Aircraft technical Basics: TM 1-406, Aircraft Electrical Systems, 1940: 2. Ignition Sytem

TM 1-406, TECHNICAL MANUAL,  AIRCRAFT ELECTRICAL SYSTEMS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON, October 18, 1940.

SECTION II - IGNITION SYSTEM

8. General.-a. When a mixture of fuel vapor and air is admitted into a cylinder and compressed, the next event in the cycle of engine operation is the ignition of the compressed charge at the proper time. The most convenient means of igniting this charge is by causing an electric spark to jump an air gap in a spark plug installed in the head or combustion chamber of the engine cylinder. The source of the electrical energy for producing the electric spark may be obtained by the use of a battery or high tension magneto. Practically all aircraft engines employ high tension magnetos, which are usually supplemented by a battery operated booster system to facilitate starting; therefor, little space will be devoted to the battery generator ignition systems in this manual.

b. A complete aircraft engine ignition system usually incorporates the following major units:

(1) High tension magneto (usually two single or one double type).

(2) Spark plugs (usually two per cylinder),

(3) Switch.

(4) Ignition wiring (usually shielded),

(5) Booster system.

(6) Storage battery.

9. High tension magneto.-a. A high tension magneto is a mechanically driven generator, incorporating all of the elements necessary to produce a high tension current for distribution to the spark plugs.

b. In any form of high tension magneto, these elements include the following :

(1) Magnetic system.

(2) Iron core wound with a primary and secondary winding.

(3) Contact breaker or interrupter to interrupt primary circuit at predetermined intervals.

(4) High tension distributor system.

FIGURE 16.-Various types of magnetos.

c. The various types of magnetos are illustrated in figure 16.

(1) When the rotor is comprised of iron segments built, in the form of a sleeve which revolves around the stationary armature as shown in figure 16(1), the machine is called a sleeve inductor type magneto.

(2) When the rotor is comprised of iron masses operating in conjunction with an external armature core as shown in figure 16(2), the machine is called a polar inductor type magneto.


FIGURE 17.-Magnetic system of high-tension magneto.

(3) When the rotating magnet and the stationary coil as shown in figure 16(3) are used, the machine is called the rotating magnet type.

(4) When the armature rotates as shown in figure 16(4), the machine is called a rotating armature or shuttle type magneto.

FIGURE 18.-Coil assembly of high-tension magneto.

d. A general description of the major units of a high tension magneto follows:

(1) The magnetic system (fig. 17) for any form of high tension magneto consists of permanent magnets which are of the horseshoe or rotating bell shaped type; the latter can be either 2-, 4-, or 8-pole depending upon requirements. Included as part of the magnetic system is the magnetic path which is made up of the stationary laminated pole pieces and the laminated iron core.

(2) The coil assembly (fig. 18) consists of a soft iron laminated core around which is wound a primary winding consisting of a few turns of comparatively coarse wire, and a secondary winding of many turns of fine wire. The condenser is included in the coil construction of some magnetos, while in others it is a separate unit.

(3) In order to obtain the maximum secondary voltage at the, proper time, it is necessary to cause a sudden interruption of the primary current when its value is maximum, which will cause an instantaneous collapse of the electromagnetic field associated with it across the secondary winding and induce the high voltage in the secondary winding at this instant. This operation is accomplished by the breaker or interrupter, a mechanical device in which two contact points are opened and closed by means of a mechanically operated cam (fig. 19) . On some magnetos the breaker assembly is the rotating member, while the cam or cams are stationary; however, on the more modern type of magnetos the breaker is stationary and the cam is rotated on a shaft. Some breaker assemblies are of the lever type, figure 19 (1) , in which the cam causes the lever to turn on an axle, opening the contact points. The most recent development is the pivotless type breaker, figure 19(2). In this design, the cam strikes a follower which in turn opens or breaks apart the contact points. As the cam rides from under the follower, a flat spring closes the contacts.

FIGURE 19.-Breaker assembly.

(4) The distributor system of a magneto (fig. 20) consists of all highly insulated parts, and is that part of the system that conducts the high-tension current from the secondary coil to the distributor, thence through the high-tension leads and out to the respective spark plugs.

