Shop Talk: The Cooling System (Part II)

By Paul Dilger

In part one of “The Cooling System” series in October 2008 OPE, we discussed radiator design and maintenance, as well as pressure testing. We also talked about cooling fans and fan clutches. Now in part two, we will be covering such topics as v-belts and pulleys, crankshaft breakage, belt deflection, testing thermostats, cylinder head and block failure, radiator hoses and coolant failure.

Fan belts or v-belts on engines should be replaced when cracking starts to appear on the sides or edges of the belts. Belts on engines working in high smog areas will need replacing more frequently due to ozone oxidation. Always use a replacement belt with the correct part number, as the belt width, depth and length are critical to proper tensioning.


Figure 1 – Correct and incorrect belt placement in a pulley grooveCorrect tensioning of the belt is important to prevent slippage of a driven component. Your maintenance manual will give you the specifications for your particular engine. Remember, under-tightening (not enough tension) will cause slipping on driven pulleys and glazing of the sides of the belts. Under-tightening belts also causes excessive heat due to the friction of slippage that reduces the life of the belts and pulleys.

Over-tightening (too much tension) reduces the life of component bearings. Excessive over-tightening of the belt around the crankcase pulley can break the crankshaft. As the crankshaft’s front main bearing wears, the flexing of the crankshaft fatigues the metal and causes stress risers that lead to cracks and failure.

Some engines have automatic belt tensioners (slack adjusters) that are spring-loaded to the correct tension to give the belt the proper deflection (see Figure 1). These automatic tensioners maintain the correct tension as the belt stretches. With manually adjusted systems, however, new belts should be re-tightened after several hours or approximately 100 miles of running.


Figure 2 – Progressive crankshaft breaksAll v-belts should ride on the sides of the pulleys as shown in Figure 1A. There is a widespread misconception that a v-belt should ride on the bottom of the pulley groove as seen in Figure 1B. Tightening the belt against the bottom of the pulley can lead to failures due to over-tightening.

To understand how over-tightened belts can lead to crankshaft failure, picture the following scenario. Tie a rope on one end of a 3-foot-long round stick and lay it on the ground. Now, step on the other end of the stick and lift up on the rope. Notice the stick will bend slightly due to the tension on the rope. Now, visualize the stick spinning or rotating. When the stick is rotated, the top side of the stick at the bend is under compression and the bottom side is under tension. As it rotates, the tension and compression points remain at the same spot, but the tension and compression on the stick continually change as it rotates. Now, think about a crankshaft rotating and bending upward due to belt tension. If a stress riser develops, a minute crack starts to form. The crack continues along the cross-section of the crankshaft until the crankshaft breaks. When you examine the crankshaft, you will see a series of crescent-shaped fracture lines usually extending 60 to 80 percent across the break (depending on the load) that looks like the drawing in Figure 3. The dark area in the drawing is where the crankshaft could no longer handle the load, thus snapping the remainder of the shaft.


Figure 3 – V-belt deflection and tensionBelt deflection is a calibration of the tension of a v-belt on the pulleys and is measured as the number of pounds required to pull a belt a specified distance of deflection (see Figure 3). Correct belt tension is dependent on the distance between pulleys or loads on accessories. A v-belt tension scale can be used to pull the tension and should be rated at least 30-40 pounds for most engines. Shop service manuals should list the specifications for the amount of deflection and the pounds of pull required to obtain the given deflection.

What are the consequences of the belt tension being too loose? The belt will slip on the crankshaft, causing the belt to rotate slower. This, in turn, will cause the water pump to rotate slower, possibly leading to engine overheating. Belts drive many accessories such alternators, power steering pumps, water pumps, air compressors, vacuum pumps, and slack adjuster pulleys. As the loads continually change on each of these devices, so will the slippage on the belts, which may result in engine overheating.

Service tip: When inspecting v-belts and hoses, look at the sides of the belts and see if they are developing a shine, burn or material disintegration (a sure sign of belt slippage). Some belts have a cloth covering over the rubber. If so, watch for any deterioration of the cloth, which is an indication that it’s time to replace it. Always follow the manufacturer’s specifications on belt tension.


 Figure 4 – Aligning pulleysOften when making modifications on an engine, you may need to add an additional pulley. It’s absolutely necessary to accurately align the pulleys. Figure 4 shows how to align two or three pulleys using a string or a straight edge like a yardstick. If the pulley you’re adding is either thicker or thinner than the others, you might have to align the grooves (centers) of all the pulleys instead of the outside edges.

