Cylinder Wall Pitting

In our last newsletter we discussed the maintenance of a cooling system. This month I would like to discuss cylinder wall pitting, which is in part a lack of cooling system maintenance. I will discuss the coolant later in this article, but first I would like to explain pitting.

The pitting which frequently appears on the outside of engine cylinder walls is the result of the combined effects of cavitation from high frequency vibrations and corrosion. Although cylinder walls are not commonly thought of as moving parts in and engine, they are subject to vibrations due to the side to side motion of the pistons as they travel up and down the cylinders.

Electrolysis, as this cylinder wall pitting condition is frequently called, is a different condition. Electrolysis generally results from a poor engine ground connection which causes an electrical current to pass through the engine’s cooling system on its way to finding a ground. Although electrolysis will cause deterioration of various parts of the cooling system, this is not the common cause of cylinder wall pitting we are discussing.

Rapid vibrations of the cylinder wall cause air bubbles to form on the coolant side of the cylinder. Cylinder wall movements are small, but because they occur very rapidly, the coolant is not able to follow the movement of the cylinder. This results in a partial vacuum which permits bubbles to form on the cylinder. When the coolant catches up with the cylinder the pressure rises causing the bubbles to break or collapse. When these bubbles break, the force exerted removes the protective film left by the coolant’s corrosion inhibitors and allows the metal to be attached by corrosion. This is the cause of cylinder wall pitting. In severe cases, this pitting can penetrate entirely through a cylinder wall allowing coolant to enter into the cylinder. This condition appears most often in engines using wet type cylinder liners, as opposed to non-sleeved engines. Cylinder wall pitting is most common in diesel engines due to the greater piston side thrust which is a result of the higher compression ratios.

Ethylene glycol is the most commonly used type of antifreeze. A mixture ranging from 50% to 68% Ethylene glycol and a good quality water should be used along with one of the commercially available corrosion inhibitors. Ethylene glycol not only lowers the freezing point, but also raises the boiling point of the coolant. A 50/50 mix of Ethylene glycol and water will provide freeze protection down to -34 degrees F and boil protection up to approximately 225 degrees F. Maximum freeze protection occurs with 68% ethylene glycol (-65 degrees F). A greater concentration of antifreeze not only results in less freeze protection, but can also cause thickening of the coolant.

Corrosion inhibitors must also be used, because antifreeze by itself does not contain sufficient corrosion inhibitors to provide the protection needed in today’s engines. Not all antifreeze and corrosion inhibitors are compatible, so be sure to check the manufacturer’s instructions before using. Mixing of incompatible products may result in the formation of sediment. Do not use chromate type inhibitors with ethylene glycol antifreeze. Mixing these two products will result in the formation of chromium hydroxide, commonly called "Green Slime". The cooling system conditioners control the acidity and alkalinity of the solution.

Inspect the cooling system before adding coolant. Excessive rust, scale and corrosion deposits must be removed by a special chemical treatment. Once the cooling system is clean, add coolant at a moderate rate to avoid trapping air in the system. The engine should be run initially without a radiator cap until the thermostat opens. Once coolant flow and fill level are at the manufacturer’s specification, install the pressure cap and operate the engine until normal operating temperature is reached and check for leaks. Inspect your pressure cap for proper pressure operation, this is a very important part of boil protection. Remember for every pound of pressure, you raise the boiling point 3 degrees F.

In engines using wet sleeves, the fit of the cylinder sleeve to the block is the primary means of controlling cylinder wall vibrations. These sleeves are generally a pressed fit at the top end. The block counter bore dimensions should be checked and reworked if necessary, to insure the engine manufacturers specified fit. The bottom end the sleeve is generally a clearance fit and is held steady by compression of the sealing rings. Check the size and condition of the lower sealing areas of the block to insure proper fit of the liner and sealing rings. Repair sleeves are available to restore the lower sealing area if damaged. Cylinder liner protrusion is also a factor relating to liner vibration, because it affects the force exerted on the cylinder sleeve by the head which holds the sleeve in place. Always check protrusion to insure meeting engine manufacturer’s specification for maximum variation under one cylinder head. Shim or rework the counter bores and interchange the parts to meet the manufacturer's specification.

With proper attention to cylinder liner fit, coolant, and cooling system maintenance, cylinder wall pitting can be controlled. Cylinder wall vibrations, the formation of air bubbles in the coolant, and corrosion are the factors responsible for cylinder wall pitting. Consequently, anything that can be done to lessen and control these factors should reduce cylinder wall pitting, although it may not be possible to totally eliminate it.

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