(5) In some magnetos a safety gap is shunted across the distributor circuit and is located as close to the secondary coil as possible. The purpose of the safety gap is to prevent breaking down of highly insulated parts by the excessive voltage of the secondary in case of an "open" circuit. Under normal conditions the resistance of the secondary current jumping across the safety gap is much greater than the resistance of the spark plug gap, and yet sufficiently low to prevent the breakdown of the parts of the secondary circuit in the event of an "open" circuit. However, in modern aircraft magnetos designed for altitude, the safety gap is eliminated by the use of greater insulation of the secondary circuit to withstand the excessive voltage resulting from an open circuit.

FIGURE 20.-Distributor system.

e. The basic electrical operating principles of all high-tension magnetos are based on the following forms of induction :

(1) Magnetic.-Magnetic induction is the causing of a magnetic substance to become a magnet by bringing it under the influence of a magnetizing force.

(2) Electromagnetic.-Electromagnetic induction is that phenomenon whereby an electromotive force is induced in any conductor that cuts across or is cut by a magnetic flux.

(3) Mutual.-Mutual induction is the electromagnetic induction produced by one circuit in a nearby circuit, due to the variable flux of the first circuit cutting the conductor of the second circuit.

(4) Self.-Self induction is that phenomenon whereby a change in the current in a conductor induces a counterelectromotive force (e. m. f.) in the conductor itself.

(5) Electrostatic.-Electrostatic induction is the inducing or setting up of electrical charges in bodies separated from the charging source by some dielectric or insulator when an e.m.f. is impressed across the conductor.

f. When the magnet assembly (fig. 21) is rotated, rapid reversals of the flux in the armature core are accomplished as follows :

FIGURE 21.-Magnetic circuits of four-pole high-tension magneto.

(1) As the rotating magnet is rotated to position (fig. 21(1)) the magnetic lines of force are conducted from the N pole of the magnet to the stationary pole piece, through the iron core to the opposite pole piece and to the magnet pole S, as indicated by the arrows. The lines of force increase in magnitude up to a maximum as the rotating magnet is turned to its 45° position (fig. 21(2)). As the magnet is rotated beyond the 45° position, the magnetic lines of force are decreasing in magnitude until the magnet reaches the neutral position, where one of the magnet poles is centered between the two stationary-pole pieces (fig. 21 (3)). When the magnet is in the neutral position, no magnetic lines of force are conducted through the soft. iron core as they are short circuited by the pole pieces. This position of the magnet is called "magnetic short circuit." As the magnet is rotated beyond its neutral position (fig. 21 (4)), the magnetic lines of force are reversing in direction through the magnetic circuit and will pass through the same cycle until its next neutral position. A study of the magnetic curve (fig. 22) will indicate that the number of neutral positions which the magnet will pass during one revolut ion is equal to the number of poles on the magnet.

(2) One end of the primary winding is grounded and the other end connected in series with the breaker contact points. As the magnet is rotated there is a flux movement across the primary winding, since the magnet flux is constantly changing, which induces e. m. f. in the primary coil. When the contact points close, current up to a certain value will flow through the primary circuit. Any increase in speed of the magnet will increase the flux movement which in turn increases the value of the e. m. f. and current flow. Inasmuch as the magnetic polarity of the iron core changes every 90° (four-pole magnet), the flux reversals result in the induction of an alternating e.m.f. When the contact points close and current is flowing in the primary winding, it will set up an electromagnetic field which tends to oppose any change of the permanent magnetic field, producing a resultant field as shown graphically in figure 23. When the contact points are open, no current is flowing in the primary winding and the permanent magnetic field curve tends to fall off as shown by the broken lines. How-ever, when the contact points are closed, the field of the current flow - ing in the primary winding opposes the permanent magnetic field and actually results in storing up magnetic flux as represented. by the resultant field curve.

FIGURE 22. -Magnetic curve representing flux travel of four-pole magneto.

