1965 Scottish Rally, R. Smith & S. Taylor
Works Imp, 1965 Scottish Rally with the engine lid gapping

The Imp Site

Cooling System

Don't blow the eau or boil the oil.


Too hot
An engine that overheats will take the temper out of the rings and they won't seal as well. It may also cause pistons to burn out and even cause a seizure. Or it may cause the block/head to warp and the headgasket to blow...
Carburetion: A dense charge of air can deliver more power. Running an engine too hot means that the incoming charge of air picks up too much heat. Therefore the charge into the cylinder will be less dense. But the engine will run smoothly anyway, because the hot air vapourizes the fuel easily.

Too cold
A cold engine has not enough heat to easily vapourize the petrol. At full throttle combustion can still take place efficiently. (Full throttle simply means everytinh gets hotter). But at low revs improper vapourization (even when it's in the cylinder) means it won't run smoothly. Cold engines tend not to tick over.
Engines with a good carburation system are usually quite capable of delivering full power even when they are little more than luke warm.

Just right
The ideal temperature for the Imp engine is 70°C. This would strike a happy compromise between such factors as

Oil needs to get hot enough to vapourize the water (blowby past rings) that gets into it. That won't happen until about 90°C...

Attend to any problems as soon as they develop.
Always let the engine warm up gradually, before you do any traffic-light racing.

    For my Hillman Imp I use Castrol oil - 1960s ad
    Oil in the carter would have to have an action temperature of about 100°C or a bit more to get rid of the water and the fuel that gets blown passed the pistons.
A busy engine will mean a temperature in the carter of up to 130°C. For some modern engines temperature in the carter has been measured to be 170°C.
source: Oils & Engine [Castrol technical course of instruction]. - ISBN 9066749245


Hot engine oil causes damage - oil, when it overheats, thins out rapidly and loses its lubricating properties (excessive wear, noise, sludge formation,fumes, etc.). This is a well known fact, and is readily rectified by fitting an oil cooler.

Cold engine oil causes damage. - oil, which is cold or running below the correct operating temperature, causes loss of power, oil contamination by condensed petrol and water and a disasterous cold sludge formation will block oilways.

Paint the dipstick handle and tube white. This will make it easier to check your oil in bad light.

Oil cooler

Higher-performance models like the Imp Sport or Stiletto had an oil cooler fitted as standard. They're the twin hedgehog type. These make an enormous difference during long, fast motorway drives, reducing the oil temperature to a level which will maintain its lubricating qality.

Fit a hedgehog oil cooler or fit a propietary after-market kit, perhaps a thirteen row oil cooler such as the one made by Serck. Mount it in a way so it gets the most cool air and won't get hit by flints or stones.
Overheating the oil can be as bad if not worse than having the water overheat. But oil temperature is easier to control.
Oil temperatures by Nick Cleak.

When you convert an Imp to Imp Sport, the easiest way to do the take off for the oil cooler is to get a sandwich plate (eg. Demon Tweeks) or a take off from a sport. Buying an Imp Sport oil cooler assembly would be the easiest and possibly the cheapest way to go. Just the oil filter block plus a cooler (but not the pipes) might be bought for £10.
When you fit the Sport oil filter housing, you have to have one longer stud because the housing is much deeper. It's readily apparent when you look at it. And if you've got a Sport exhaust manifold already attached you can't fit the Sport housing without taking off (or at least largely loosening) the manifold.
The stud isn't found anywhere else on the engine.

If you were to fit a oil pressure gauge, you would know about it, if your oil pressure would be less than adequate. If the engine gets hot and the (air) cooling isn't good enough, the oil pressure may go down as low as 25 lbs psi. Engine wear results. An oil cooler is probably a good investment.
On the other end: too high oil pressure - keep the revs reasonable while the engine is still cold.

