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XPAG Water Flow–Cooling

This article can also be read in conjunction with :

XPAG Engine Temperatures and Cooling.
XPAG Overheating.
Overheating or Vaporisation.
Warming Up times.


The Cooling System.


When Claude Bailey redesigned the water flow of Morris Motors 10hp engine in 1937-38, to that of the ‘X’ Series, he adopted a system still current today. On the older pre-war cars the engine was cooled by the action of hot water rising, called ‘thermo-syphon’, which was quite satisfactory for the feeble side-valve cars about then. That was why these pre-WW2 cars all had such tall radiators. The water was fed into the bottom of the block, to rise up through the head, and up into the radiator header tank. The block would be cool, and the head very hot.


XPAG Water Flow.


Claude added a water pump to the Morris engine to boost the natural action of the hot water to rise, but he only pumped the water through the cylinder-head. The cylinder block was also cooled by water, but only half way down the bores. The cooling area was that of the surface wiped by the piston rings. There was no water flow through the block on the original 1140cc engine, it relied entirely upon thermo-syphon. The water was collected from the bottom of the radiator, cooled by its fall down the copper tubes, to be pulled through the water pump driven off the fan belt. The water pump then assists the water, ( note, not forcing it as the oil pump does with the sump oil) along a duct on the offside of the engine underneath the manifolds, to the rear of the cylinder block. The water then enters the rear of the block to be fed directly up into the back of the cylinder head. It comes up through the large slotted holes at the back of the block, which can be seen when the head is off. In this way the hottest part of the engine is cooled first, that of number four cylinder. Due to the way an in-line engine is laid out the rear always runs hotter. Later XPAG engines had odd holes drilled into the cooling core around the cylinders to assist in removing the sand core after casting as well as to aid cooling of hot areas.


XPAGcooldiag2bThe water flows forwards through the head, to the front where it rises into a thermostat housing. The thermostat will control the engines running temperature at about 82 degrees ‘C’. The fact the block has no flow other than the natural rising of hot water, and the head has all the flow, keeps those exhaust valve seats cool. The major cooling of the engine is done virtually all through the head. This area is the most important on the engine, and the hottest area. The block will be kept pretty hot, and the overall temperature graph for the head and block will be about the same, ie no big differences between the metal’s temperature, leading to less stress and more even expansion. It also means the exhaust valve can be run pretty hot with less risk, getting more power from the fuel. In fact all very up-to-date in its design.



The Thermostat.


From the thermostat the water rises to the header tank, where it cools and drops through the cooling tubes in the radiators matrix, to start all over again. The thermostat is a bellows controlled affair, that expands as it is heated up. On the ‘X’ series it starts by stopping any flow through the radiator, by blocking the entry up to the radiator. Instead it gives access to a small by-pass pipe on the side of the thermostat housing, to ‘recirculate’ the water back to the water pump entry, missing out the radiator. This is to get the water and engine heated up quickly upon starting it. The cool water will run round the engine, via the by-pass pipe until it gets to the temperature that will open up the thermostat, to let the now hot water up into the radiator. On hot days the thermostat will be fully open, on very cold days it may only open up a little, controlling the water flow to keep the engine at its optimum working temperature. When the thermostat is fully open, it closed off the by-pass hose.


If you modify your engine to run with a modern waxstat type thermostat, you need to blank off the old by-pass hose, or otherwise it will operate all the time, and cause very hot running, which will cause exhaust valves to burn out. The modern unit is tiny, and does not have the necessary sleeve to close the by-pass hose aperture. Note that a waxstat can be had for a few pounds, where as the correct Nuffield thermostat will now cost tens of pounds, and is no where near as reliable or long lasting.


XPAG in Old Age.


Normally, the system is no problem to look after. The correct ratio of anti-freeze solution will stop any cylinder blocks being cracked in the winter, but a good by product is that good quality antifreeze also reduces internal corrosion. The cast iron of the engine will corrode slowly, and produce sludge and iron oxide flakes. The will often sink into the cylinder block and slowly block up the bottom of the water jacket. After a time this can cause hot-spots and therefore stress areas in the casting. Some can collect at the rear of the block, where the water is fed in via the off-side duct. Luckily, the cylinder head has a pair of rear and front alloy bolt on covers. This is excellent for giving the head a good wash-out. A hint this is wash-out required is when you drain the radiator, and the last few pints of water are a thick brown colour. You may well have been suffering a hotter engine, and perhaps even boiling occasionally. The radiator will need a good flushing out, best when it is removed from the car, and washed out in the reverse direction to the normal flow. Using a proprietary radiator flushing mixture will help, follow the instructions on the packet. Beware people who suggest you drill holes inside the rear cylinder block water duct to improve cooling of the cylinders. They obviously have little understanding of the water flow as designed.