FIGURE 23.-Magnetic primary and secondary current curve for four-pole magneto

(3) Wound on the outside and grounded through the primary winding is the secondary winding. During the period that the contact points are closed the primary current reaches its maximum value, and when the contact points open, the electromagnetic field set up by the primary current collapses. Consequently, the stored up magnetic flux previously mentioned instantly collapses and results in a very rapid magnetic flux movement across the secondary winding, inducing a very high secondary e. in. f. and current flow in the secondary circuit. This causes the spark at the spark plug gap, which ignites the compressed charge in the cylinder. Each spark actually consists of one peak discharge after which a series of small oscillations occur, as represented by the secondary voltage curve in figure 23, until the voltage becomes too low to maintain the spark. In the two- and four-pole types of magnetos, ample time is given for the complete discharge of the spark before the contact points close, starting another cycle.

(4) When the contact points open, the collapsing stored up flux, in addition to producing the spark at the spark plug, causes self-inductance in the primary winding. The current, due to self-inductance, would arc across the contact points at their point of opening and would burn away the contact point material unless counteracted. A condenser connected in parallel across the contact points absorbs the effect of self-inductance thereby supressing the arc.

g. The various types and models of standard aircraft magnetos are designated by certain letters and figures whereby a description of the magneto can be obtained at a glance. An explanation and example of the symbols are given below :

Example: Type SF 14 L designates a single type magneto, flange mounted, 14 cylinder and anticlockwise direction of rotation. In cases where an additional figure or letter is noted in the type designation, such as SF 14 L-1 a, it merely denotes that some minor change or some new feature has been incorporated in the magneto.

h. A diagram of the electric and magnetic circuits of a conventional single four-pole aircraft magneto is illustrated in figure 24. The rotating magnet (1) has four poles joined together inside the laminated ends into pairs. The two N poles make up one pair and the two S poles make up the other pair. The rotating magnet (1) rotates between the laminated pole pieces (2), producing an alternating field in the iron core (3) causing the flux changes across the primary winding (4) and inducing the primary e. m. f. One end of the primary winding (4) is grounded to core (3). The other end of the primary winding is connected to the primary bridge (6), thence to the insulated contact spring (7) and grounded contact on the breaker support (8), completing the primary circuit.

(1) When the current in the primary winding reaches its maximum value, the breaker cam (9) is timed with the cam follower (10) to lift the main spring (7) and open the contact points, interrupting the primary circuit.

FIGURE 24.-Typical single aircraft magneto, four-pole construction.

(2) The interruption of the primary circuit causes induction to take place in the secondary winding (5). One end of the secondary winding is connected to ground through the primary winding, and the other end is attached to the secondary coil contact which is an integral part of the coil. From the coil contact, the secondary current is conducted to the high tension button contact (11) , to the carbon brush (12) , thence to segment (13) of the distributor finger (14). The distributor finger (14) is fixed on the large distributor gear (15) in a definite position relative to the opening of the contact points. The distributor finger segment (13) makes contact with the distributor block electrodes (16), thereby transmitting the current through the high tension cable and spark plugs.

(3) The condenser (17) is connected in parallel across the contact points to absorb the current from self-inductance in the primary winding, thereby suppressing arcing when the contact points open.

i. A diagram of the electric and magnetic circuits of a single eight-pole magneto is illustrated in figure 25. Except for a secondary condenser, eight-pole magnet, and eight-lobe cam, the electric and magnetic circuits are very similar to the four-pole magneto.

FIGURE 25.-Typical single aircraft magneto, eight-pole construction.

j. An aircraft magneto which has the same characteristics as other magnetos, except that it is a true double ignition system incorporated in a single unit, is illustrated in figure 26. This magneto employs the principle of the rotating magnet and stationary coils. The rotating magnet is mounted between two pairs of pole shoes and produces four instantaneous reversals of the magnetic flux through the cores of the two coils, each located on opposite sides of the magneto housing. There are two breaker assemblies; consequently, this magneto produces four sparks from each coil for every revolution of the magneto drive shaft. The high tension current from the coil is distributed to two independent distributor heads mounted externally on the engine.

FIGURE 26.- Double aircraft type magneto.

10. Spark plugs.-a. One of the most vital units of an ignition system is the spark plug installed in the engine cylinder. It is a very highly insulated unit, which provides an air gap across which the secondary voltage of the ignition system causes current to flow for igniting the combustible mixture in the cylinder.

b. The general construction of the spark plug (fig. 27) consists of the following parts:

(1) Spark plug shell or base.

