I recently replaced the water pump on my roadster. You can read of my trials and tribulations here and here. Below is a summation of the experience. It’s by memory, so it’s almost surely wrong. Your mileage may vary.
There are two kinds of water pumps that go in these motors. Some have a longer impeller that protrudes further into the engine block for increased flow. These have been known to “rub” the vanes depending on which motor the pump is going into. I believe up to early 1275cc motors took the short impeller, but the longer impeller will fit earlier motors with a bit of grinding. I believe (Moss Motors part number) 434-540 is short-impeller and 434-545 is long-impeller, but I do not know for sure. Looking around, I also saw some with iron impellers vs steel, iron being a bit stronger (though usually it’s the bearings that go, so I’m not quite sure what the benefit is).
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:
- Big hose that goes from the pump to the bottom of the radiator (#7)
- Little, ribbed bypass hose that goes from the pump to the engine head (#11). 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.
- Six-bladed plastic fan in favor of four-bladed metal fan (#12). The fan is $50 and 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 at most hardware stores (Home Depot, OSH, etc).
- Water pump gasket (#15, usually comes with the pump)
- Fan belt (#13). 460-870 is type AVX 10x865LA, 460-990 is type AVX 10x900LA. Compare to your existing belt. I believe the numbers are dimensions in mm (earlier cars want the shorter belt–460-870).
- Radiator cap (#3)
- Radiator tappet (#2). If yours isn’t leaking, don’t bother replacing it. It can be a bear.
- 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.
- Thermostat and gasket (#9 and #10). 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 might make sense to change it (they’re cheap).
- 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.
- Disconnect the two hoses
- 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.
- 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.
- Unclamp and detach any hoses attached to the radiator.
- Pull the radiator straight up. It may require some man-handling, but make sure nothing is snagging when you pull it out.
- 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.
- 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.
- Loosen the two generator hinge bolts, and remove the front bolt.
- Remove the four fan bolts, fan, and pulley.
- Unclamp and remove any hoses attached to the water pump.
- Remove the four bolts from the water pump–The two longer bolts go into the two holes in the thicker side of the pump.
- 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.
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 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, give the pump a few spins.
- 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.
- 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.
- 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.