Hoses do not last forever, and the very large diameter one from the thermostat housing to the radiator on the non-pressurised engines, ( pre TF & 4/44,) will show crazing around the jubilee clip areas. The others will just age-harden till they crack and leak. rummage around the average auto-jumble will usually provide enough pipes to renew the straight sections, but the large top one you will need to get from a M.G specialist or your Register. The YB and TD top pipes are shorter than the YA, TB, & TC. The wire type clips that you find on these hoses are often of poor quality, and will cut into the hose. It is better to use the flat jubilee type clips.


Other than the TF and Wolseley 4/44, the cooling system of the ‘X’ series of Morris engines, in this case the XPAG version, run un-pressurised. This means the boiling temperature is that of the ambient air pressure, 100 degrees. If the thermostat operates at 82 degrees, the water around the exhaust valve seats will be very, very close to boiling point. As the system is open to the atmosphere, it is natural that the hot water will evaporate. That is why on these early M.G’s the radiator level needs checking weekly. On modern cars with pressurised, sealed systems, the water cannot evaporate, so need less topping up. This is one of those old facts lost to modern drivers, and one to bear in mind. Once that water level drops too far, the engine will run even hotter and boil. This can cause exhaust valve to burn, and even the engine to seize up. Therefore, check the water level at least every week of a parked car, and daily if in use. The TF and 4/44 run with a 4psi sealed system, so do not suffer water loss naturally. Ensure you do not alter the antifreeze to water ratio, by weakening it. Have a mixture made up for use for topping up.


The Water Pump.


XPAGCoolingdThe water is assisted on its way around the system by the water pump. This is driven off the fan belt, and is a simple impeller type, relying on the centrifugal action to move the water. This is just like the water pump in your washing machine. It cannot displace positively, as the oil pump does. The oil pump can actually wreck itself if allowed to, where as the water pump would just ‘stall’ and not pump if its exit was blocked. The water pump ‘assists’ movement. It runs in two ball bearings that require an occasional bit of grease from your grease pump. To keep the water inside the pump it has a carbon faced seal. These usually last for many years, but the first signs of it wearing out is water dripping from under the pump. If you miss this, the water gets into the bearings and then rusty water will drip out, and it makes a nasty noise. Usually before this though, enough water had dripped away for you to notice the water level has suddenly begun to drop rather often. Over-tightening of the fan belt is a sure way to ruin both the water pump and dynamo bearings quickly.


To check the pump, it is worth trying to rock the fan blades, these are far enough away from the bearings to give quite a good indication of bearing wear. If they do rock a little, and the pump does not leak water, it could last for ages, but I would carry a spare in the boot, ( include a gasket as well. ) The rules of life say the pump will fail on a busy road, probably in France, or deepest Cornwall.


A squeaking water pump is one of two faults. The first is a loose fan belt, slipping in the pulley groove. The other is a noisy carbon seal. This can be cured by putting a capful of brake fluid into the radiator. The fluid mixes with water easily, turning white like machine cutting fluid. ( It also removes paint very efficiently, take care.) Modern antifreeze that is also a corrosion inhibitor will cure a squeaking water pump carbon seal as well.


The Radiator.


Old age problems of the cooling system are discussed above, in the accumulation of sludge in the block and head, and the age hardening of the hoses. Another is the rusting away of the radiator fins between the radiator down-tubes. Not only do they rust away, the fins get blocked with dead flies. Using a thin PLASTIC tie-wrap to poke out the flies is one way, another is to soak the lot with water, then blast the flies out from inside the engine bay, forward through the matrix. Take care to straighten any bent fins, though if you fin they break off through corrosion, start saving for a re-cored radiator.

The Fan.


Although you need to be odd to do so, there have been cases of people assembling the fan the wrong way around. It can be fitted so it tries to blow through the radiator, instead of pulling the air towards the engine. Whilst this does not have mush effect when stationary, as the fan is still ‘cooling’ the water, once you drive away you get a stalemate. The air coming in from the forward motion of the car is met by the air going backwards from the fan, the result is a boiled engine. The fan only cools the engine below about 25 mph. Above this speed the ram-air effect of the cars speed is responsible for most of the cooling effect. Driving over 30mph the car would cool itself quite happily without the fan being fitted.That is why many modern cars have viscous couplings on their fans, or temperature sensors to switch them on if and when the engine gets hot. Alas, on the XPAG the fan works all the time, using about two brake horse power at its maximum speed. Like all early cars, the flat pressed steel fan blade is very un-aerodynamic, and wastes energy in its inefficiency. It is basically a paddle, not a fan. But without it the engine would boil at slow speeds and when the car is stopped in traffic.


