Back to the Technical Articles main page
Back to Cooling section

Engine cooling

Author: John Hopwood

Click on the images to see a larger version.

Let's start off from basics – please bear with me:

There are two main sources of heat generation in our engines – Friction and Combustion. There are also two main methods of heat dissipation – Lubricating oil and Coolant.

Let's consider Friction first, as this is by far the least of our problems (or should be!). Heat is generated by rotary friction in bearings but mainly by linear friction between pistons and cylinder walls, cam followers, etc. This heat is carried away by the circulating lubricating oil, although some is transmitted through the cylinder walls to the coolant. Lubricating oil is then cooled by heat conduction through the crankcase and sump to the outside air. Now, let's consider the effects of combustion. In simplistic terms – each cylinder receives a charge of fuel and air once each cycle, it is then compressed and ignited, producing a rapidly expanding 'fire' of hot gasses which pushes the piston down. This is our power source. These hot gasses are then expelled on the next upward piston stroke, out through the exhaust valve and port into the exhaust system. During these two strokes the combustion chamber and cylinder wall are exposed to very high temperatures, which are kept within acceptable limits by heat conduction through the metal into the coolant.

OK so far? – well, I did say it was a simplistic explanation!

We now have a dollop of hot coolant in the engine, which rises to the top of the radiator where it is then cooled by air-flow through the fins and by so doing drops to the bottom due to the increased density (weight). This cooler coolant then goes back into the bottom of the cylinder block and the process is repeated – this is known as the Thermo-siphon Cycle. Now that we understand what is going on, we can immediately see the limitations of this very basic system. The hotter the engine gets, due to working hard, the more the coolant temperature rises and the need for a higher flow-rate through the engine and radiator to keep things in check. Sadly we don't have the benefit of a water-pump to compensate for this, so any restriction to coolant flow in our thermo-siphon will produce a result halfway up Park Rash that resembles a steam loco, blowing its cylinders down before leaving the platform. I have just realised that there may be members out there thinking Park Rash is some kind of affliction requiring the application of a special cream!! Not so – it is that delightful hill, climbing out of Kettlewell towards Leyburn – 3 miles of bottom gear (1in4) with a couple of hairpin-bends thrown in for good measure – a challenge to man and machine (and great fun). Many are the Austin 7's (and others) I have seen with a 'boil on' here, including my Pearl before I got her cooling sorted.

Now let me give you the advice I promised you and this is all about maximising the coolant flow-rate and heat transfer through the radiator – and later on, the engine. It is essential that the radiator is not partially clogged with lime-scale crud and the easy way to check this is to warm the engine up, then rub your hand sideways across the surface starting from the top – take care it will be hot. If you have a cowled rad, (Ruby type) and need to do this 'engine side', make sure you stop the engine first, or you may find out why the fan blades are painted red (to hide the blood?)!! Repeat this at intervals downwards until you reach the bottom, where it will be cooler. There should be an even temperature across the radiator on each pass. If there are any colder areas, this indicates a flow restriction or blockage. It may be possible to clear this by back-flushing – best done with the radiator off the car and a mains pressure hose-pipe. It is a good idea to do this anyway on a regular basis, especially if you had to poke a wire up the drain tap to get it flowing! If not, it is a job for the radiator specialists. I would like some feed-back here from anyone who knows of another DIY method of restoring flow. I have heard mention of filling the radiator with vinegar or Johnsons Toilet Duck lavatory cleaner, but not had cause to try! Well, I suppose if the Toilet Duck doesn't shift the lime-scale you will have the satisfaction of knowing it has killed 99.9 % of all the known germs in there!! Anybody tried such 'dodges'? Please let us know.

It is equally important to keep the fins clear of flies etc. to maximise air-flow (and heat transfer) through the radiator and this can also be done whilst you have the radiator off the car. Just squirt water through from back to front – you will be amazed at the muck that comes out. We also need a good fan-belt, at the correct tension, to drive the fan. When I know I am going to be doing some serious hill climbing, as on the Yorkshire Experience, I fit a 4 blade fan and this has proved to be very effective at compensating for the reduction in forward motion and lower air-flow through the radiator.

Now let's think about what is going on inside the engine: Coolant flows from the radiator into the block via the side water connection, where it flows upwards and through between the cylinders to the hotter side of the engine – or it will providing there is a way through!

Clear gaps between the cylindersMake sure the gaps between the cylinders in the casting are not scaled up – they should look clear between, as shown in Picture 1. If they are not, then it will be necessary to chip away through using a thin chisel – I find a broken machine hacksaw blade does the trick nicely. Ask if you want one.

I mentioned 'the hotter side of the engine', this is where the manifold sits and it is due to the hot gasses passing through the exhaust ports. The worst area by far is in the centre, because there are two exhaust ports side-by side here and the cooling way between them is often blocked with scale and/or the manifold stud – I don't think this cast hole was big enough in the first place! If you do not want to disturb the side water connection, at least do the following – this can be done easily with the head off whilst doing a de-coke.

Water-way sizesDrill out the water-ways in the block and head to the sizes shown in Picture 2. When drilling the block, particular attention should be given to the holes marked 3/8 and more especially the 5/16 one in the middle of the row between the two exhaust ports. If you are doing this with the manifold studs in place, great care will be needed to drill past them, as the drill will tend to 'snag' – start off slowly with a smaller drill first.

How far to drill downPicture 3 shows how far down you need to drill to clear a way through to the transverse water-way below the ports and just above the tappet chest – it is essential you get this far and it is indicated by the drill 'going easy'. Do make sure you stop here or the consequences will be dire!! If you have the manifold studs out, then make sure when re-fitting that they do not protrude into the water-ways or you will undo all the good work – threads in liquid do nothing anyway – other than obstruct coolant flow!

So, that is all there is to it and really all we can do to ensure maximum coolant flow in our basic system, whilst retaining authenticity.

There are other benefits in having a cool-running engine – not just saving you the embarrassment of stopping on a hill wreathed in steam – a cooler engine produces more power due to the increased density of the in-coming charge of fuel and air.

But that could be the topic of another dissertation sometime in the future?

Download the article