Technical Information for Gunsmiths and for Classic Car Enthusiasts

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MG Restoration

Cooling System Inspection


Les Bengtson


This time of year in Arizona is a good time to do a complete cooling system inspection to prepare for the hot summer driving season. This is exactly the reverse of the system used up north where the cooling system is checked before the onset of winter. The possibility of freezing is limited in the Phoenix area; our problem is running hot in summer. Thus, we need to make sure the cooling system is in good working order to prevent overheating.


The first step is a physical inspection. Look at the fan belt for cracks or glazing on the part that runs between the pulleys. If it is cracked it will break. If it is glazed, it will slip. Also check the tension. You should be able to move the fan belt about ½" when you push on it between the alternator and crankshaft pulley. This is a fairly firm push, not just move it with slight pressure. Tighten or replace the belt as necessary.


Then, inspect all the hoses for cracking. Merely being fairly new to you does not mean they are "new". I had a hose split that was less than a year old. I suspect it had been sitting in the warehouse for a while before being sold. If the hoses look good, look at the ends where they attach to the radiator, water pump and the thermostat housing (water elbow in Brit.). If you see a build up of crud or an actual leak, tighten the hose clamp.


Look for signs of water leakage around the water pump pulley. This indicates a bad seal. Then, try to shake the pulley or fan back and forth. If it moves, the water pump bearings are beginning to go and you should replace the pump.


Lastly, check the pressure cap to see that the rubber is not cracked and is still flexible. If it is cracked or not flexible, replace it. MG's use the same sized pressure cap as the 70s vintage Chevrolet small block engines. The 16 psi cap can be used on the later Rubber Bumper cars as the required 15 psi cap is hard to find.

The above checks have been performed with the engine cold. Removing the pressure cap when the system is hot or under pressure can scald you. Always wait until the radiator and engine block are cold before removing the pressure cap. We also may have a system that leaks when the motor is hot and pressure builds up in the system or we may have a pressure cap that is bad and not allowing the system to pressurize properly. To check the system out under pressure requires a radiator pressure tester. This is a hand pump with a pressure gauge on it and a flexible line running to a cap which is attached to the radiator filler neck on the chrome bumper cars and the expansion tank on rubber bumper cars.


When the pressure checker is attached to the radiator system, it is pumped up to approximately the system operating pressure. How do you know the system operating pressure? It is governed by the pressure cap whose function is to vent when the system operating pressure is exceeded. Thus, the early cars, which had a seven pound pressure cap, are pumped up to 7 psi, while the later rubber bumper cars, with their 15 pound pressure caps, are pumped up to 15 psi. This puts the system under pressure and any leaks will cause the gauge to drop. Ideally, the gauge should hold pressure for at least 30 seconds. If it starts dropping, it indicates there is a leak somewhere in the system. This should be visually obvious as you should see water dripping (or in some cases streaming) from the leak. If the pressure gauge drops and you cannot see a leak, look at the heater and under the dash. If you are fortunate, you have a bad heater core. If you are not lucky, you have a leak inside the engine, probably a bad head gasket or a cracked cylinder head.


The pressure checker is also used to check the radiator pressure cap by using an adapter which hooks to the end of the pressure checker (similar to a radiator neck with a flair at both ends), then the cap is attached to the adapter. Once again, you pump up the tool and see that the cap vents at the proper pressure and holds pressure below that setting. This indicates that the pressure cap is working correctly. Not all do, even when new. The 16 pound cap I just put on Maria's LE vents at 13 psi. Not a significant problem, but something to be aware of as it reduces the boiling point of the coolant in the system.


Every two years, you should flush out the cooling system and replace the anti-freeze. It will probably last longer and will certainly provide anti-freeze protection for a longer period, but anti-freeze serves several different functions in the cooling system.


To flush the cooling system, get a commercial cooling system flush and follow the directions on the container. This normally involves draining a small amount of cold coolant, adding the flush, running the engine for a period of time to allow the flush to clean out the cooling passages, allowing the engine to cool, draining the system, refilling with fresh water, running the car, allowing the system to cool again, draining again, then refilling the system with anti-freeze and water to the proportions you decide.


The water and anti-freeze are an often neglected area of study. Especially here in Arizona with our hard water and hot summers with mild winters. With this in mind, I would like to discuss them in depth.