Shop tip: Very minor mis-alignment accelerates bearing wear by loading one side of the bearing instead of evenly distributing the load across the entire bearing surface.

Thermostats: The water pump pulls coolant from the bottom of the radiator and sends it into the cylinder block. Coolant is warmed as it flows around the cylinders and up into the cylinder head, then forward to the thermostat. The thermostat opens or closes according to the coolant temperature. When the thermostat is open, coolant flows from the cylinder head to the upper radiator. When the thermostat is closed, on some engines, the coolant is circulated within the block through a by-pass system to equalize the temperature around the cylinders. On other engines, when the thermostat is closed, the coolant can only move by convection to equalize the temperatures.


Figure 5 – Testing a thermostatThermostats need to be tested periodically (see Figure 5). To measure the thermostat’s opening temperature, suspend the thermostat by a string in a pot of water. Heat the water gradually, and read the temperature at which the thermostat valve begins to open. Continue heating the water (being careful not to let the thermostat touch the bottom of the hot pan) until the valve opens approximately 1/4-inch. If the valve does not open to manufacturer’s specification at the correct temperature, always replace the thermostat.

Service tip reminder: When refilling a cooling system, be sure to remove all the air from the cylinder head. Most thermostats have an air bleed hole, but some do not. This might require loosening the thermostat housing to remove the air. Bleeding the air could take more than an hour by venting through the bleed hole, so be patient.

Water pumps: Many water pumps last the life of the engine. Two wear-points on the water pump can be reduced by good service and maintenance practices. The first wear-point is the water pump bearing. Bearings will have their longest life when v-belts are not over-tightened.

The second wear-point on a water pump is the water pump seal. A seal’s life may be extended by using only the antifreeze recommended by the manufacturer. The wrong coolant, or mixing coolants, may swell the seals and cause accelerated wear.

Cylinder head and block: Three major problems normally occur with the cylinder head water jackets. The first problem is scale build-up from minerals dissolved in tap water that is used in the cooling system. Because of the salt content in normal tap water, de-ionized or distilled water is highly recommended for the dilution of 50-percent antifreeze. Tap water contains calcium and magnesium salts that form mineral scale coatings in the water jackets. Salts can be eliminated by never adding tap water for “make-up” water and changing the coolant that comes with new machines, tractors, autos, etc.


Figure 6 – The effect of scale on heat transferEngineers have found that 1/16-inch of salt scale reduces heat transfer by 40 percent (see Figure 6). When there is insufficient heat removal by the water jacket, the water temperature indicator will not show a temperature rise in the combustion chamber because the coolant will actually be cooler when the heat cannot be transferred to the coolant. The excess heat will now leave through the exhaust system. This, in turn, raises the exhaust temperature and increases oil oxidation on the cylinder walls and valve guides. Increased oil oxidation reduces the service life of the engine. There are various reasons why engines need rebuilding, but scaled water jackets are one of the reasons engine rebuilders’ warranties are seldom for more than a few months or minimal miles. I have never found an auto mechanic or technician who evaluated the water jacket for scaling before rebuilding. Large industrial engine rebuilding facilities automatically pickle (hot acid bath) used engine blocks before rebuilding to restore the water jacket by removing the salt deposits. In the past, I have seen truck engine rebuilding programs where the rebuilds, using distilled or deionized water in the coolant, were getting 30 percent more miles than the new engines.

Secondly, when the anti-rust additives in the coolant have been exhausted, the water jacket starts rusting. Rust is not as good a conductor of heat as gray cast-iron. Engine wear accelerates in older engines because the cooling system cannot conduct the heat away. With some of the cooling system cleaning products available in auto parts stores, individuals can acid-clean a cooling system without disassembling the engine. Rust and scale can be removed by acid flushing with approved chemicals. An important precaution is not to leave the acid solution in any longer than recommended by the product’s manufacturer to prevent damage to weaker components like the heater core. There is always a neutralizer provided with an acid flush, so take care to properly neutralize the water jacket to prevent continuing oxidation of the accessory components such as hoses, radiator and water pump. Oxalic acid is commonly used to flush cooling systems and can cause severe bodily injury to eyes and skin if precautions are not followed. Be sure to have a safety shower and eye wash, along with a water hose, adjacent to the radiator service area.


