TypePressurised radiator. Thermo-siphon, pump assisted and fan cooled.
Thermostat setting (Standard)82°c (180°F)
Thermostat setting (Cold climates)87°C (1880F)
Blow-off pressure of radiator cap (early models)7 lb/sq.inch. (.49kg/0m2)
Blow-off pressure of radiator cap (later models)13 lb/sq,inch
Cooling system capacity51/4 pints (61/4 pints with heater)

General Description

The engine cooling water is circulated by a thermo-siphon, water pump assisted system, and the coolant is pressurised. This is both to prevent the loss of water down the overflow pipe with the radiator cap in position and to prevent premature boiling in adverse conditions. The radiator cap is pressurised to 7 lb./sq. in. and increases the boiling point to 226°F.

On later Cooper models the pressure was increased to 13 lb/sq.in. If the water temperature exceeds this figure and the water boils, the pressure in the system forces the internal part of the cap off its seat, thus exposing the overflow pipe down which the steam from the boiling water escapes thus relieving the pressure. It is, therefore, important to check that the radiator cap is in good condition and that the spring behind the sealing washer has not weakened. Most garages have a special machine in which radiator caps can be tested.

The cooling system comprises the radiator, top and bottom water hoses, by-pass hose to return water to the block when the thermostat is closed, heater hoses (if heater / de-mister fitted), the impeller water pump, (mounted on the front of the engine it carries the fan blades and is driven by the fan belt), the thermostat and the two drain taps.

The system functions in the following fashion, Cold water in the bottom of the radiator circulates up the lower radiator hose to the water pump where it is pushed round the water passages in the cylinder block, helping to keep the cylinder bores and pistons cool.

The water then travels up into the cylinder head and circulates round the combustion spaces and valve seats absorbing more heat. Then, when the engine is at its proper operating temperature, the water travels out of the cylinder head, past the open thermostat into the upper radiator hose, and so into the radiator header tank. The water travels down the radiator where it is rapidly cooled by the rush of cold air through the radiator core.

As the radiator is mounted in the wheel arch the fan pushes cold air through the radiator matrix. The water, now cool, reaches the bottom of the radiator, when the cycle is repeated.

When the engine is cold the thermostat (which is a valve which opens and closes according to the temperature of the water) maintains the circulation of the same water in the engine by returning it via the by-pass hose to the cylinder block. Only when the correct minimum operating temperature has been reached, as shown in the specification, does the thermostat begin to open, allowing water to return to the radiator.

Cooling System – Drainage

With the ear on level ground drain the system as follows:

  1. If the engine is cold remove the filler cap from the radiator by turning the cap anti-clockwise. If the engine is hot having just been run, then turn the filler cap very slightly until the pressure in the system has had time to disperse. Use a rag over the cap to protect your hand from escaping steam. If with the engine very hot, the cap is released suddenly the drop in pressure can result in the water boiling. With the pressure released the cap can be removed.
  2. If anti-freeze is in the radiator drain it into a clean bucket or bowl for re-use.
  3. Open the two drain taps. When viewed from the side the radiator drain tap is on the bottom right-hand side of the radiator, and the engine drain tap is halfway down the rear right-hand side of the cylinder block. A short length of rubber tubing over the radiator drain tap nozzle will assist draining the coolant into a container without splashing.
    NOTE:- On some later models a drain plug is fitted at the bottom of the radiator, rather than a drain tap.
  4. When the water has finished running, probe the drain tap orifices with a short piece of wire to dislodge any particles of rust or sediment which may be blocking the taps and preventing all the water draining out.

Cooling System – Flushing

With time the cooling system will gradually loose its efficiency as the radiator becomes choked with rust scales, deposits from the water, and other sediment. To clean the system out, remove the radiator cap and the drain tap and leave a hose running in the radiator cap orifice for ten to fifteen minutes.

In very bad cases the radiator should be reversed flushed. This can be done with the radiator in position. The cylinder block tap is closed and a hose placed over the open radiator drain tap. Water, under pressure, is then forced up through the radiator and out of the header tank filler orifice.

The hose is then removed and placed in the filler orifice and the radiator washed out in the usual fashion.

Cooling System – Filling

  1. Close the two drain taps.
  2. Fill the system slowly to ensure that no air locks develop. If a heater unit is fitted, check that the valve to the heater unit is open, otherwise an air lock may form in the heater. The best type of water to use in the cooling system is rain water, so use this whenever possible.
  3. Do not fill the system higher than within 8inch of the filler orifice. Overfilling will merely result in wastage which is especially to be avoided when anti-freeze is in use.
  4. Only use anti-freeze mixture with a glyceride or ethylene base.
  5. Replace the filler cap and turn it firmly clockwise to lock it into position.

Radiator Removal – Inspection & Cleaning

The radiator on Mini Cooper models is renewed in a slightly different manner to that on standard models. The radiator on standard models is removed by different methods depending on whether a one or two piece cowling is fitted.

