The cylinder block of liquid-cooled engines forms the basic framework of the engine. Other parts are attached to the cylinder block or are assembled in it. The block is cast in one piece from gray iron or iron alloyed with other metals, such as nickel or chromium. Some blocks are cast from aluminum. The block contains not only the cylinders but also the water jackets that surround them. In aluminum blocks, cast iron or steel cylinder sleeves (also called bore liners) are used. These metals have better wearing qualities than aluminum and can better withstand the wearing effect of the pistons and rings moving up and down in the cylinders. For most engines, cast iron has been found to be a satisfactory cylinder-wall material. However, in some small engines, the cylinder walls are plated with chromium, a very hard metal, to reduce wall wear and lengthen their life.