Figure 7 – Crack between two valve insertsSilicon-silicate gel drop-out occurs in diesel engines when antifreezes designed for gasoline engines are used. Silicon is added to some antifreezes to help protect gas engines against oxidation where aluminum is commonly used in the engine and cooling system. Because diesel engine manufacturers use very little or no aluminum in their engines, less silicon is needed. A high silicon concentration results in the precipitation or settling of the silicate in radiator tubes and passageways of the cylinder block and cylinder head. Scale, silicate gel and rust reduce heat transfer from the cylinder walls to the water jackets. This excess heat also oxidizes the oil in the cylinders and leads to piston scuffing from a lack of good lubrication. Furthermore, poor heat transfer may lead to cracked cylinder blocks and heads.

Figure 7 shows a crack between two valve inserts caused by overheating due to scale insulating the coolant jacket inside the head of a farm tractor engine.


Figure 8 – Cracks between valve inserts and injector tubeFigure 8 shows cracks between all four valve insert counterbores and an injector tube caused by overheating due to scale insulating the coolant jacket inside the head of a Detroit Diesel in a generator set.

There are cylinder head and block welding facilities that can weld cracks in cast iron. They build an igloo with fire bricks and then heat the

entire iron casting red hot. While the head or block is still red hot, they weld the cracks, close up the igloo chamber, and let it cool for a day. Once cooled, they surface-machine the repaired area so it comes back looking brand new.

In Figure 9, notice the destruction to the sides of the piston when the cylinder and piston swell as a result of overheating. The best lubrication cannot prevent this kind of damage where there are heavily scaled water jackets.


Figure 9 – Piston damage caused by overheatingRadiator hoses: Radiator hose failure can normally be detected by feeling for soft, swollen or mushy spots on the hose surface. Oxidation of the inside and outside surfaces of the hose causes deterioration of the rubber and reduces rubber hose life. Keeping hoses free of oils will increase the life of rubber radiator hoses. Ozone will cause surface cracking of the hoses, but should not normally lead to hose failure. Surface cracking normally stops on quality hoses when the crack reaches the fabric reinforcement.

Radiator hoses without vacuum suppression springs should not be installed on the bottom radiator outlet tube. This bottom radiator hose must be designed to withstand high vacuums created by the water pump. An incorrect lower hose will collapse and can lead to coolant blockage and high operating temperatures.

Coolant cavitation can occur from a high vacuum on the water pump inlet, eventually causing enough damage to destroy the water pump seals due to the imploded particles released in the coolant. The high vacuum results from a combination of restricted coolant tubes and/or a partially collapsed bottom radiator hose.

Service tip: Coating radiator inlet and outlet tubes with silicon grease before installing hoses prevents the hoses from cementing to the radiator tubes.

Hose clamps: Wire squeeze clamps are a pain if you do not have the right pliers, but are the most efficient type compared to adjustable clamps. Adjustable clamps are quite often selected because they are easier to install, remove, and adjust with a screwdriver or nut driver. The down side is that adjustable clamps normally need to be re-tightened a couple of times over their service life as the soft rubber hoses harden. As the rubber hoses harden, they lose their resilience to expand and develop a tight seal. Wire squeeze-type hose clamps are self-adjusting with constant pressure as the rubber loses its resilience, thus they have less potential for leakage.


Figure 10 – Engine coolant/water filterFleetguard, like some other manufacturers, sells a coolant/water filter (see Figure 10). It spins on like an oil filter, but there are a couple of design differences. Under the base plate is a sacrificial zinc plate to reduce the electrolysis that occurs when there are different metals that can cause an electrical potential, which leads to pitting in the engine water jacket. These filters quite often contain chemical inhibitors such as water-softening agents that retard cavitation pitting. In addition, Fleetguard adds water-softening agents to control scale formation and buffering agents to control the coolant’s acid/base balance.


Paul Dilger is a retired professor of agricultural engineering at Cal-Poly State University. He worked as a mechanic in the U.S. Army before attending college. After graduating from college, he taught mechanics for 25 years. He is currently a private consultant helping companies develop quality service training programs.

EDITOR’S NOTE: For more comprehensive studies on OPE engine, electrical and hydraulic certification, visit Paul Dilger’s Web site at These study programs develop professional mechanics.



Leave a Reply

Your email address will not be published. Required fields are marked *