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a. Dense chromium plating of the crankpin and main journals of some small engine crankshafts has been found satisfactory except where the crankshaft is already marginal in strength. Plating to restore worn, low-stress engine parts, such as accessory drive shafts and splines, propeller shaft ends, and seating surfaces of roller and ball-type bearing races, is acceptable but requires compliance with FAA-approved data.
b. Porous chromium-plated walls of cylinder barrels have been found to be satisfactory for practically all types of engines. Dense or smooth chromium plating without roughened surfaces, on the other hand, has not been found to be satisfactory.
(1) Cylinder barrel pre-grinding and chromium plating techniques used by the military are considered acceptable for all engines, and military-approved facilities engaged in doing this work in accordance with military specifications are eligible for approval by the FAA.
(2) Chromium-plated cylinder barrels have been required for some time to be identified in such a manner that the markings are visible with the cylinder installed. Military-processed cylinders are banded with orange enamel above the mounting flange. It has been the practice to etch on either the flange edge or on the barrel skirt the processor’s initials and the cylinder oversize. Most plating facilities use the orange band as well as the permanent identification marks.
(3) A list of engine and maximum permissible cylinder barrel oversize are referenced in Table 8-1.
TABLE 8-1. CURRENT ENGINE AND MAXIMUM PERMISSIBLE CYLINDER
BARREL OVERSIZE Engine Manufacturer Engine Series Max. Oversize (in) Aircooled Motors (Franklin) No oversize for sleeved cylinders.
Solid cylinders
0.017 Continental Motors R-670, W-670, R9A 0.010 to 0.020 GTSIO-520, 550 0.005 All others 0.015 Jacobs All 0.015 Kinner All 0.015 Pigman, LeBlond, Rearwin, Ken Royce All 0.025 Lycoming All 0.010 to 0.020 Menasco All 0.010
Par 8-11 Page 8-7 Engine Manufacturer Engine Series Max. Oversize (in) Pratt & Whitney R-2800B, C, CA, CB 0.025 *R-959 and R-1830 0.030 All others 0.020 Ranger 6-410 early cyls. 6-390 0.010 6-410 late cyls. 6-440 (L-440) series 0.120 Warner All 0.015 Wright All 0.020 *(The above oversize limits correspond to the manufacturer’s requirements, except for P&W R-985 and R-1830 series engines.)
NOTE: (Check for latest manufacturer specifications.)
(4) Cylinder barrels, which have been plated by an agency whose process is approved by the FAA and which have not been worked beyond maximum permissible limits, will be considered acceptable for installation on certificated engines. It will be the responsibility of the owner or the repairing agency to provide this proof. In some cases, it may be necessary to remove cylinders to determine the amount of oversize since this information may be etched on the mating surface of the cylinder base flange.
8-12. CORROSION. Accomplish corrosion preventive measures for temporary and long-term storage in accordance with the instructions issued by the pertinent engine manufacturer. Avoid the use of solutions which contain strong caustic compounds and all solutions, polishes, cleaners, abrasives, etc., which might possibly promote corrosive action (see Chapter 6, Corrosion, Inspection, and Protection).
8-13. ENGINE RUN-IN. After an aircraft engine has been overhauled, it is recommended that the pertinent aircraft engine manufacturer’s run-in instructions be followed. Observe the manufacturer’s recommendations concerning engine temperatures and other criteria. Repair processes employed during overhaul often necessitate amending the manufacturer’s run-in procedures. Follow the approved amended run-in procedures in such instances.
NOTE: Do not run up engines on the ground for long periods of time with the cowling off. The engine will overheat because cylinder cooling has been disrupted.
8-14. COMPRESSION TESTING OF AIRCRAFT ENGINE CYLINDERS. A test to determine the internal condition of the combustion chamber cylinder assembly by ascertaining if any appreciable internal leakage is occurring is compression testing of aircraft engine cylinders.
