The definition of fin area becomes important in the  examination of fins for broken areas. It is a determining  factor for cylinder acceptance or removal. For example, on a certain engine, if more than 12 inches in length of any one  fin, as measured at its base, is completely broken off, or if  the total fins broken on any one cylinder head exceed 83  square inches of area, the cylinder is removed and replaced.  The reason for removal in this case is that an area of that size  would cause a hot spot on the cylinder; since very little heat  transfer could occur. the air velocity. The outlet at the bottom rear of the cowling  produces a low-pressure area. As the air nears the cowl exit, it  is speeded up again and merges smoothly with the airstream.  The pressure differential between the top and the bottom of the  engine forces the air past the cylinders through the passages  formed by the deflectors. The baffles and deflectors normally  are inspected during the regular engine inspection, but they  should be checked whenever the cowling is removed for any  purpose. Checks should be made for cracks, dents, or loose  hold down studs. Cracks or dents, if severe enough, would  necessitate repair or removal and replacement of these units.  However, a crack that has just started can be stop-drilled, and  dents can be straightened, permitting further service from  these baffles and deflectors. Where adjacent fins are broken in the same area, the total  length of breakage permissible is six inches on any two  adjacent fins, four inches on any three adjacent fins, two  inches on any four adjacent fins, and one inch on any five  adjacent fins. If the breakage length in adjacent fins exceeds  this prescribed amount, the cylinder should be removed and  replaced. These breakage specifications are applicable only  to the engine used in this discussion as a typical example.  In each specific case, applicable manufacturer’s instructions  should be consulted. This system usually consists of an indicator, electrical  wiring, and a thermocouple. The wiring is between the  instrument and the nacelle firewall. At the firewall, one end  of the thermocouple leads connects to the electrical wiring,  and the other end of the thermocouple leads connects to  the cylinder. The thermocouple consists of two dissimilar  metals, generally constantan and iron, connected by wiring  to an indicating system. If the temperature of the junction is  different from the temperature where the dissimilar metals are  connected to wires, a voltage is produced. This voltage sends  a current through wires to the indicator, a current-measuring  instrument graduated in degrees. Reciprocating engines use some type of intercylinder and  cylinder head baffles to force the cooling air into close  contact with all parts of the cylinders.  Figure 6-50  shows a  baffle and deflector system around a cylinder. The air baffle  blocks the flow of air and forces it to circulate between the  cylinder and the deflectors.  Figure 6-57  illustrates a baffle  and deflector arrangement designed to cool the cylinder  head. The air baffle prevents the air from passing away  from the cylinder head and forces it to go between the head  and deflector. Although the resistance offered by baffles to  the passage of the cooling air demands that an appreciable  pressure differential be maintained across the engine to obtain  the necessary airflow, the volume of cooling air required is  greatly reduced by employing properly designed and located  cylinder deflectors.  As shown in  Figure 6-55 , the airflow approaches the nacelle  and piles up at the top of the engine, creating a high pressure  in the top of the cylinders. This piling up of the air reduces  The thermocouple end that connects to the cylinder is either  the bayonet or gasket type. To install the bayonet type, the  knurled nut is pushed down and turned clockwise until it is  snug.  [Figure 6-58]  In removing this type, the nut is pushed  down and turned counterclockwise until released. The gasket  type fits under the spark plug and replaces the normal spark  plug gasket.  [Figure 6-59]  When installing a thermocouple  lead, remember not to cut off the lead because it is too long,  Cylinder Temperature Indicating Systems Cylinder Baffle and Deflector System Inspection Deflector Air baffle Cowl panel air seal The nut lock the probe in the cylinder head socket This part of the probe presses against  the cylinder head when installed Figure 6-58.  Bayonet type CHT probe.  Figure 6-57.  Cylinder head baffle and deflector system.  6-38