(2) Outside insulator, cylindrical in shape, which is a covering for the main insulator or cigarette that surrounds the central electrode.

(3) Central and ground electrodes.

(4) Spark plug bushing or barrel assembly.

(5) Copper asbestos gasket.

(6) Spark plug terminal.

c. Due to the various conditions under which a spark plug operates, its construction must meet the following requirements:

(1) The electrical insulation must be constructed to withstand excessively high voltage, not only against direct puncture of the insulation, but against flash-over at high altitudes and for proper operation in rain or snow.

(2) For radio shielding it is of the greatest importance that the spark plug capacity losses be as low as possible.

(3) Heat conduction in a spark plug must be balanced to carry away the heat absorbed from the hot gases of combustion to prevent pre-ignition and to maintain an operating temperature of the nose of the core below a certain value to prevent lead fouling as a result of fusing or melting of the deposit that collects when leaded gasoline is used. It is also important that the spark plug heat flow be balanced to permit a quick warm-up when starting a cold engine and to operate warm enough to condition the fuel around it on the compression stroke of the engine, insuring positive and rapid flame travel when the spark occurs at the electrodes.

(4) The spark plug must also seal the high combustion pressure developed within the cylinder and absorb the mechanical vibration resulting therefrom without deterioration of its electrical or thermal characteristics.

FIGURE 27.-Typical spark plug.

d. There are two conventional types of spark plugs in use: the porcelain and the mica insulated plugs. The majority of aircraft spark plugs at the present time are of the latter type; however, both types are made to fulfill the requirements of modern aircraft engines which operate at extremely high pressures and temperatures.

e. Aircraft engine manufacturers recommend certain types of spark plugs which have been found to be most suitable and should be used i f available. Spark plugs are usually classified, according to their construction and purpose, as hot or cold operating plugs (fig. 28).

(1) A hot operating spark plug is defined as one in which the insulation is maintained at a comparatively high temperature. This plug is usually employed in low power output engines where they are not subjected to extremely high operating temperatures. The hot operating spark plug has a large area of insulation exposed to the cylinder temperature which maintains the exposed insulation at a high temperature. This feature tends to reduce oil fouling tendencies at low operating speeds of the engine.

FIGURE 28.-Types of aircraft spark plugs in order of heat range.

(2) A cold operating plug is designed to provide for rapid dissipation of heat. This type plug is desirable for use in highly super-charged engines which operate at high temperatures. The cold operating spark plug has a much smaller area of insulation exposed to combustion temperatures as compared to a hot operating spark plug. This feature tends to reduce pre-ignition tendencies at high operating speeds of the engine.

f. Although specific spark plugs are tested and recommended for a particular engine, there are certain operating conditions which must be considered.

(1) On closely cowled engines and where ventilation to hot running plugs is limited, the engine should not be idled over long periods of time; otherwise, overheating of the spark plug will result which will increase the temperature of the nose and the electrodes to such an extent as to cause probable lead fouling.

(2) In case cold operating plugs are used, idling an engine too long may result in oil fouling and malfunctioning.

(3) Spark plugs operating above normal temperature may result in pre-ignition which may be accompanied by knocking or pounding of the engine. The first indication of pre-ignition is a decrease in r. p. m. or power, which in some cases may only drop a certain amount and remain constant. Pre-ignition may also cause after-firing of an engine when the ignition is cut off.

11. Ignition switches.-a. The ignition system for starting and stopping the engine is controlled by the ignition switch.

b. Switches used with the magneto ignition system merely open the primary circuit through the switch and ground when the switch is placed in the "on" position, and close or complete the primary circuit through the switch to the ground when placed in the "off" position. Magneto ignition switch operation differs from the battery ignition switch in that the battery type closes or completes the primary circuit when the switch is placed in the "on" position, and opens the primary circuit when placed in the "off" position. A comparison of a magneto switch and a battery switch circuit is shown in figure 29.

e. In many types of magneto ignition switches used on single engine installations, provisions are made for a battery circuit to furnish current for the operation of the booster system and starter solenoids when the switch is placed in the "both on" position, and when in the "off" position acts as a safety switch for the booster and starter switch circuit. Figure 30 illustrates the various positions of this type of switch.