Coollist: Oil Pressure (Feb./Mar. 1999); Oil pressure problem (Nov. 1998); Oil pressure (Apr. 1998)

Oil thermostat

On racing engines an oil thermostat is not a good idea. It would reduce the oil flow and under arduous conditions, it would allow the temperature of the oil to get a little too high. Anyway, racing engines will see very regular oil changes and their bearing life is probably more affected by thin oil than a road engine.
Normal Imps, that are used both summer and winter, may benefit from an oil thermostat. The valve is governed by viscosity and temperature, and regulates oil to around the 75°-80°C mark. It opens at 77° (Serck). Or get a four row radiator and a 71° thermostat... Of course any radiator only works if enough cool air goes through.


Oil as lubricant


Warning light
If your Imp has only a Sensor-switch temperature warning lamp, it is wise to make sure it is functional. Sometimes it's just plain defective, sometimes the temperature sensor for a gauge system gets fitted, in stead of one for the switch system. This would make the warning light glow more, the faster you drive.
It would be better to install a temperature gauge like the Chamois has.


new radiator in the making

Keep the radiator clean. When you can see your radiator is blocked up, it's too late. Prevent damage and make it routine maintenance to clean it every 5 months or 5,000 miles or so.

    clogged pipes
    coventry radiator
   into a new radiator
   into a new radiator

New radiators are expensive, so try to keep yours in good condition. Use antifreeze as the manual says. Don't use tap water for coolant, it will fur-up the waterways. Look inside your kitchen kettle to see what tap water can do.

Keep the cooling system clean externally as well as internally in the battle against overheating. Take it out and clean it thoroughly once a year (Spring clean). If you really don't want to take it out: work plenty of engine cleaner into the blocked fins with a soft brush, then hose it off with water to clean out the grunge.

The early Imp Sport and Stiletto were fitted with a 4 row radiator, but later ones got a cheaper 3 row, which was less efficient.
If your engine gets too hot, you may consider putting in a 4 row higher efficiency radiator. Or a three row one that has the vertical tubes more widely spaced and more cooling fins per inch. But radiators don't come cheap. And a badly designed four row-radiator will be worse at getting rid of heat than a well designed three-rower. Pipes too narrow, pipes too close together, that sort of thing.

Some suppliers rebuild standard radiators.

Reversing the air flow / D. Henshaw. - Impressions 1984, Jan.
With graphs.
and more on the subject by Steve Williams. - Impressions 1985, August

The rear radiator relies on the fan for airflow. Keep the fan clean, too.

The radiator cap should release at 7 psi (according to the Workshop Manual), hence the water doesn't boil until 106°C (±3°).
(Other source: around 9psi.)
Early Imps lost water without an obvious trace. It was caused by the first production batch of radiator filler caps. They weren't holding their stated 4 psi. For a period 7 psi caps got fitted (so production wasn't held up). On some cars the extra pressure soon caused the water-pump seals to blow.

Front radiator

If your Imp has any overheating problems, putting a front radiator in should cure them. Unfortunately it is awkward, time consuming and expensive.

For a conversion to a front radiator, an Alfa Sud item might do, mounted at 45 degrees. Or if you don't want to cut the car around, or have pipes running through the footwell you might decide on a Custom build radiator to use all the available space behind the fan. Add another fan behind it to suck the air through. (The mega rad / D. Couldry. - Impressions 8 (1988), no. 3)

For race Imps a front mounted radiator (usually Hillman Hunter/Avenger) was often used in combination with a swirl pot. With a front radiator it is essential that some provision is made to allow to vent air-locks front the cylinder head.

If you want to do a front radiator conversion on an imp it may be worth looking at some of the different motorbike radiators. Yamaha bike alloy radiator up front with alloy pipes underneath which are the same diameter as the heater pipes at the engine end.
Darcy and Russell Maddock both run Yamaha bike radiators up front in the air box (open topped) piped into the heater pipes. They still have our normal rads in position.

The fitting of a front mounted radiator to an Imp has several advantages. The most important is that the rear fan can be removed, saving up to 7bhp on a highly tuned engine (@ 7000rpm). A front radiator also adds a certain amount of stability to what can otherwise be an over-light front end.
Use one that has an thermostatically controlled electric fan.
Mount the rad as close as possible to the front of the car.
The rear radiator can be retained in its original position for use as a header tank. Join the two rads together, using aluminium pipe of at least 1" internal diameter. (copper or rubber pipe can be used...) Lead the lines as near to the centre line of the car as possible, inside or under the floor. Inside is safer, outside is cooler.
Fill the system up and jack up the rear to bleed it.
Use a 7 pounds pressure cap.
Remove the bonnet rubber to let the hot air out.

