Aircraft technical Basics: TM 1-405, Aircraft Aircraft Engines, 1941: IV - Repair Principles

SECTION IV - REPAIR PRINCIPLES

35. General.    a. The information in this section pertaining to engine repair principles is limited to the repairs which may be accomplished in the field, provided the necessary tools and equipment are available. Although the following instructions are given with the assumption that the engine has been removed from the aircraft for "top" overhaul, the procedure is equally applicable to a large number of repairs which can be accomplished with the engine installed in the aircraft.

b. The various major units of aircraft engines are very much the same in general design and construction, the principal difference being in their size and arrangement. In view of this similarity in construction, the same general procedure of repair may be followed for all types with few exceptions. The most important items that are necessary in connection with the repair of a particular type of engine are a complete set of special tools and the engine specifications. For detailed information on repairs reference must be made to technical publications on the equipment involved.

36. Preparation of engine for disassembly.-Mount the engine on a suitable revolving stand that will permit the rotation of the engine to any desirable position. As the various units and parts are removed from the engine they should be placed on a bench or portable stand for cleaning and inspection. Small parts such as bolts, nuts, washers. etc.. should be placed in suitable containers where they will not be misplaced. Cleaning of engine parts may be accomplished by suitable cleaning solutions and compressed air. In either case the cleaning should be done in some part of the shop where the dirt and refuse may be readily disposed of, preferably in separate room. A mixture of carbon tetrachloride and naphtha (50 percent by volume of each) makes tin excellent noninflammable cleaning solution.

37. Disassembly. Due to the difference in construction of the various types of engines, the order and procedure of disassembly will vary somewhat for each type; therefore only general instructions that may be applied to all types will be given here.

a. Usually all accessories should be removed first. This equipment, on the average engine, consists of the magnetos, carburetors, generator, starter, spark plugs, etc. Removal of these items eliminates possible breakage during disassembly. Next. remove all cylinder attachments. This usually includes the push rods, manifolds, ignition wiring. fuel, oil and coolant lines etc.

b. Dismount the cylinder assemblies with valve mechanism intact. Extreme care should be exercised in removing cylinders to prevent damage to pistons and piston rings. If possible, have the piston at the top of the stroke when the cylinder is removed and disassemble the piston from the connecting rods immediately upon removal of the cylinder. When dismounting the cylinders from radial-type engines, the cylinder in which the master connecting rod is located should be removed last, thus preventing its free movement during disassembly of the remaining cylinder. Screw drivers or similar tools should never be used as a pry under cylinder flanges as such will mar the mating surfaces or spring the cylinder hold-down flanges.

c. During. disassembly of the various units, all marks and fits should be carefully observed so is to facilitate assembly and adjustment.

38. Clearance specifications. - The correct clearance to be allowed between the various moving parts of an engine depends upon the material of which the part is fabricated, the stresses to which it is subjected, and the temperature at which it operates. The manufacturer uses certain clearance specifications for factory assembly and adjustment of each type of engine; these specifications have been determined through careful experimentation, and should be used by the mechanic during repair. The clearance tables are usually given in four columns of dimensions, for example, minimum clearance for new parts, desired clearance for new parts, maximum clearance for new parts, and maximum clearance allowable due to wear. In addition, each dimension is usually followed by the letter "L" or "T," the letter "L" indicating a loose fit and the letter "T" indicating a tight fit. For example, the dimension 0.002-inch L indicates that the I. D. of the outer mating surface is 0.002 inch larger than the O. D. of the inner mating surface, and the dimension 0.002-inch T indicates the I. D. of the outer mating surface is 0.002 inch smaller than the. O. D. of the inner mating surface.

39. Cylinder inspection and repair. - a. The cylinder barrels should be inspected for scores, scratches, and wear. If scores, scratches, or excessive wear are found in any one cylinder, it should be replaced.

b. In the case of liquid-cooled engines, the cylinder jacket should be tested under pressure with air or water.

c. Check the valve springs for general condition and tension. In some instances valve springs may lose their tension due to overheating and fatigue; therefore, they should be carefully checked with a valve-spring tester.

d. Check the valves for condition of the stem and face. If the valve stem is worn or the valve face contains pits and scratches, the valve should be replaced. (For various fits and tolerances, consult hand book on individual engines.) Check the valve seat for condition. If it contains pits or scratches, it should be resurfaced with a special reseating tool. After resurfacing the valve seat, the valve should be carefully lapped or ground into the seat. Use a medium grade grinding compound for this purpose. After the valves have been lapped in, they should be installed in the cylinder and tested for proper seating by pouring gasoline into the combustion chamber. Any irregularity in the seat or valve face will be indicated by the leaking of gasoline into the valve ports.

e. Check the valve stem guides for general condition and wear. In the event that new valve stem guides are installed, the valve seats should be refaced after installation of the guides.

f. The remaining valve mechanism should be given a thorough inspection, and care must be taken to see that all parts are marked before removal, so that they may be replaced exactly as removed. A systematic check up should be made in the following order:

(1) Rocker arms - for correct side clearance, fit of shaft, condition of roller, and condition of ball socket.

