IN analyzing causes of cylinder-bore wear it is found that they fall generally into one of the following classifications: (1) Abrasion - wear due to foreign particles in the oil film. (2) Erosion - wear due to metal contact between the pistons or rings and the cylinder bore. (3) Corrosion - oxidization or chemical action of the cylinder wall by the products of combustion. Although these classifications place the three main factors in the approximate order of their importance, the conditions of operation, the design of the engine, the design of the air cleaner, the piston and ring equipment, or the lubricating oil used, may change the order completely.Four ways are described for abrasives to get into the engine, and three design factors are mentioned that will reduce cylinder wear in the presence of dust. Keeping the dirt out, however, is stated to be the only real solution, and air cleaners are recommended as the best method.Ample space is devoted to a discussion of erosion and corrosion with various causes and cures.