There is the problem of a luggage compartment full of rain-water. But there are ways around that...

As hot air goes up, the best solution for venting a front radiator would appear to be bonnet vents. If you have the courage to do painful things to your expensive bonnet, do plan it carefully. And have a look at the vents off a Fiesta Turbo or so.
If the Imp does not have a tendency to overheat, perhaps because you are one careful driver, it may be enough to just raise the bonnet slightly and cut out the spare wheel well.

1999 June: coollist discussion

The Simca 1000 also used a rear radiator, but the airflow went in the opposite direction. While this radiator was much the same size as the Imp one, Simca used a 1000cc engine (also high compression) and overheating problems of any kind were unknown... (?)

Using plain water in the radiator will lead to corrosion and build-up of sediment in the waterways. Use antifreeze all year round to prevent internal corosion silting up the cooling system. The composition of the coolant is critical. It must be a mixture of water and a good quality ethylene glycol-based antifreeze, and a corrosion inhibitor (Not something like 'Bar's Leaks' which seals leaks automatically).

Haynes: use fluid which conforms to BS 3150 specifications. Use 2.75 pints of anti-freeze with 8.5 pints of water for temperatures down to -12 °C.
Manual: Bluecol

The more anti-freeze you add to the coolant, the more viscous it becomes. Put more than 50% anti-freeze in the mixture and it may even refuse to pump around if you have a front radiator. The Imp water pump does not build up a lot of pressure but simply provides a certain pressure out. More anti-freeze means less cooling after a point.

  Imps came with one of three different radiator tags:
  • filled with
    cooling inhibitor
    Do Not Drain
  • Rootes
    Do Not Empty
    Bluecol AA
  • filled with Chrysler
    cooling inhibitor
    Do Not Drain

Take the weekly check of radiator fluid level very seriously, because a falling coolant level gives an early warning.

Fitting a 'restrictor' in the bypass hose of the cooling system will

Glycol coolant - corrosive?
Soluble oil - rots the hoses?
Penrite Classic Car Coolant is non-glycol based and they claim it is not corrosive.

The original factory specification coolant for temperate climes was a straight inhibitor - Geigy 'C' - coloured blue. (Not oily). It went into all NZ Imps ex factory. It was discontinued in the mid-80s, as antifreeze with a better reputation became available.

Loosing water
If the Imp looses water but has no visible leaks (you've checked the heater too), then run a pressure test on the system - using a radiator cap with a handpump (like a blood pressure rubber bulb). Look for puddles. (maybe put the Imp on old newspapers first, to make wet spots more noticeable).

Consider bypassing the heater control valve (location: on the rear wall of the luggage compartment, below the rear edge of the bonnet). This can leak badly. It does not restrict the flow of the coolant: it is rated at 2 gallons/min. @ 2000rpm.
Bypassing it means you can't shut off the supply of hot air to the cabin. (Once the weather starts to warm up, you would disconnect the heater air intake pipe.)

Check whether water is mixing with the oil - there would be 'mayonaise' in the oil filler neck.

Trying to cure coolant loss by converting to a fully pressurized system (using a separate catch tank) will not be successful. Find the underlying cause of the problem.

Radiator drain plug
The plastic type fitted to other Chrysler UK cars and some Fords can be used as a direct replacement even though it looks completely different, also try your local motor factors or radiator repair shop as both brass and plastic types are very common parts for a variety of cars and trucks...


  The thermostat doesn't really keep the temperature constant, but it maintains a temperature between ± 85 and 106 °C., depending on how much power (and therefore heat) the engine produces.
If more heat is produced, temperature increases and the thermostat opens wider. More water going through the radiator cools the engine down. The thermostat won't close as much as before extra power was produced. A new balance has come to be, at a higher temperature.

