gases take place, and it houses the piston and the connecting  rod. There are four major factors that need to be considered in  the design and construction of the cylinder assembly. It must: The cylinder head of an air cooled engine is generally made of  aluminum alloy because aluminum alloy is a good conductor  of heat and its light weight reduces the overall engine weight.  Cylinder heads are forged or die-cast for greater strength. The  inner shape of a cylinder head is generally semispherical.  The semispherical shape is stronger than conventionalist  design and aids in a more rapid and thorough scavenging of  the exhaust gases. The cylinder used in the air cooled engine is the overhead  valve type.  [Figure 1-17]  Each cylinder is an assembly of two  major parts: cylinder head and cylinder barrel. At assembly,  the cylinder head is expanded by heating and then screwed  down on the cylinder barrel, which has been chilled. When  the head cools and contracts and the barrel warms up and  expands, a gastight joint results. The majority of the cylinders  used are constructed in this manner using an aluminum head  and a steel barrel.  [Figure 1-18] openings in the cylinder head to provide guides for the valve  stems. These are generally located at an angle to the center  line of the cylinder. The valve seats are circular rings of  hardened metal that protect the relatively soft metal of the  cylinder head from the hammering action of the valves (as  they open and close) and from the exhaust gases. The purpose of the cylinder head is to provide a place for  combustion of the fuel/air mixture and to give the cylinder  more heat conductivity for adequate cooling. The fuel/air  mixture is ignited by the spark in the combustion chamber  and commences burning as the piston travels toward top  dead center (top of its travel) on the compression stroke. The  ignited charge is rapidly expanding at this time, and pressure  is increasing so that, as the piston travels through the top dead  center position, it is driven downward on the power stroke.  The intake and exhaust valve ports are located in the cylinder  head along with the spark plugs and the intake and exhaust  valve actuating mechanisms. After the cylinder head is cast, the spark plug bushings, valve  guides, rocker arm bushings, and valve seats are installed in  the cylinder head. Spark plug openings may be fitted with  bronze or steel bushings that are shrunk and screwed into the openings. Stainless steel Heli-Coil spark plug inserts  are used in many engines currently manufactured. Bronze or  steel valve guides are usually shrunk or screwed into drilled  1-15 Figure 1-18. The aluminum head and steel barrel of a cylinder. Cylinder Heads Piston Cast aluminum head Exhaust valve Intake valve Connecting rod Forged steel barrel Combustion chamber Crankshaft Piston pin Figure 1-17. Cutaway view of the cylinder assembly. 1. Be strong enough to withstand the internal pressures  developed during engine operation. 2. Be constructed of a lightweight metal to keep down  engine weight. 3.  Have good heat-conducting properties for efficient  cooling. 4. Be comparatively easy and inexpensive to manufacture,  inspect, and maintain.