If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine and procedures in subparagraphs 8-14b(5)(i) and (j) fail to raise the compression reading, the cylinder must be removed and inspected. To determine the cylinder’s problem area, have someone hold the propeller at the weak cylinder’s top dead center (TDC) and, with compressed air still being applied, listen. If air is heard coming out of the exhaust pipe, the cylinder’s exhaust
Par 8-14 Page 8-8 valve is not seating properly. If air is heard leaking out of the air cleaner/carburetor heat box, the intake valve is leaking. With the oil dipstick removed, and air is rushing out, the piston rings are defective. Remove and repair/overhaul the defective cylinder.
a. Differential Compression Test. The most common type of compression tester currently in use is the differential pressure-type tester. It provides a cross-reference to validate the readings obtained and tends to assure that the cylinder is defective before it is removed.
Before beginning a compression test, consider the following points:
(1) When the spark plugs are removed from the engine, identify them to coincide with the cylinder and location from which they were removed. Close examination of the plugs will reveal the actual operating conditions and aid in diagnosing problems within each individual cylinder.
(2) The operating and maintenance records of the engine should be reviewed. Records of previous compression tests are of assistance in determining progressive wear conditions and help to establish the necessary maintenance corrective actions.
b. Differential Pressure Compression Test. The differential pressure tester is designed to check the compression of aircraft engines by measuring the leakage through the cylinders caused by worn or damaged components. The operation of the compression tester is based on the principle that for any given airflow through a fixed orifice, a constant pressure drop across that orifice will result. The restrictor orifice dimensions in the differential pressure tester should be sized for the particular engine as follows:
(1) For an engine cylinder having less than a 5.00-inch bore, 0.040-inch orifice diameter, 0.250-inch long, and a 60-degree approach angle.
(2) For an engine cylinder with 5.00-inch bore and over 0.060-inch orifice diameter,
0.250-inch long, and a 60-degree approach angle.
(3) A typical schematic diagram of the differential pressure tester is shown in Figure 8-1.
Par 8-14 Page 8-9 FIGURE 8-1. SCHEMATIC OF DIFFERENTIAL PRESSURE COMPRESSION TESTER (
4) As the regulated air pressure is applied to one side of the restrictor orifice with the air valve closed, there will be no leakage on the other side of the orifice, and both pressure gauges will read the same. However, when the air valve is opened and leakage through the cylinder increases, the cylinder pressure gauge will record a proportionally lower reading.
(5) While performing the check, the following procedures are listed to outline the principles involved and are intended to supplement the manufacturer’s instructions for the particular tester being used.
(a) Perform the compression test as soon as possible after the engine is shut down to ensure that the piston rings, cylinder walls, and other engine parts are well lubricated.
(b) Remove the most accessible spark plug from each cylinder.
(c) With the air valve closed, apply an external source of clean air (approximately 100 to 120 pounds per square inch (psi)) to the tester.
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(d) Install an adapter in the spark plug bushing and connect the compression tester to the cylinder.
(e) Adjust the pressure regulator to obtain a reading of 20 psi on the regulator pressure gauge. At this time, the cylinder pressure gauge should also register 20 psi.
(f) Turn the crankshaft, by hand, in the direction of rotation until the piston (in the cylinder being checked) is coming up on its compression stroke. Slowly open the air valve and pressurize the cylinder to 80 psi.
CAUTION: Care must be exercised in opening the air valve since sufficient air pressure will have built up in the cylinder to cause it to rapidly rotate the propeller if the piston is not at TDC.
(g) Continue rotating the engine against this pressure until the piston reaches TDC.
Reaching TDC is indicated by a flat spot or sudden decrease in force required to turn the crankshaft. If the crankshaft is rotated too far, back up at least one-half revolution and start over again to eliminate the effect of backlash in the valve operating mechanism and to keep piston rings seated on the lower ring lands.
(h) Open the air valve completely. Check the regulated pressure and readjust, if necessary, to read 80 psi.
(i) Observe the pressure indication of the cylinder pressure gauge. The difference between this pressure and the pressure shown by the regulator pressure gauge is the amount of leakage through the cylinder. A loss in excess of 25 percent of the input air pressure is cause to suspect the cylinder of being defective; however, recheck the readings after operating the engine for at least 3 minutes to allow for sealing of the rings with oil.
(j) If leakage is still occurring after a recheck, it may be possible to correct a low reading. This is accomplished by placing a fiber drift on the rocker arm directly over the valve stem and tapping the drift several times with a hammer to dislodge any foreign material between the valve face and seat.