Water is water-except when it isn't. Our water in Arizona is "hard" water, containing a large number of minerals. At the cooling system tech session, I displayed a water pump I had pulled off an "Arizona" car. It was filled with a white "crud" which blocked the flow of the pump. This is due to the build up of minerals over time. You may have noticed the same thing in your toilet tank or swamp cooler. NEVER use tap water in the cooling system (or the battery). The best thing to use is distilled water (available by the gallon from any grocery store) or "demineralized" water (available from grocery stores, water stores or the water machines located around the valley). Flushing every two years and using the proper water will save you an expensive radiator hot tanking in the future. You can, if you desire, purchase pre-mixed water anti-freeze in gallon jugs. This contains 50% distilled water and 50% anti-freeze. I do not recommend this. The cost of the mixture is only slightly less than the cost of a gallon of anti-freeze. Anti-freeze costs about $6-$7 per gallon. Distilled water costs about $0.80 per gallon in the grocery store and about $0.25 from a water store or water machine. You can save a significant amount by doing it yourself.


Anti-freeze is more than a product to reduce the freezing point of water. While it gets its name from that feature, it has three more functions, each of which is very important. First, it contains anti-rust products which inhibit rusting inside the cooling passages of the engine. Secondly, it contains a water pump lubricant which may help slightly. Lastly, and very importantly, it contains flow modifiers. Flow modifiers? Have you ever ridden in a boat propelled by an outboard motor? One where you could see what the water looked like behind it? The bubbles that form from the prop's movement through the water mean that the flow of water behind the prop is not flowing efficiently. Your water pump impeller (the thingy that moves the water) is like the prop on an outboard motor. One of the features of anti-freeze is that it cuts down on the number of bubbles and their size, making the coolant flow more efficiently within the closed cooling system of an automobile. Racers run pure water and add a product called "Water Wetter" to provide the proper flow of water through the system.

Water Wetter, made by Redline, is available from Moss Motors and Pep Boys. It was designed to modify the flow characteristics of the pure water used in race cars to allow a more efficient flow. The company claims that it also lowers the operating temperature of the coolant by up to 10 degrees F. I did not notice a significant change in operating temperature when I added it, but other people have. It is probably worth adding a bottle when refilling the cooling system. Pep Boys has a better price than Moss.


The mixture of water and anti-freeze is a subject for serious discussion. Many sources (especially the manufacturers) recommend a 50%-50% mix of water and anti-freeze. This is probably a good idea in the north. The mixture provides a good margin of anti-freeze capability-down to -34 degrees F in protection. In Arizona, however, our problem is not freezing, it is cooling.


Water has been assigned a value of 1.0 as a cooling medium. Using water as the standard, anti-freeze has a cooling value of 0.6. This means that anti-freeze is only 60% as efficient at transferring heat as water is. What this means is that a 50-50 mix is not as efficient as straight water. In point of fact, is only 80% as efficient. And MG's tend to run hot in Arizona summers. (As an interesting side light, Colin Campbell's book, "The Sports Car Engine" gives advise on how to set up the cooling system so it will run cool in everything but "Arizona Summers". This book was written back in the mid-60s, so our problems have long been known.) The formula to figure out the cooling system efficiency is percent water X 1.0 plus percent antifreeze X 0.6 to equal the cooling system efficiency. ( .50 x 1.0=.50 plus .50 x 0.6= .30 equals 80% system effectiveness.) I recommend (and use) 75% water and 25% anti-freeze which results in 90% system effectiveness. (.75 x 1.0 = .75 plus .25 x 0.6= .15 equals .90% system effectiveness.) If you add a bottle of Water Wetter to this, you have about as effective a coolant as you can find which will still provide adequate rust protection, water pump lubrication and proper coolant flow.


There is another area of concern within the cooling system-the thermostat. The thermostat, most basically, determines the minimum temperature the engine will run at. Often, people will run a 180 degree or a 195 degree thermostat in winter and a 160 degree thermostat in summer. There reason for this is to get the engine hot enough to operate efficiently in winter. This is not a significant factor here in Arizona, but was of great value when I lived in Wyoming, Alaska and Germany. One of the better reasons to live in Arizona. The thermostat is an often mis-understood thing and worth spending some time discussing.


The thermostat is a mechanical device which keeps the coolant from flowing through the radiator until it opens and allow water flow through the cooling system. Our MG's use the same thermostats as the small block Chevys, so if your parts store does not have a listing for the MG, use the 327 or 305 Chevy listing. Thermostats have a rating such as 160 degrees, 185 degrees, etc. This is the temperature at which the thermostat begins to open. Thermostats are built to "the rule of twenty". This means that the thermostat begins to open at the temperature listed, but is not fully open (hence full coolant flow) until about 20 degrees above that rating. The rule of twenty has little bearing up north, but can be a significant factor in Arizona summers.