  1. Undo and remove the two bolts and spring washers from the bonnet side of each of the two hinges. Carefully lift the bonnet off and place it to one side.
  2. Drain the engine as previously described.
  3. Models up to car No. 3940 had a one piece cowling, which is removed with the radiator as below:
    1. Undo and remove the cowling upper support bracket, and take off the two bolts which hold the bottom of the cowling to the bracket on the engine mounting.
    2. Unscrew the clip on the upper radiator hose at the thermostat housing outlet pipe, and re- move the hose off the pipe.
    3. The bottom water hose incorporates a bonded in take-off tube which carries water to the heater (where fitted). Disconnect this tube and then completely remove the bottom hose by unscrewing the clips at each end.
    4. Undo and remove the four bolts which hold the cowling to the radiator, and lift the radiator and cowling out.
  4. The two piece cowling was fitted from car No.3941 and is removed with the radiator as described below:
    1. Undo the bolts holding the radiator support bracket to the thermostat housing.
    2. Undo the clip holding the top hose to the radiator, pull off the hose, undo and remove the six bolts which hold the top half of the cowling to the radiator and remove the top half of the cowling.
    3. Undo and remove the two bolts holding the cowling to the bracket on the engine mounting.
    4. Undo the clip holding the lower hose to the water pump, and pull the hose outside the bottom half of the cowling. Note:- Disconnect the heater hose from its take off to the lower hose (where fitted).
    5. Lift the radiator and bottom part of the cowling out of the car.
  5. The Mini Cooper radiator is removed as at ‘4= ‘ above, after undoing the eight Phillips sore ‘s which hold the front grille in place, and removing the grille. Before the radiator is lifted out of the car, it is also advised to remove the fan. Pull the radiator back against the wind valance, undo the four bolts and washers which hold the fan in place, and remove the fan.

With the radiator out of the car any leaks can be soldered up or repaired. Clean out the inside of the radiator by flushing as detailed in the section before last. When the radiator is out of the car it is advantageous to turn it upside down for reverse flushing. Clean the exterior of the radiator by hosing down the radiator matrix with a strong jet of water to clear away road dirt, dead flies, etc.

Inspect the radiator hoses for cracks, internal or external perishing, and damage caused by over- tightening of the securing clips. Replace the hoses as necessary. Examine the radiator hose securing clips and renew them if they are rusted or distorted. The drain taps should be renewed if leaking, but ensure the leak is not because of a faulty washer behind the tap. If the tap is suspected try a new washer to see if this clears the trouble first.

Radiator Replacement

To replace the radiator proceed as follows:

  1. Fit the radiator bottom hose to the bottom pipe (except on cars before No. 3941) but do not tighten the clip completely.
  2. Fit the top hose in position on the thermostat outlet pipe. Do not tighten completely.
  3. Replace the radiator and bottom cowling. (Complete cowling in the case of cars before No. 3941. Replace the fan on Mini Cooper models).
  4. Fit the lower hose to the input side of the water pump (fit the lower hose to the radiator outlet pipe as well on pre- 3941 models), fit and do up the engine mounting to radiator support bracket and tighten the lower hose clips.
  5. Reconnect the heater hose to its take off on the I bottom hose (where applicable).
  6. Replace the top half of the radiator cowling and
    do up the securing bolts.
  7. Refit the top radiator support bracket to the then – no stat housing.
  8. Fit the top hose in position on the top radiator pipe and tighten the clips at each end of the hose.
  9. Replace the bonnet.
  10. In the case of Mini Cooper models refit the radiator grille.
  11. Fill the system with water. Start the engine to
    pressurise the system, and check for leaks.

Thermostat Removal, Testing & Replacement

To remove the thermostat partially drain the cooling system (4 pints is enough), loosen the upper radiator hose at the thermostat elbow end and pull it off the elbow. Unscrew the three set bolts and spring washers from the thermostat housing and the two bolts and spring washers from the radiator cowling. Lift off the radiator support bracket and the advance and retard pipe clip. Lift the housing and paper gasket away. Take out the thermostat.

Test the thermostat for correct functioning, by immersing it in a saucepan of cold water together with a thermometer. Heat the water and note when the thermostat begins to open. This should be at 82°C. It is advantageous in winter to fit a thermostat that does not open until 88°C. Discard the thermostat if it opens too early. Continue heating the water until the thermostat is fully open. Then let it cool down naturally. If the thermostat will not open fully in boiling water, or does not close down as the water cools, then it must be exchanged for a new one. If the thermostat is stuck open when cold this will be apparent when removing it from the housing.

Replacing the thermostat is a reversal of the removal procedure. Remember to use a new paper gasket between the thermostat housing elbow and the thermostat. Renew the thermostat elbow if it is badly corroded and eaten away.