NOTE: When correcting a low reading in this manner, rotate the propeller so the piston will not be at TDC. This is necessary to prevent the valve from striking the top of the piston in some engines. Rotate the engine before rechecking compression to reseat the valves in the normal manner.
8-15. SPARK PLUGS. The spark plug provides the high-voltage electrical spark to ignite the fuel/air mixture in the cylinder. The types of spark plugs used in different engines will vary with regard to heat range, reach, thread size, and other characteristics required by the particular installation.
a. Heat Range. The heat range of a spark plug is the principal factor governing aircraft performance under various service conditions. The term “heat range” refers to the classification of spark plugs according to their ability to transfer heat from the firing end of the spark plug to the cylinder head.
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(1) Spark plugs have been classified as “hot,” “normal,” and “cold.” However, these terms may be misleading because the heat range varies through many degrees of temperature from extremely hot to extremely cold. Thus, the words “hot,” “cold,” and “normal” do not necessarily tell the whole story.
(2) Since the insulator is designed to be the hottest part of the spark plug, its temperature can be related to the pre-ignition and fouling regions as shown in Figure 8-2.
Pre-ignition is likely to occur if surface areas in the combustion chamber exceed critical limits or if the spark plug core nose temperature exceeds 1,630 ºF (888 ºC). However, fouling or short circuiting of the plug due to carbon deposits is likely to occur if the insulator tip temperature drops below approximately 800 ºF (427 ºC). Since spark plugs must operate between fairly well-defined temperature limits, they must be supplied in various heat ranges to meet the requirements of different engines under a variety of operating conditions.
FIGURE 8-2. CHART OF SPARK PLUG TEMPERATURE RANGES (
3) From the engineering standpoint, each individual plug must be designed to offer the widest possible operating range. This means that a given type of spark plug should operate as hot as possible at low speeds and light loads and as cool as possible under cruising and takeoff power. Plug performance, therefore, depends on the operating temperature of the insulator nose with the most desirable temperature range falling between 1,000 ºF and 1,250 ºF (538 ºC and 677 ºC).
(4) Fundamentally, an engine which runs hot requires a relatively cold spark plug, whereas an engine which runs cool requires a relatively hot spark plug. If a hot spark plug is installed in an engine which runs hot, the spark plug tip will be overheated and cause pre-ignition. If a cold spark plug is installed in an engine which runs cool, the tip of the spark plug will collect unburned carbon, causing fouling of the plug. The principal factors governing the heat range of aircraft spark plugs are:
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(a) The distance between the copper sleeve around the insulator and the insulator tip;
(b) The thermal conductivity of the insulating material;
(c) The thermal conductivity of the electrode;
(d) The rate of heat transfer between the electrode and the insulator;
(e) The shape of the insulator tip;
(f) The distance between the insulator tip and the shell; and
(g) The type of outside gasket used.
(5) “Hot” plugs have a long insulator nose; thereby, creating a long heat transfer path, whereas “cold” plugs have a relatively short insulator to provide a rapid transfer of heat to the cylinder head (see Figure 8-3).
FIGURE 8-3. HOT AND COLD SPARK PLUGS
b. R each. The spark plug reach (see Figure 8-4) is the threaded portion which is inserted into the spark plug bushing of the cylinder. A plug with the proper reach will ensure that the electrode end inside the cylinder is in the best position to achieve ignition. Spark plug seizure or improper combustion within the cylinder will probably occur if a plug with the wrong reach is used. Shell threads of spark plugs are classified as 14- or 18-mm spark plug diameter, long reach or short reach, thus:
Par 8-15 Page 8-13 FIGURE 8-4. SPARK PLUG REACH Diameter Long Reach Short Reach 14 mm 1/2 in (12.7 mm) 3/8 in (9.53 mm) 18 mm 13/16 in (20.64 mm) 1/2 in (12.7 mm)
c. Insta llation Procedures. When installing spark plugs, observe the following procedure.
(1) Visually inspect the plug for cleanliness and the condition of the threads, ceramic, and electrodes.
NOTE: Never install a spark plug which has been dropped and always use new gaskets every time you install a spark plug.
(2) Check the plug for the proper gap setting using a round wire feeler gauge as shown in Figure 8-5. In the case of used plugs, procedures for cleaning and regapping are usually contained in the various manufacturers’ manuals.