A 160 degree thermostat will begin to open at 160 degrees F. and will be fully open at 180 degrees F. A 195 degree thermostat will begin to open at 195 degrees F. and will be fully open at 215 degrees F-three degrees over the boiling point of water. (The boiling point is modified by the addition of anti-freeze which raises the boiling point and the pressure cap which raises the boiling point by 3 deg. F. per 1 psi of pressure.) What does this mean? It means that if your cooling system runs, with full coolant flow, at 200 degrees, by using a 195 degree thermostat, you will run hotter. The 195 degree thermostat will only be just beginning to open and you will not have full coolant flow until a somewhat higher temperature. This means your car will run hotter than it might. The 180 degree thermostat seems to be a good compromise for use in Arizona, especially if you do not want to change thermostats with the seasons.


Changing a thermostat is not a big deal and is an ideal project for the less experienced "serious students of MG's". (Most of us are not trained mechanics, but as serious students of MG's, have picked up a fair amount of mechanical knowledge over the years.) The first thing to do is loosen the air pump (not necessary if you are fortunate enough to own a Mark I). You may have to disconnect the line to the air rail and gulp valve to get the pump out of the way. Drain the cooling system. Remove the three bolts holding the thermostat housing in place and remove the thermostat housing. Sometimes, tapping (or bashing) the housing is required. Remove the thermostat. Scrape the old gasket material from the head and thermostat housing. Put in the new thermostat, install the new gasket and install the thermostat housing. (There are two types of thermostat housing gaskets-paper and cork. If paper, use some non-hardening gasket compound and tighten firmly. If cork, use a thin layer of RTV and tighten gently.) Re-install the air pump and re-fill the cooling system. When doing the cooling system flush and re-fill is a good time to replace the thermostat. It should be replaced about every two years to prevent failure.


Thermostats fail in two ways-fully open and fully closed. In the former case, there is no real problem. In winter, the car does not fully warm up and the heater runs cool. More common is the thermostat failing closed-it never opens. In this case, the engine quickly begins to overheat and, if not caught, can result in engine damage. This fact alone makes bi-annual replacement a very inexpensive form of preventative maintenance. If you believe you have a thermostat problem, you can remove it and test it for proper functioning.


To test the thermostat requires a heat source (the family stove), a container (wife's best pot or pan), some water (OK to use tap water for this) and a meat thermometer. (Best to send the wife off for an afternoon's shopping when you do this. My wife has figured it out and I had to get my own pan and thermometer.) Fill the pan with water sufficient to cover the thermostat and put it on the stove to heat. Watch when it begins to open and measure the water temperature at that point. Then, watch until it quits opening (fully open) and measure that temperature. If it does not open, it is stuck in the closed position and will cause the engine to overheat. If, when removed, it is already open, it is stuck in the open position and the engine will never warm up fully. This may not be a significant problem in the Arizona Desert. In fact, the factory made a "blanking plate" to replace the thermostat for certain applications.


When the factory began to race and rally the MGB, they found that the engine could begin to overheat. They also found that, if they ran without a thermostat, too much water flowed through the radiator too fast-it was not cooling efficiently. They decided that the system needed some form of constriction to function correctly without the restriction of the thermostat. The developed a "blanking plate" which restricts the coolant flow less than the thermostat but more than running completely without the thermostat. The blanking plates are available from Moss Motors and the other "usual suspects". I have not tried them and cannot comment on their effectiveness. I do have one and will try it out this summer in the GT which always runs hot in the summer. How hot does it run?


We are "fortunate" enough to have several MG's in the family. (Daddy gets to work on all of them.) My GT runs about ¾- 7/8ths of the way up the scale when driven in stop and go traffic. The cooling system has been flushed and it has a new water pump and radiator. Theresa's 77 roadster runs between ½ and ¾ of the way up the scale in traffic and has a new water pump and radiator. Maria's 79 LE runs about mid way on the scale (but we have not had the opportunity to spend twenty minutes or more in stop and go traffic yet). It would appear, from my experience, that these cars run fairly hot in city traffic in the heat of summer. Thus, a cooling system tune up is a good investment when compared to the cost of overheating and, possibly, warping the head or blowing the head gasket. Plus, you get to learn a little more about your MG.


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Copyright © 2001 by Les Bengtson