove the radiator cap, open the tappet on the bottom of the radiator, and wait for the coolant to run out. Failing that, just detach the lowest hose and do your best with a bucket and the spray.
   

   Radiator draining through tappet

2. Disconnect the two hoses
3. Remove the front grill. Four screws on top, and then two screws on the bottom that are accessible by sticking a screwdriver down behind the grill after pulling it forward a bit. This will provide access to the top screws on the radiator.
   

   Grill screws

   

   Behind-the-grill screws

4. Unbolt the radiator. There are four bolts, and each is a bit sneaky. I don’t think the ones on my car were OEM, so your mileage may vary.
   

   Bottom rad bolt

   

   Bottom radiator bolt

5. Unclamp and detach any hoses attached to the radiator.
6. Pull the radiator straight up. It may require some man-handling, but make sure nothing is snagging when you pull it out.
7. At this point, I took a garden hose, stuck it in the top hose leading to the thermostat, and sprayed water through the motor. The bottom hose spit green, then brown, then finally clear, at which point I stopped. Very satisfying. It will go much faster if you remove the thermostat.
   
   

   Garden hose to the motor

8. Loosen the fan belt tensioner bolt (it’s in a sliding metal track beneath the generator). If you don’t loosen it enough, you won’t be able to get the belt off the pulleys.
9. Loosen the two generator hinge bolts, and remove the front bolt.
10. Remove the four fan bolts, fan, and pulley.
11. Unclamp and remove any hoses attached to the water pump.
    

    Old pump

12. Remove the four bolts from the water pump–The two longer bolts go into the two holes in the thicker side of the pump.
13. The water pump is held on straight by two small posts that protrude from the engine block into capped holes in the water pump. My pump required some persuasion for removal, so I tapped it with a mallet–from the engine side directly towards the front of the car–and it popped right off.
    

    Pump socket (with posts)

Inspection

At this point, you might as well have a look around and clean off any gunk you see. I found more corrosion in the motor than I wanted to, but nothing that terrified me. You might want to shoot a hose through the engine block at this point, just to clean out any lingering old coolant. I took the time to flush the radiator with a hose, and clean all the mating surfaces with a power drill and a wire bristle bit. To tell the truth, I did this with every bolt that came off, but I’m kind of a lunatic.

I attempted to replace the tappet on my radiator and failed. When I tried to thread the new one in (it was a very tight fight to my wrenches), I ended up stripping some threads. I believe this was because the tappet was closed (and the valve piston was pushing outwards on the bolt). I got nervous and simply put the old one back in.

Installation

Installation is the reverse of removal, with a few notes:

• Clean the mating surface behind the pump to provide a good seal for the gasket.
• Before adding coolant, spin the pump by hand for a 20 seconds to seat the carbon race water pump seal against the back face of the impellor. This discourages any leaks at start-up with your new water pump.

When you install the pump, there is a bit of a dance with the bypass hose. Install the hose onto the pump, put the gasket in place on the motor (some like to use a sealant on the gasket as well, I went without), and then slide the bypass hose onto the motor head will sliding the water pump straight forward on the posts.

• When you put the pump bolts in, and get each one hand tight, then each one snug with a wrench, then each one torqued to about 25 ft-lb or so..
  

  Look at that rock star

• Tensioning the fan belt is part art, part science…You want to get the hinge bolts just tight enough where you can still move the generator, but barely. Then, you just cram a big screwdriver back there, leverage the generator out to where you think the tension is correct, then tighten the adjustment bolt (on the metal slide). Once tight, check your tension. It should be 1/2″ on the longest side. As long as the belt isn’t slipping, they like to be loose. When you have good tension, tighten up the hinge bolts.

• When installing the pulley/fan bolts, increase the torque gradually as you work your way around and around to each bolt multiple times. The plastic fan seems to squish a bit, and you want everything even.
  

  Honking, new-hotness fan

• Some will install the bottom radiator hose before installing the radiator. I couldn’t see how they did it, so I opted to install the radiator, then install the hose from under the car. All it takes is a clever position, yelling, and swearing. Remember to put the clamps on before getting the hoses on!

After that, your little British car will be problem free forever with unicorns sitting shotgun.