FIGURE 8-5. METHOD OF CHECKING SPARK PLUG GAP
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(3) Check the plug and cylinder bushing to ascertain that only one gasket is used per spark plug. When a thermocouple-type gasket is used, no other gasket is required.
(4) Apply anti-seize compound sparingly to the shell threads but do not allow the compound to contact the electrodes since the material is conductive and will short out the plug. If desired, the use of anti-seize compound may be eliminated on engines equipped with stainless steel spark plug bushings or inserts.
(5) Screw the plug into the cylinder head as far as possible by hand. If the plug will not turn easily to within two or three threads of the gasket, it may be necessary to clean the threads.
NOTE: Cleaning inserts with a tap is not recommended as permanent damage to the insert may result.
(6) Seat the proper socket securely on the spark plug and tighten to the torque limit specified by the engine manufacturer before proceeding to the next plug.
CAUTION: A loose spark plug will not transfer heat properly and, during engine operation, may overheat to the point in which the nose ceramic will become a “hot spot” and cause pre-ignition. However, avoid over-tightening as damage to the plug and bushing may result.
(7) Connect the ignition lead after wiping clean with a dry, lint-free cloth. Insert the terminal assembly into the spark plug in a straight line. (Care should be taken as improper techniques can damage the terminal sleeves.) Screw the connector nut into place until finger tight, then tighten an additional one-quarter turn while holding the elbow in the proper position.
(8) Perform an engine runup after installing a new set of spark plugs. When the engine has reached normal operating temperatures, check the magnetos and spark plugs in accordance with the manufacturer’s instructions.
8-16. OPERATIONAL PROBLEMS. Whenever problems develop during engine operation and appear to be caused by the ignition system, it is recommended that the spark plugs and ignition harnesses be checked first before working on the magnetos. The following are the more common spark plug malfunctions and are relatively easy to identify.
a. Fouling.
(1) Carbon fouling (see Figure 8-6) is identified by the dull black, sooty deposits on the electrode end of the plug. Although the primary causes are excessive ground idling and rich idle mixtures, a cold heat range may also be a contributing factor.
Par 8-16 Page 8-15 FIGURE 8-6. TYPICAL CARBON-FOULED SPARK PLUG (
2) Lead fouling is characterized by hard, dark, cinder-like globules which gradually fill up the electrode cavity and short out the plug (see Figure 8-6a). The primary cause for this condition is poor fuel vaporization combined with a high tetraethyllead content fuel. A cold heat range may also contribute to this condition.
FIGURE 8-6a. TYPICAL LEAD-FOULED SPARK PLUG
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(3) Oil fouling is identified by a wet, black carbon deposit over the entire firing end of the plug as shown in Figure 8-6b. This condition is fairly common on the lower plugs in horizontally opposed engines and both plugs in the lower cylinders of radial engines. Oil fouling is normally caused by oil drainage past the piston rings after shutdown. However, when both spark plugs removed from the same cylinder are badly fouled with oil and carbon, some form of engine damage should be suspected and the cylinder more closely inspected. Mild forms of oil fouling can usually be cleared up by slowly increasing power while running the engine until the deposits are burned off and the misfiring stops.
FIGURE 8-6b. TYPICAL OIL-FOULED SPARK PLUG
b. F used Electrodes. There are many different types of malfunctions which result in fused spark plug electrodes; however, most are associated with pre-ignition as either the cause or the effect. For this reason, any time a spark plug is found with the following defects, further investigation of the cylinder and piston should be conducted.
(1) Occasionally, the ceramic nose core will crack, break away, and remain trapped behind the ground electrode. This piece of insulation material will then build up heat to the point it will ignite the fuel/air mixture prematurely. The high temperatures and pressures encountered during this condition can cause damage to the cylinder and piston and ultimately lead to fusing and shorting out of the plug (see Figure 8-6c).
Par 8-16 Page 8-17 FIGURE 8-6c. TYPICAL SPARK PLUG WITH CRACKED CORE NOSE (
2) Corrosive gases formed by combustion and the high voltage spark have eroded the electrodes. Spark plugs in this condition require more voltage to fire—often more than the ignition system can produce (see Figure 8-6d).
FIGURE 8-6d. TYPICAL WORN OUT SPARK PLUG