From engine No. 9F-SA-H-4167 on Mini Cooper models a modified non-bellows type thermostat is fitted When replacing this unit ensure that the threaded stem faces upwards.

Water Pump Removal

  1. Drain the cooling system.
  2. Remove the top radiator support bracket by un- doing the bolts securing it to the radiator and thermostat housing. Remove the top half of the cowling.
  3. Either remove the top and bottom radiator hoses and remove the radiator and cowling completely as previously described, or just remove the two bolts which hold the bottom of the radiator and cowling to the support bracket and pull the radiator back against the wing valence to give clear – once.
  4. Loosen the dynamo securing bolts and remove the fan belt. Note:- On later, post August 1961, engines fitted with sixteen blade fans it is recommended that the four bolts holding the fan to the pulley are 11 redone and the fan and pulley removed. The easiest way to remove these bolts is to hold the head of a bolt with a spanner, and to then rotate the fan clockwise.
  5. Undo the clips on the by-pass hose and the clip holding the lower radiator hose to the water pump inlet pipe.
  6. Unscrew the four bolts which hold the pump to the front of the cylinder block and lift the water pump away.

Water Pump Dismantling & Replacement

If the water pump starts to leak (make certain it is not the bypass hose) the pump can be dismantled and rebuilt, or an exchange reconditioned pump fitted.

To dismantle the pump proceed as follows:

  1. Remove the four bolts and spring washers which hold the fan blades and fan pulley in place. With these removed, pull or tap off the hub from the end of the spindle, taking great care not to damage it.
  2. Then pull out the bearing retaining wire.
  3. The spindle and bearing assembly are combined (and are only supplied on exchange as a complete unit), and should now be gently tapped out of the rear of the water pump.
  4. The oil seal assembly and the impeller will also come out with the spindle and bearing assembly.
  5. The impeller vane is removed from the spindle by judicious tapping and levering, or preferably, to ensure no damage and for ease of operation, with an extractor. The oil seal assembly can then be slipped off.

Reassembly of the water pump and replacement on the front of the block is a reversal of the above sequence. Five points should be noted which have not already been covered:

  1. If the oil seal assembly shows any sign of dam_ age or wear it should be renewed, and the gasket between the water pump and the cylinder block should be renewed every time the pump is removed.
  2. There is a small hole in the bearing body cover. When assembled it is vital that this hole lines up with the lubrication hole in the pump body. To check that this is so, prior to reassembly remove the greasing screw and check visually that the hole is in the correct position directly below the greasing aperture.
  3. Re-grease the bearing by pushing a small amount of grease into the greaser and then screwing in the greasing screw. Under no circumstances should grease be applied under pressure as it could ruin the efficiency of the oil seal.
  4. The fan belt tension \”must be correct when all is reassembled. If the belt is too tight undue strain will be placed on the water pump and dynamo bearings, and if the belt is too loose it will slip and wear rapidly as well as giving rise to low electrical output from the dynamo.
  5. The bypass hose should always be replaced at the same time as the pump. It is very difficult to replace the hose after the pump is in position. It can, however, be done, and the trick here is to slip the securing clips over each end of the tube, compress the tube in a vice, and then quickly fit the hose before it has time to expand to its normal length again.

Fan Belt – Removal & Replacement

If the fan belt is worn or has stretched unduly it should be replaced. The most usual reason for replacement is that the belt has broken in service. It is therefore recommended that a spare belt is always carried. Replacement is a reversal of the removal sequence, but as replacement due to breakage is the most usual operation, it is detailed below:-

  1. Loosen the two dynamo pivot bolts and 13898 met Qn the adjusting link and push the dynamo in towards the engine.
  2. Fit the belt by manoeuvring it over each fan blade in turn, through the small gap at the top front side of the radiator.
  3. Slip the belt over the crankshaft, dynamo, and water pump pulleys.
  4. Adjust the belt as detailed in the following section and tighten the dynamo mounting nuts. Note:- After fittings new belt it will require adjustment 250 miles later.

Fan Belt Adjustment

It is important to keep the fan belt correctly adjusted and although not listed by the manufacturer, it is considered that this should be a regular maintenance task performed every 6,000 miles. If the belt is too loose it will slip, wear rapidly, and cause the dynamo and water pump to malfunction. If the belt is too tight the dynamo and water pump bearings will wear rapidly causing premature failure of these components.

The fan belt tension is correct when there is 2 inch. of lateral movement at the midpoint position of the belt between the dynamo pulley wheel and the water pump pulley wheel.

To adjust the fan belt, slacken the dynamo securing bolts and move the dynamo either in or out until the correct tension is obtained. It is easier if the dynamo bolts are only slackened a little so it requires some force to move the dynamo. In this way the tension of the belt can be arrived at more quickly than by making frequent adjustments. If difficulty is experienced in moving the dynamo away from the engine a long spanner placed behind the dynamo and resting against the block serves as a very good lever and can be held in this position while the dynamo bolts are tightened.