The thermostat maintains the temperature. When the engine's normal operating temperature is exceeded, the thermostat opens, allowing water to flow through the radiator. Once the temperature drops, it closes, and so on.
A lower temperature thermostat will not compensate for a blocked radiator. If the radiator cannot keep the temperature of the water below that of the thermostat, the thermostat cannot keep the engine from overheating.

Standard an Imp is fitted with an 88°C. This means the engine temperature is well into the 90's (up to 106°C ?) before it is fully open.

For the summer, for nice warm days, a 71° thermostat would be best. But not in winter, as the heater wouldn't be quite as effective and you'd freeze. If the car is slow to reach the operating temperature, this will cause more engine wear. It used to be common practice to fit different units summer and winter.

Perhaps a compromise: fit an 82° stat, as this will still give you a good heater in the winter. Purchase a new 82° thermostat and gasket.
While the radiator is out of the car it will be a little easier to get at the thermostat.

Thermostat cover
How to go about removing the thermostat cover (part no. 7010105): Richard McIntosh, Impressions 8 (1980), no. 6

Smear a little vaseline on the mounting edge of the thermostat, as this will help keep it in place when you replace the cover. - Martyn Jones. - Impressions June 1986

Water pump (QC574)

The Imp water pump is quite a complex unit with the shaft running right through. It doesn't often give problems. Eventually it can leak coolant through the main impeller-shaft seal. Such a problem will sooner turn up if the drive belt has been overtightened (causing premature failure of the bearing and excessive up-and-down movement of the shaft).
The body can crack if it's treated roughly.

To ensure long life:

  1. Make sure that the alignment of radiator to fan-cowl is correct, so that the rubber sleeve doesn't deform the cowl enough to cause the fan-blades to scrape under engine movement
  2. Give it a twirl dry (by hand), to commence bedding the seals
  3. Do the initial running with the pressure-cap cracked loose, or else use a sabotaged cap (so that there's minimal leakage before the seals can bed in)
  4. Keep the pump assembly in its normal working position (i.e. with axis horizontal) for ever after. If you take the unit off for any reason, DON'T park it face-down on the fan-cowl - this is a death-sentence to the front bearing. (A little water will inevitably get to it.)
Given the above, maybe even a Q-H pump might last a while... A good overhaul of a 1970-ish pump would make most of these precautions other that 4) unnecessary, because about then they had generous water-grooves. The late Q-H ones have the same exterior, but the grooves are filled in.

Keep the correct fan belt tension (1" total movement on the belt's longest run), or the water pump will give trouble. Overtightened belts will drastically reduce life expectancy of the pump bearings and seals.
When you replace the water pump, don't take off the fan by undoing the centre nut. Undo the four bolts securing the fan to the pump boss. The replacement pump comes with a new boss.

Ferodo fan belts ref V916 listed as fitting Sunbeam Imp Sport and Chamois 1970 -on, Bond Equip GT 1968-1970 (some, V917 is also fitted to these cars), Triumph Vitesse 2 litre 1966-1967 (to eng H37937E) Commer FC 2266cc petrol 1957-1967

After replacing the water pump don't forget to tighten the centre bolt properly onto the fan. When you have a garage replace it for you, check the bolt yourself.

New: about £75.
Repair kits: about £25
The Imp Club has got a stock of reconditioned pumps: £45, or £40 exchange with your old one.

Water pump bearings (number 6202 EE - double seal) are common and cheap. They are identical to those used in Lucas dynamos and alternators.

There are two water-pump pulleys available. Fitting the smaller one would increase the speed of the pump, as well as the fan.

Coollist archive February 1998: water pump
Coollist archive December 1998: water pump

Electric water pump

Electric cooling pumps are (were?) sold by Ben Boult.

Demon Tweeks sell a kit electric water pump, manufactured by Davies Craig of Melbourne, Australia. It is supposed to be suitable for an Imp. Prices excl. VAT:
Pump £120
Controller £80
Electric thermal switch £48.43
Mechanical thermal switch £20.04
This pump can be set to run with an ignition feed or with an electronic controller cutting in and out like an electric fan. Engine temperature can be adjusted to run cooler for racing or hotter for efficiency. The pump can be set to continue for a short time after engine shutdown, which could cure a heat-soak/cold slug problem.

