Corrosion in gasoline engines is generally believed to be due to sulphuric acid formed by the combination of sulphur carried in low-grade fuels and oils with water that enters or is generated in the engine. Much of this trouble occurs in winter and may be traced directly to the action of water that condenses on the inside of the cylinders and crankcase when a cold engine is started. The water destroys the oil-film and comes into direct contact with metal of the pistons, cylinders and other parts, causing them to rust. If this occurs and the lubricating system does not supply more oil to the surfaces immediately upon the restarting of the engine, scored cylinders and pistons are likely to result, or, if the engine is stopped before it is warmed up, condensation and rusting will be rapid and will result in excessive wear.The only completely successful method of dealing with the condensation and rust problem is to provide a lubricating system that will begin to function as soon as the engine is started. The splash system has been found to meet this requirement best. If pressure-feed systems are used, it is recommended that (1) The oil-pump be located in the sump (2) No oil-screen finer than 30 mesh be used over the intake and it should have a bypass (3) Oil lines be as straight and as short as possible and not less than ½ in. in diameter (4) Connecting-rods have a diametrical clearance of 0.0015 in. and a side clearance of 0.0060 to 0.0080 in. for oil (5) Light oils be used in winter Road-tests have not confirmed the common belief that use of thin or diluted oils results in rapid wear of pistons and cylinders. A castor-oil film is more resistant to the action of water than a mineral-oil film and it is suggested as an inside coating in engines that are stored during cold weather.Corrosion in gasoline engines can be traced, in practically all cases, to condensation of the water vapor in the gases of combustion and of moisture in the air upon the cold surfaces of the cylinders and crankcase walls. It is found in the form of rust in crankcases and cylinders and is made evident by etched wristpins, valve tappets, timing chains and other engine parts.The most commonly accepted theory for the cause of corrosion is the action of sulphuric acid, which is formed by the combination of sulphur in the fuel and lubricating oil with water entering or generated in the engine. According to A. Ludlow Clayden, water collects in the engine at the rate of 80 cc. per hr. at zero temperature. The rate at which the sulphur collects would, no doubt, depend largely upon the sulphur content of the fuel and oil used. Sulphuric acid as a corrosive agent will not be dealt with in this paper, which is concerned only with water, its sources and effects, and means of avoiding its presence or of neutralizing its effects.