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box covers for flatness; re-surface them if necessary. After installing the gaskets and covers, tighten the rocker-box cover nuts to the specified torque. Always follow the recommended safety procedures. 

## Cold Cylinder Check 

The cold cylinder check determines the operating characteristics of each cylinder of an air-cooled engine. The tendency for any cylinder, or cylinders, to be cold, or to be only slightly warm, indicates lack of combustion or incomplete combustion within the cylinder. This must be corrected if best operation and power conditions are to be obtained. The cold cylinder check is made with a cold cylinder indicator. 

Engine difficulties that can be analyzed by use of the cold cylinder indicator are [Figure 10-55] : 

1. Rough engine operation 

2. Excessive rpm drop during the ignition system check 

3. High manifold pressure for a given engine rpm during the ground check when the propeller is in the full low- pitch position 

4. Faulty mixture ratios caused by improper valve clearance 

In preparation for the cold cylinder check, head the aircraft into the wind to minimize irregular cooling of the individual cylinders and to ensure even propeller loading during engine operation. 

Operate the engine on its roughest magneto at a speed between 1,200 and 1,600 rpm until the cylinder head temperature reading is stabilized. If engine roughness is encountered at more than one speed, or if there is an indication that a cylinder ceases operating at idle or higher speeds, run the engine at each of these speeds, and perform a cold cylinder check to pick out all the dead or intermittently operating cylinders. When low power output or engine vibration is encountered at speeds above 1,600 rpm when operating with the ignition switch on both, run the engine at the speed where the difficulty is encountered until the cylinder head temperatures have stabilized. 

When cylinder head temperatures have reached the stabilized values, stop the engine by moving the mixture control to the idle cutoff or full lean position. When the engine ceases firing, turn off both ignition and master switches. Record the cylinder head temperature reading registered on the flight deck gauge. As soon as the propeller has ceased rotating, apply the instrument to each cylinder head, and record the relative temperature of each cylinder. Start with number one 

<img src='content_image/572106.jpg'>

and proceed in numerical order around the engine, as rapidly as possible. To obtain comparative temperature values, a firm contact must be made at the same relative location on each cylinder. Note any outstandingly low (cold) values. Compare the temperature readings to determine which cylinders are dead (cold cylinders) or are operating intermittently. 

Difficulties that may cause a cylinder to be inoperative (dead) when isolated to one magneto, either the right or left positions, are: 

1. Defective spark plugs 

2. Incorrect valve clearances 

3. Leaking intake pipes 

4. Lack of compression 

5. Defective spark plug lead 

6. Defective fuel-injection nozzle 