FIGURE  29. - Typical ignition switch circuits.

FIGURE 30. - Switch positions of single engine magneto switch

d. The twin engine magneto ignition switch (fig. 31) incorporates an emergency switch which, when in the "off" position, opens the battery circuit and closes the primary magneto circuits through the switch to ground. In this type switch, before starting the engines it is necessary to place the emergency switch in the ``on" position to complete the battery circuit to the auxiliary electrical equipment and to open the primary circuit of the magnetos through the switch and ground. The magnetos on each engine are controlled independently by various switch positions.

FIGURE 31.-Dual-engine magneto switch.

12. Ignition wiring and shielding.-a. The purpose of the high tension wires or cables is to form a conducting path for the high-tension current from the magneto distributor blocks to the spark plugs. They are made of stranded tinned wire and covered with an insulating compound, homogeneous in character, placed con-centrically around the conductor. An additional braid is added over the insulation, and consists of closely woven cotton yarn, with a protective coating of nonhydroscopic compound, resistant to oil, water, and gasoline. This coating also has the property of with-standing high temperatures without injuring its flexibility.

b. The entire ignition system must be shielded to prevent interference with the radio equipment installed in the aircraft. This is accomplished by enclosing the ignition system with a metallic can or radio shield. This shield, being grounded to the engine, picks up the uncontrolled wave lengths produced by the ignition system and conducts them to the ground, eliminating interference with radio reception. Figure 32 illustrates the method of shielding magnetos by using special metal covers over the distributors. All aircraft engines are equipped with special ignition harnesses which incorporate the necessary shielding for the high-tension cable between the magneto and spark plugs.

FIGURE 32.-Shielded type magnetos.

FIGURE 33.-Radio shielded type spark plugs.

c. Spark plugs on all radial engines incorporate a shielding feature integral with the plug (fig. 33). V-type engines do not require spark-plug shields, as the engine cowling prevents the radiation of spark plug disturbances.

d. The low-tension ignition wires, such as the magneto and booster switch wires, are separately shielded throughout their length. High-tension wires are never included in a shield with low-tension wires.

13. Ignition boosters.-a. The purpose of a booster in an ignition system is to aid in engine starting. This system operates in con-unction with the starter and the magneto that are installed on the power plant. The coming-in speed, or the speed at which the magneto fires consistently, is at a certain r. p. m., and the inertia starter, when engaged, may crank the engine at a lower speed than the coming-in speed of the magneto; therefore, the booster system is added to aid in starting the engine.

b. (1) One type of booster system in use is in the form of an auxiliary high-tension magneto. The construction, as well as the electrical operation of the booster magneto, involves the principle of any high-tension magneto. However, the booster magneto is not coupled to the engine but is installed externally, and its operation is accomplished by hand. A typical booster magneto is illustrated in figure 34.

FIGURE 34.-Typical booster magneto.

(2) The turning of a large gear in mesh with a small gear results in the rapid make and break of the primary current, resulting in the inducing of a succession of impulses in the secondary winding. The secondary current of the booster magneto is conducted to the booster terminal of the engine magneto through a high-tension wire to the trailing segment on the distributor finger, thence to the segment on the distributor block, and through the lead to the spark plug.

c. (1) Where a battery current is available, a battery or vibrator booster system (fig. 35) is used. This system consists of a combination, in one unit, of a primary and secondary winding, contact points, and a condenser connected in parallel with the points. This unit when in operation will supply a rapid succession of sparks to the booster terminal on the magneto for starting.

FIGURE 35.-Battery booster system.

(2) When switch (E) is closed, the current from the battery is completed through the contacts (A), through the primary coil (B) wound around the soft iron core (C), and back through the ground to the battery negative terminal. During the period that the current is flowing through the primary coil, a magnetic field is being produced. When the magnetic effect of the iron core is great enough, the spring contact is attracted to the core, opening the contact points. At the instant of this break in the primary circuit, the electromagnetic field is collapsed across the secondary winding (D) and induces a high voltage sufficient to produce a spark impulse. When the contact points opened, the battery current ceased flowing through the coil, releasing the spring and closing the contact points. This rapid make and break of the primary circuit produces a rapid succession of spark impulses from the booster system to the engine magneto.

