THE problem of cylinder wear has now become very serious in commercial and public service vehicles, according to an article in the January issue of the Nickel Bulletin. This is due to the new designs giving higher efficiency, and also the use of aluminium pistons. Wear is considered to be due to a combination of two causes: mechanical abrasion and the corrosive attack of the cylinder walls by the products of combustion. The former predominates in vehicles running for long periods with a hot engine such as motor-buses used for long-distance runs in daily service. The latter effect is the more important for vehicles which run intermittently with long waits between runs, like delivery vehicles. In these the engines never get properly warm. The development of nickel cast iron has provided a satisfactory solution. The addition of one or two per cent of nickel to a suitable composition of base iron has produced cylinder castings which are readily machinable and yet have a high Brinell hardness. Recently the manufacturers of many commercial vehicles have obtained excellent results by using heavily alloyed iron. The cylinders of both petrol and Diesel vehicles are now being regularly made of irons containing 4-5 per cent of nickel with or without proportions of chromium and molybdenum. Improved machinability is obtained by slowly cooling the castings from a temperature of about 650 ° C. When this course is followed, the original hardness is restored after machining by air-cooling the material from a temperature of about 850 ° C. In France, this hard type of iron has been used for several years with satisfactory results for air-cooled motor-cycle cylinders.