MECHANICAL OVERHAUL

of AC's post-war 2 Litre Saloon

Cooling System


This section will be completed at a future date.

The AC's cooling system is non-pressurised and thus runs at a slighlty lower temperature than a modern system. Typically around 75 deg.C. The original radiator caps were brass with a very fine thread. It takes ages to unscrew! AC modified a lot of ACs to take more modern quick-release caps, but without pressurising the system. There is an over-flow pipe, which is where steam issues if the coolant boils over.


Original brass radiator cap, and its sponge rubber seal.


The alloy passages corrode eventually, depending on how well maintained it is. Coolant tends to stagnate in the rear of the water jacket, causing any debris to be deposited in a fine honey-comb structure. Thus, circulation gets worse and the risk of head-gasket failure increases.

As part of a restoration, it will be a matter of routine to have the radiator re-conditioned with a new core and also remove limescale from the cylinder-head.

The water pump is mounted on the side of the block, and pumps coolant into the engine. This pump can be a source of trouble.

The top radiator hose is very short, and should have a large bulge to permit lateral movement of the engine.

Corrosion

If the cylinder head has been removed for attention, then that is a golden opportunity to clean out all the debris in the water-jacket. Then take measures to avoid further corrosion. Ideally, use purified water, such as reverse osmosis ultra-purified water. If there's no risk of freezing, you can put in some anti-corrosion additive. If you do need antifreeze, keep the percentage down to the minimum required, since water is better at cooling on those hot days. And don't use long-life anti-freeze which can wreck vintage engines. There is some debate over the pros and cons of waterless coolant, and I would avoid it unless some positive data emerges from any experience in AC engines? Using a corrosion resistant coolant can be a problem if the water-pump leaks - as it often does in an AC! See further down the page. One of the worst things for causing corrosion, is draining the coolant out and leaving it without filling straight away. Also, constant topping up with water, if there is leakage, also introduces more dissolved oxygen. Oxygen dissolved in water is largely driven out when it heats up, which is why you don't want fresh water added too often.



Thermostat(s)



The engine thermostat is of the bellows type, probably filled with alcohol. It is rated at 78 deg.C. which refers to the temperature at which it is fully open. The valve disc is screwed onto the stem and then soldered to lock it in place. If the solder joint fails, it might unscrew slightly and never fully close. It is easy to repair if this has happened, using a small blow torch, some plumber's solder and some flux paste. Melt off the old solder and clean up the thread and surrounding areas thoroughly (with abrasives). Apply some flux to both threads. Screw the disc on until the valve is closed - but only gently so. Then heat up with the blow-torch until solder will melt into the thread and surrounding area, and quickly remove the heat.


Thermostat with valve disc removed. Note temperature rating.


The thermostat can be tested over a kitchen stove (especially if you are single/divorced!). Strictly speaking, it should be suspended in a pan of water, but I only had a shallow pan. So I rested the unit on the bottom of the pan. Having the thermostat sitting directly against the pan, means that heat conducts through the metal casing causing the thermostat to begin to open early as you heat up the water. It then hesitates to open further, because it conducts less heat when open. It will then continue to open from about 55 deg.C. until fully wide at 78 deg.C.



While the water cooled, I tested the electric thermostat for the cold-start carb. You need to keep water out of this unit. In fact, the heat has to conduct through the alloy casing from the water passage to the thermo unit. This causes a time delay, which might explain why it's rated so low at 35 deg.C? On mine, 70 deg.C. water took about 30 seconds to trigger the thermo to open. It finally closed again as the water got down to 36 deg.C. The effective time delay, as heat conducts through the casing, no doubt stops it from opening and closing repeatedly. All very clever low-tech stuff!


Electric thermostat for cold starting.


I haven't opened up the electric thermostat, so I don't know how easy it is to repair. For the bellows thermostat, it is probably not worth trying to repair a failed bellows. Either seek out a second-hand replacement, or see if you can convert to a more modern wax thermostat?

The engine thermostat has a locating screw in the side. Unfortunately, this is steel, and its tip is exposed to coolant, where it goes into a slot in the brass part of the unit. Mine broke off (as did the stud extractor I tried!), hence the over-sized hex screw (stainless steel) seen in the photos.

There is a stub pipe on the right of the engine thermostat with a steel adaptor screwed in. This is for the temperature gauge bulb to fit into. The adaptor tends to get encrusted with scale and debris, clogging the small clearance between it and the bulb. So, remember to clear it out so that the bulb will be surrounded by water for a reliable gauge reading.

Water Pump



A problem with the water-pump design, is that the pulley is not bell-shaped as seen on most cars. That means that the fan-belt load is off-set from the ball-race bearing, which in turn, places a side-loading on the plain bearing at the rear of the spindle. Any grit getting into that plain bearing will accelerate its wear. The seal is made from carbon, and is spring-loaded, pressing against a bronze ring. Both of these wear out. Sealing is impaired if the shaft is skewed due to bearing wear. There is an oil-seal behind the ball-race bearing, but this is more useful for keeping water off the bearing. If water leakage is bad, this seal won't last long, and the bearing will fail.

Leakage is most likely at low revs, since the water seal relies partly on centrifugal force to keep the water from escaping. A new seal needs bedding in, and this can be done on the bench using an electric drill to spin the pump. Even so, it is worth rust-proofing the chassis just under the pump!

Useful tip: Carry a spare fan-belt that is short enough to drive the fan only. If the water-pump bearing(s) fail, you can fit the short belt, and continue on your journey at reduced speed and topping up coolant if necessary. The radiator is tall enough for thermo-syphon circulation.

AC were very strict about fitting a very thin paper gasket where the pump outlet meets the block. Mine was reluctant to seal, and one might need to check the mounting studs and lands. Possibly add a thin shim washer to one of the studs, to get the gasket joint to seal better?

The pump cannot be lubricated once it is assembled, unless it has been modified.

I hope to add more to this section if/when I can suggest any improvements to the pump.


Inside a water-pump with the shaft partially
extracted, to reveal the seal assembly.


Electric Fan?

Depending on your local climate, an electric fan might be a useful added extra. This might be automatic or manual, with or without the original mechanical fan.

There's a common mis-understanding about power consumption of a fan. It is usually assumed that power input is proportional to the RPM cubed. But that does not hold true if the fan is in a forward moving car. The faster the car is travelling, the greater the reduction of power input to the fan. That means that removing the mechanical fan, won't make a big improvement in fuel economy.

A dis-advantage of the original fan. is that it is too far away from the radiator to move the air efficiently, and also has a tendency to draw it in near the centre, and fling it out like a radial fan. The more obvious drawback, is that it runs slowly when it is needed the most. That is, the time lag in removing engine heat, means that slowing (or stopping) the car after a fast run, releases a lot of heat when the fan and pump have slowed. The pump is less of an issue on the AC, because it functions well in thermo-syphon 'mode', because the radiator top tank is high up. An electric fan can assist during those "heat soak" moments. It can also help to lower the under bonnet temperature, to cool the intake air, fuel system... and driver's feet! The actual heat-flow will increase at first, as the engine cools, although the air temperature might be lower.

The electric transmission for such a fan is inefficient, but offset by being switched off most of the time. You would need to buy a fan with a fairly low current draw. The AC's generator is rated at 13 amps, and so you want to find a fan that draws far less (continuous rating).

I'm planning to keep the mechanical fan, but also install an electric one to the front of the radiator (hidden and safe), with manual control (and maybe a timer to switch it off?).

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Website started 29th December 2006