Water pump seal
You may check whether the pump seal at the fan is leaking by shining a powerful pencil-beam torch among the fan blades. Look for signs of dirty water on the moving blades and around the inside of the cowling.
If a pump rattles when the engine is running, it may be leaking (it may have lost the grease out of the bearing because of hot water leaking through the seal).
As seals harden with age (and hardened seals leak), don't buy pumps that have been on the shelf a long time.

The carbon ring can crack (not uncommon) when putting the waterpumps halves back together, making a rebuilt pretty expensive and time consuming.
It is possible to convert the imp pump to use an industrial seal say from a Grunfus water staged pump (CR range).

Radiator fan

The fan needs a considerable amount of power so to pass a good lot of air through the radiator. Imp fans consume something like 3bhp@5000rpm. (it changes with the revs). It would use far more if it weren't such a very well designed fan with air-foil section blades.
The largest gain in power which can be gained through modification of the cooling system is: getting rid of the fan. The fan is needed because the radiator is not in a favourable position for cooling. Moving the radiator to the front of the car would position it in the way of unaided air flow, if ...


Opinions differ on this one. Use at your own peril.

In stead of using mini fans for reverse flow cooling, one could use the plastic fans from the Austin Maxi. These work well and are easier to fit. They also have the same effect as the Imp fan, thus run closer to the radiator than a mini fan.
The Maxi fan has three holes for bolts like the Imp fan. If the metal collar is removed from these holes, it will bolt straight on to the pump hub - no drilling is required.

    To fit:
  1. Remove old fan and metal disc
  2. Grind down metal disc so it fits the Maxi fan
  3. Remove metal collars from Maxi fan holes and trim blades to clean the cowl
  4. Cut out alternate plastic cowl ribs to improve air flow
  5. Bolt up the Maxi fan and the metal disc and re-fit pump assy
John Doughty. - Impressions Oct. 1986

Impers try all sorts to get more air through the radiator, but a good, well-kept standard system works ok.

To work at its best, the fan must be kept clean. You don't want the air to find resistance from dirty blades. Polishing the fan with T-Cut, Brasso or something would be good. I find that nothing works better to clean it than Biotex Blue. Tough luck if you don't have it in the UK.

More blades make a quieter fan. Usually a fan has 5, 7 or 11 blades: prime numbers to keep the amount of resonance to a minimum. The Imp fan has nine... but still.
You could put a second fan behind the radiator, one that sucks the air through...

A modification that was tried at the factory, involved lining the inside of the fan housing with closed cell foam. When the fan was assembled, it chewed away the excess and created a tight seal. This improved the performance of the fan and, as a bonus, made it quieter.

Kenlowe made deservedly popular fans for other makes. They made the 'Airomatic' fan for the Imp and Chamois, a device which controls engine coolant temperature automatically without the need to spend all your life wasting power on a permanent 18-driven mechanical fan. The Airomatic is supplied as a complete kit which can be bolted on in place of the existing fan blades, using the same bolts.
It is completely automatic in action and was introduced to provide the advantages of the Thermomatic electric/thermostatic device, already popular, at less cost. What happens in this case is that, instead of an electric switch operating the fan and controlled by temperature, the Airomatic's blades feather, absorbing minimum power, until the engine water temperature gets higher than the determined point. Then the blades automatically go into increasing pitch to draw more air until, under really hot conditions, the fan is actually having a greater effect than the fixed blades of the standard component. Advantages? Faster warm-up, less power consumed and a good deal less noise. Write to them at Burchetts Green, Maidenhead, Berks.


Back-flush the heater radiator and check it for leaks. Loosing all the muck will help the engine stay a little bit cooler and (in the winter) will help you stay a little warmer.

  change the bleed valve from an early Imp

The first type of bleed valve (right in the front of the boot) almost invariably left a little air in the system. Using a heater bleed unit from a Mk 2 Imp may improve heater efficiency. To fit the later valve:

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