14. Battery ignition systems.-a. A battery ignition system (fig. 36) is somewhat similar to a magneto ignition system except for its primary source of energy. The essential elements in a battery generator ignition system are

(1) Storage battery.-The battery furnishes the primary current for starting purposes; however, a mechanically driven generator is used to furnish current, after the engine has attained a certain r. p. m., for ignition and for maintaining the battery in a charged condition.

(2) Interrupter or breaker mechanism .-The breaker mechanism incorporates the mechanically operated contact points and condenser. Some breaker mechanisms are provided with an automatic device for advancing or retarding the spark.

(3) Induction coil.-The induction coil transforms the low potential energy to a high voltage required to jump the spark plug gap and is similar in construction to a magneto armature or coil.

(4) Distributor.-The distributor conducts the high-tension current to the spark plugs, as in the case of magneto ignition distributors.

FIGURE 36.  - Typical battery ignition system.

b. When the contact points in the breaker mechanism are closed, a steady or constant voltage is applied to an inductive circuit which comprises the primary winding in series with a certain resistance, and the circuit will increase from zero in conformity with the wellknown law I=E/R. During the interval of time that the current is increasing, a magnetic field is also being produced and stored in the primary winding. At the instant of the break caused by the opening of the contact points, the electromagnetic field suddenly collapses and induces in the secondary winding a high voltage, which is sufficient to produce a spark at the spark plug electrodes. The action is analogous to what takes place in a high tension magneto, however the characteristics of the coil can be much more easily investigated because a constant voltage is being dealt with at all times, whereas in a magneto the voltage is alternating and is de-pendent upon the speed and strength of the flux reversals through the armature.

15. Typical aircraft engine ignition systems.-There are several typical aircraft ignition system hook-ups used in various types of aircraft, the difference being in the various auxiliary units used in conjunction with the system. Figure 37 illustrates a typical ignition system circuit used in single-engine aircraft and figure 38, one used in twin-engine aircraft.

FIGURE 37.-Typical single-engine ignition system.

FIGURE: 38.-Typical twin-engine ignition system.

16. Maintenance.-The following instructions pertaining to the maintenance of various units of the aircraft ignition systems are confined to the minor repairs and adjustments which can be accomplished in the field with the facilities on hand. In case of emergency, when it is evident that it requires less time to replace a unit than to repair it, such procedure is advisable, providing the replacement unit is readily available.

a. Magnetos.-Check the magnetos for security of mounting and condition as follows :

(1) Check the breaker assembly by removing the breaker cover and inspecting for general cleanliness, damaged or worn cam follower, and proper felt lubrication. If major defects are found, replace the entire breaker assembly. In the pivotless type breaker, cam follower wear is indicated by a small depression where it lifts against the end of the main spring. The clearance between the cam follower and the main spring is checked with the breaker assembly removed. Felt lubrication is satisfactory if oil appears on the surface when the felts are squeezed with the fingers. If the felts are dry and require lubrication, do not apply too much oil as the excess may be thrown off during operation on the contact points, causing them to burn and pit.

(2) Check the main breaker spring for proper tension with a special spring tension gage.

(3) With the breaker assembly installed, check for worn or loose cam and cam bearings by turning the engine crankshaft and noting the contact point opening on all lobes of the breaker cam.

(4) When checking the condition of the contact points of the pivotless type breaker, do not raise the main breaker spring beyond a point of % cinch clearance between the contacts. A further tension weakens the spring, which may result in faulty magneto operation. If contacts are burned or pitted they must be replaced. The con-tact points are adjusted to open at the proper position on the cam in relation to the timing marks on the breaker housing and not for any fixed clearance. To check this adjustment, loosen the two screws (A), figure 39, which hold the movable contact point, and by means of the eccentric screw (B) set the contact points so that they will just begin to open when the straight edge (C) coincides with timing marks (D) on the breaker housing. Tighten the screws (A) and check the adjustment by placing a strip of .0015-inch shim stock between the points and pulling against it slightly. When the shim stock can be released with a slight pull as the crankshaft is turned, the contact points are just opening. Permissible service tolerances are allowed for various magnetos, that is, the distance between the straight edge and the timing marks on the breaker housing. These tolerances should not he exceeded; therefore it is necessary to refer to the tolerance chart of the magneto which is installed.

