Cooling System

The cooling on the K series engine is somewhat sensitive to all the components functioning well and correctly.  The two main issues consist of the relatively small water capacity of the cylinder head and poor flow round the liners in the block.  This means that the rest of the cooling system has to be well designed, to avoid the dreaded head gasket failure.

The K series cooling system has the thermostat on the inlet, right next to the water pump, and uses a bypass line to control the thermostat.  The system in the Midget is like the following diagram:

Using this diagram, the hot water exits the head at the top right of the engine.  This goes into the pipe known as the coolant rail, which splits the hot water into the bypass pipe and the pipe to the radiator.  With the thermostat closed, the hot water circulates around the back of the engine (top in diagram).  In my system, this is made from ½ inch copper pipe and is routed through the heater matrix, incorporating a bleed screw at the highest point.  From the heater matrix the hot water goes into the sensing side of the thermostat housing, passes past the thermostat bulb and enters the water pump and engine.  When this water in the bypass reaches 88 degrees C then the thermostat will start to open.  This allows cold water to exit the bottom of the radiator and mix with the hot, thus providing cooler water to the water pump and the rest of the engine.  When the thermostat is fully open, the flow is almost entirely through the radiator with a little through the bypass allowing the heater to work.

The two black lines are the air bleeds, the one from the head being the most important pipe in the system.  This exits the inlet manifold and must be allowed to bleed air out of the front of the cylinder head. There is a little ball bearing in the inlet manifold in this line which stops the head from filling with air from the expansion tank.  It is vitally important that this does not jam shut, or the head will not bleed. With my system, I can squeeze the top radiator hose and hear the little bearing clicking about, so that is a good check.  The bleed from the radiator is less important, I have it as the expansion tank I use is from a metro and has 2 inputs at the top.

Everything in this system is designed to get the air out; the head bleed comes upwards to the expansion tank, the bypass goes upwards to the bleed point at the back near the heater, the coolant rail goes upwards to the radiator.

The sensors (one for the ECU and one for the gauge) are situated in the bypass line.  This means that they always give an accurate representation of the temperature of the water exiting the head.  The fan is controlled by the ECU, and switches on at 103 degrees C and off at 98 degrees C.  This temperature will not be reached if there are any leaks causing pressure loss, so if there is boiling where there should not be, check the expansion tank cap.  This can, and does, stick open, meaning that the water only ever gets to 100 degrees and boils in the tank.  Coolant should be 50:50 water : antifreeze.

The coolant rail:

The bypass pipe:

I have an 82 degree thermostat fitted instead of the standard Rover 88 one.  This means that the radiator is introduced into the cooling system at a lower temperature.  If the thermostat sticks shut, the engine will overheat in about 5 minutes.  If it sticks open, then the engine will take a long time to get to operating temperature.  I am also using an alloy thermostat housing from the same metro as the expansion tank.


The matrix used is the original Midget one.  The placement of the heater in the bypass pipe results in a fast heater warm-up time, and also means that the heater can be used as an extra radiator if needed.  The heater used the motor and fan from the Rover.

It has to be nice and narrow to fit between the distributor and whatever the air intake system is. It doesn’t work very well.  This is because I didn’t leave enough space between the axial fan and the casing of the heater, so it can’t push enough air through. I need to re-make the heater surround to have a bit more space.

It’s also on radiator number 3 – number one corroded away (no idea how old it was to start with) and I managed to get a stone in the replacement within a week. Number 3 still in and going strong, and still the original radiator.  Track work makes it run warmer, but not too hot.