The cylinder heads of air cooled engines are subjected to  extreme temperatures; it is therefore necessary to provide  adequate cooling fin area and to use metals that conduct heat  rapidly. Cylinder heads of air cooled engines are usually cast  or forged. Aluminum alloy is used in the construction for a  number of reasons. It is well adapted for casting or for the  machining of deep, closely spaced fins, and it is more resistant  than most metals to the corrosive attack of tetraethyl lead in  gasoline. The greatest improvement in air cooling has resulted  from reducing the thickness of the fins and increasing their  depth. In this way, the fin area has been increased in modern  engines. Cooling fins taper from 0.090" at the base to 0.060"  at the tip end. Because of the difference in temperature in  the various sections of the cylinder head, it is necessary to  provide more cooling-fin area on some sections than on  others. The exhaust valve region is the hottest part of the  internal surface; therefore, more fin area is provided around  the outside of the cylinder in this section. is always the front end, and the accessory end is the rear end,  regardless of how the engine is mounted in an aircraft. When  referring to the right side or left side of an engine, always  assume the view is from the rear or accessory end. As seen  from this position, crankshaft rotation is referred to as either  clockwise or counterclockwise. Inline and V-type engine cylinders are usually numbered from  the rear. In V-engines, the cylinder banks are known as the  right bank and the left bank, as viewed from the accessory end.  [Figure 1-19]  The cylinder numbering of the opposed engine  shown begins with the right rear as No. 1 and the left rear as  No. 2. The one forward of No. 1 is No. 3; the one forward of  No. 2 is No. 4, and so on. The numbering of opposed engine  cylinders is by no means standard. Some manufacturers  number their cylinders from the rear and others from the front  of the engine. Always refer to the appropriate engine manual  to determine the numbering system used by that manufacturer. Single-row radial engine cylinders are numbered clockwise  when viewed from the rear. Cylinder No. 1 is the top cylinder.  In double-row engines, the same system is used. The No. 1  cylinder is the top one in the rear row. No. 2 cylinder is the  first one clockwise from No. 1, but No. 2 is in the front row.  No. 3 cylinder is the next one clockwise to No. 2 but is in the  rear row. Thus, all odd-numbered cylinders are in the rear  row, and all even-numbered cylinders are in the front row. The cylinder barrel in which the piston operates must be  made of a high-strength material, usually steel. It must be  as light as possible yet have the proper characteristics for  operating under high temperatures. It must be made of a  good bearing material and have high tensile strength. The  cylinder barrel is made of a steel alloy forging with the inner  surface hardened to resist wear of the piston and the piston  rings which bear against it. This hardening is usually done  by exposing the steel to ammonia or cyanide gas while the  steel is very hot. The steel soaks up nitrogen from the gas,  which forms iron nitrides on the exposed surface. As a result  of this process, the metal is said to be nitrided. This nitriding  only penetrates into the barrel surface a few thousands of an  inch. As the cylinder barrels wear due to use, they can be  repaired by chroming. This is a process that plates chromium  on the surface of the cylinder barrel and brings it back to new  standard dimensions. Chromium-plated cylinders should use  cast iron rings. Honing the cylinder walls is a process that  brings it to the correct dimensions and provides crosshatch  pattern for seating the piston rings during engine break-in.  Some engine cylinder barrels are choked at the top, or they  are smaller in diameter to allow for heat expansion and wear. The firing order of an engine is the sequence in which the  power event occurs in the different cylinders. The firing order  is designed to provide for balance and to eliminate vibration  to the greatest extent possible. In radial engines, the firing  order must follow a special pattern since the firing impulses  must follow the motion of the crank throw during its rotation.  In inline engines, the firing orders may vary somewhat, yet  most orders are arranged so that the firing of cylinders is  evenly distributed along the crankshaft. Six-cylinder inline  engines generally have a firing order of 1-5-3-6-2-4. Cylinder  firing order in opposed engines can usually be listed in pairs  of cylinders, as each pair fires across the center main bearing.  The firing order of six-cylinder opposed engines is 1-4-5- 2-3-6. The firing order of one model four-cylinder opposed  engine is 1-4-2-3, but on another model, it is 1-3-2-4. In some instances, the barrel has threads on the outside  surface at one end so that it can be screwed into the cylinder  head. The cooling fins are machined as an integral part of  the barrel and have limits on repair and service. Occasionally, it is necessary to refer to the left or right side  of the engine or to a particular cylinder. Therefore, it is  necessary to know the engine directions and how cylinders of  an engine are numbered. The propeller shaft end of the engine  On a single-row radial engine, all the odd-numbered cylinders  fire in numerical succession; then, the even numbered  cylinders fire in numerical succession. On a five-cylinder  radial engine, for example, the firing order is 1-3-5-2-4, and  on a seven-cylinder radial engine it is 1-3-5-7-2-4-6. The firing  order of a nine-cylinder radial engine is 1-3-5-7-9-2-4-6-8. 1-16 Single-Row Radial Engines Cylinder Numbering Cylinder Barrels Firing Order