(5) When checking the contact point clearance on the lever type breaker, rotate the engine crankshaft until the breaker lever is resting on the peak of any cam lobe and adjust the contact points according to specifications.

(6) Check the distributor head and distributor finger for cracks and signs of electrode arcing and clean if necessary. Check the distributor finger mounting screws for security, sticking or broken brushes. Re-place all defective parts.

FIGURE 39.-Adjusting contact points on pivotless type breaker.

(7) All magneto ball bearings and gears contain an adequate amount of grease and do not require lubrication between overhaul periods.

b. Spark plugs.-(1) Carefully check the terminal connections of unshielded spark plugs for condition and security.

(2) Check the barrel core nut for proper tightness, and if found loose the plug is removed for tightening with special tools. After tightening, the gap clearance is checked. Before reinstalling, check the threads on the electrode gap end of the plug for evidence of damage and proper lubrication.

(3) Check the shielded spark plug elbow terminals and shielding nuts for condition and security. A snug but not, too tight fit is desirable.

(4) When spark plugs are removed from an engine, careful visual inspection may assist in determining unsatisfactory condition.

(a) If the plug is clean and the metal parts show signs of over-heating, the cylinder has been running hot, indicating pre-ignition and detonation from improper fuel or poor cooling.

(b) If covered with fresh oil, the plug has not been firing and may be defective or shorted by failure of the ignition cable or shield.

(c) The presence of caked carbon is evidence of the beginning of excessive oil consumption.

(5) Check shielded type spark plug terminals for mechanical or insulator failure and for excessive accumulation of moisture.

(6) Use proper tools when removing or installing spark plugs. In-stall new gaskets, and do not tighten the plugs excessively into the cylinder bosses. The reconditioning of spark plugs which involves disassembly is not advisable.

c. Ignition switches.-Check the functioning of the ignition switches as follows:

(1) With the engine running at about one-third throttle, turn the ignition switch momentarily to the "off" position. If engine does not entirely cease firing, a defective functioning of the switch or its connections is indicated. During this test, the engine must not be excessively hot, and the period during which both switches are "off" must be brief so that the engine speed does not appreciably decrease. Warning: If the engine does not cease firing when the switches are placed in the "off" position, stop the engine by turning off the fuel. After the engine stops, do not touch the propeller until the difficulty is found and corrected, as the engine may start or kick-back causing serious injury. When operating the engine on only one switch, do not exceed cruising manifold pressure, otherwise overheating may occur.

(2) Check the switch mounting, terminal connections, cannon plugs, and leads for condition and security.

d. Ignition boosters. - (1) The magneto-type ignition booster does not require any maintenance between overhaul periods. A defective booster magneto is replaced.

(2) In the battery-type ignition booster, check the contact points for condition. Dirty or pitted points may be resurfaced with an oilstone, then washed in carbon tetrachloride. If no vibrating sound can be heard when the switch is closed, check the battery circuit at the switch and coil terminals. This trouble, provided the battery circuit is complete to the coil, is probably due to defective contact points or open primary winding, in which case the booster is replaced. If the vibrating sound can be heard when the switch is closed, but does not furnish secondary current, the probable trouble is a defective secondary winding, and replacement is required.

e. Ignition cable.-(1) Check the ignition cable for condition, tight connections and terminals, exposed ends, proper anchorage, and for chafing due to vibration.

(2) Troubles in the ignition cables are usually due to rough handling of spark plug leads and to vibration or loose cable clamps. In some instances, abnormal engine heat and weather elements may develop trouble inside the ignition manifold assembly. The electrical functioning of the high-tension harness assembly requires testing with special instruments and high-voltage testers, usually not available in the field.

(3) If fault is found in the shielding or bonding, the radio mechanic is called upon to make the necessary repairs.