Figure 3-2.—Requirements of a cylinder. Aluminum is more expensive than cast iron. Aluminum is not as strong as cast iron. Because of its softness, it cannot be used on any surface of the block that is subject to wear. This necessitates the pressing, or casting, of steel sleeves into the cylinder bores. Threaded holes must  be  deeper.  This  introduces  extra  design considerations and increases production costs. Aluminum  has  a  much  higher  expansion  rate than iron when heated. This creates problems with maintaining tolerances. The CYLINDERS are bored right into the block. A good cylinder must be round, not varying in diameter by more than approximately 0.0005 inch (0.012 mm) (fig. 3-2). The diameter of the cylinder must be uniform throughout  its  entire   length.  During  normal  engine operation, cylinder walls wear out-of-round, or they may become cracked and scored if not lubricated or cooled properly. The cylinders on an AIR-COOLED engine (fig. 3-3) are separate from the crankcase. They are made of forged steel. This material is most suitable for cylinders because of its excellent wearing qualities and its ability to withstand high temperatures that air- cooled cylinders obtain. The cylinders have rows of deep fins cast into them to dissipate engine heat. The cylinders are commonly mounted by securing the cylinder head to the crankcase with long studs and sandwiching the cylinders between the two. Another way of mounting the cylinders is to bolt them to the crankcase, and then secure the heads to the cylinders. A– B– C– D– E– F– G– H– J– K– O-RING  GASKET EXHAUST   VALVE INTAKE  VALVE CYLINDER CYLINDER BARREL NUT LOCK NUT DOME FIN DEFLECTOR (LH) BOLT LOCK  WASHER PRIMER  NOZZLE  ASSEMBLY L– M– N– P– Q– R– S– T– U– V– INTAKE  VALVE  GUIDE EXHAUST  VALVE  GUIDE CAMSHAFT BEARING CAP ROCKER  SHAFT ROCKER ROCKER  SUPPORT  BRACKET WASHER SLOTTED  NUT ROCKER BOX COVER PLATE TAB  WASHER W– X– Y– Z– AA– BB– CC– DD– EE– FF– GG– VALVE ROCKER COVER BOLT LOCK  WASHER WASHER VALVE  LOCK VALVE   SPRING   RETAINER OUTER VALVE SPRING INTERMEDIATE  VALVE  SPRING INNER  VALVE  SPRING VALVE  RING  SEAT DOME FIN DEFLECTOR (RH) CMB10054 Figure 3-3.—Air-cooled cylinder. 3-3