A Heater?


On a few cars a heater is fitted. These are usually of the round heater-matrix type seen in thousands of Morris Minor 1000’s, with a small fan behind worked by a rotary rheostat switch. The heater should be connected across the water pump, ie so the pump assists the flow through it. In summer it can be an extra cooling device for a hot engine, but you will suffer from roasted feet. Poor heater output will be an air-lock in the long hoses. To cure this, disconnect one hose by the engine end, ( plugging the pipe you took it off, ) and fill the hose full of water whilst holding it about level with the header tank, until you affect the header tank level. Often, the very disconnecting of the hose, and the resultant cooling water escaping, cures the air-lock. If it is still cool, the thermostat may well be broken in the open position, and not controlling the engine temperature. This is often accompanied by water vapour inside the rocker cover not being ‘boiled off’. It looks like mayonnaise, and is water and oil mist mixed. It indicates the engine is running too cool. You should not be able to put your hand on the radiator cap, without pain. If you can, then check that thermostat. To run cool wears the engine bores quickly, uses more petrol, ( the extra petrol washes the oil off the cylinder walls, ) and makes your exhaust gas fail any roadside pollution test, with excessive hydrocarbon particles.


On the TF and 4/44 the hot water from the engine is taken via the rear cylinder head alloy plate, to the heater, and feeds back into the system at the cast iron elbow just before the water pump. Kits sold in the 1950’s take the water from a junction below the thermostat housing, and feed back at the same cast iron elbow. The latter heater system runs hotter as its flow has been via the head as well.

Temperature Gauge?


The ‘Y’ Type never had such a gauge fitted. On the other M.G’s with XPAG engine fitted, be aware that these simple gauges are notoriously inaccurate, and only a guide. Often the temperature sensor is actually fitted in the radiator header tank. This is miles away from the engines hot area, so you will get a cool reading indicated. The sensor is far better fitted to the cylinder head. There you will get a more accurate reading, and see any faults sooner. The header tank fitting has a serious fault, as when the water level drops below it, it reads cool on the gauge. When in fact the water is leaking away and the engine is actually boiling, and about to seize up! The reading on these gauges is often the reason owners think their car is running hot. If an un-pressurised cooling system runs hot, it boils. This is obvious as lots of steam will be seen. If the gauge reads high, but the engine is not boiling, your gauge is lying.




XPAGheadgasketsbOverall the cooling system of the ‘X’ series of engines has no vices. During the engines production life the internal core of the cooling spaces between the cylinders and around the exhaust valve, was modified. The holes between the head and block were also modified, causing the problems of an owner needing to know what head gasket to fit. To be safe, the oval hole gasket can be used on both round hole and oval hole engines, or if a round hole head is fitted to an oval hole block, and visa-versa. But never use a round hole head gasket if either the head or block has oval holes, or if both are oval holes. The reason being, the folded edges of the copper over the steel backing, is the seal. If the ‘hole’ in the gasket is not gripped between the head and block faces, then you will get water leaking into the sump via the push rod holes, etc. On both types of head gasket, the actual combustion chamber ‘holes’ are the same, ( though bigger on the TF1500, of course.) The improved cooling around the exhaust valve seats by making the core area bigger, resulted in the head getting long reach plugs.


To the uninitiated, when stripping down the cylinder-block with the core plugs removed, they will see small holes drilled through into the main core around the cylinders. The main water-flow in the XPAG is through the head, from the rear end-fed from a gallery under the exhaust manifold from the water pump. These holes were drilled by the foundry to remove the sand used in the casting process of the core and to permit the block to drain completely of water during servicing. They are not normally part of the cooling flow, they are far too small anyway to seriously affect it. They also stop any air-locks forming around the almost static water around the cylinders where thermo-syphon cooling is used. Should you be tempted to drill out these holes as they will look blocked with corrosion, they are only about 3/16” diameter, and behind them old rust particles may have consolidated into a solid mass. This is dangerous as you will try to drill through this hard ferrous oxide thinking it is the casting, only to find you have drilled right through into the cylinder bore. Be warned.


Neil Cairns.