(2) Push rods   for condition of ball ends and alinement (care must be taken to avoid reversing the push rods on reassembly).

(3) Cam followers - for proper functioning, fit in guide, and condition of ball socket.

(4) Overhead lubrication system - for freeness from restrictions and particles of foreign matter.

(5) The above procedure applies particularly to radial engine installations and must be modified to include: a check of the overhead camshafts etc., on most liquid-cooled engines. An indication of failure in any of the units calls for immediate replacement.

40. Piston inspection and repair. - a. Remove all carbon deposits from the piston by means of an approved carbon solvent.

b. After the piston is thoroughly cleaned, it should be checked for scores, scratches, and wear. Light scores and scratches are not serious and may he dressed down by an oilstone. Deep scores and scratches are cause for replacement of the piston. Wear on one side of the piston skirt usually indicates bent or twisted connecting rods and, if excessive, necessitates replacement of the piston as well as the rod. A new piston should weigh within a specified amount of the old piston when replacement is made.

r. Piston rings should be checked for tension, clearance, and wear. Rings are removed from the cylinder when checking them for tension. Those with less than 1/4-inch gap, or which require less than 5 pounds tension to close the gap, are weak; and ordinarily should be replaced. Rings should fit snug but not tight in the piston groove. The gap is checked by inserting the ring near the lower end of the cylinder barrel and measuring with a thickness gage, being careful to keep the ring perfectly square in the cylinder bore. In fitting new rings, if the gap is less than specified, it may be increased by dressing down the butt ends with a small, smooth file. If the gap is more than specified by the table of limits, the ring should not be used.

d. Check the piston pins as well as the piston-pin bushings for wear. The piston pin should be a hand push fit in the piston at room temperature. Its fit in the connecting rod depends entirely upon the type of piston pin used. Piston pins in 0.005 and 0.010 inch oversize are usually held in stock by the engine manufacturer for replacement purposes. When oversize pins are used, the piston and connecting-rod bushings must be reamed to size.

41. Manifolds and pipes.--a. Intake pipes of radial engines should be carefully inspected for cracks and dents, especially at their attaching ends. Intake manifold assemblies should be checked for alinement at their cylinder attachment flanges with the use of a surface plate. If out of alinement, they may be lapped or dressed down on a lapping plate.

b. Check the attaching flanges of exhaust manifolds for warpage and cracks. Cracked tubing may be welded; however, if the flanges are warped, the unit must be replaced.

42. Assembly. - In view of the difference in construction of the various types of engines, the order and general procedure of assembly will be somewhat different for each type. However, if the order and procedure of disassembly were carefully observed, no difficulty should he experienced during assembly. The following points apply generally to all types:

a. Keep all parts scrupulously clean.

b. Lubricate all bearings, bushings, shafts. etc., during assembly.

c. Replace all gaskets, packings, etc.

d. Safety all bolts, nuts, etc., with new cotter pins or safety wire.

e. Assemble all parts and units in the reverse order of disassembly and according to the manufacturer's marks.

f. Never use parts that are defective or worn beyond the limits given in the table of clearance specifications.

43. Checking valve timing.  a. If the engine parts were assembled according to the manufacturer's marks, the valve timing should be correct; however, in view of the importance of correct valve timing and the possibility of errors during engine assembly, a complete and thorough check of the valve timing after assembly is advisable. The exact procedure for timing each type of engine differs somewhat; therefore only instructions that may be generally applied to all types will be given here.

b. The following valve-timing specifications with respect to the crankshaft position should be checked:

(1) Intake valve opening position.

(2) Intake valve closing position.

(3) Exhaust valve opening position.

(4) Exhaust valve closing position.

c. All valve-timing specifications are given in degrees of crankshaft  travel and usually refer to the position of the crankpin, before or after top and bottom center. In tables of specifications the following abbreviations are generally used:

(1) Top center (TC).

(2) Bottom center (BC).

(3) Before top center (BTC),

(4) After top center (ATC).

(5) Before bottom center (BBC).

(6) After bottom center (ABC).

d. The following is the general procedure for checking the valve timing of the average engine:

(1) Adjust the valve tappets to the timing clearance specified by the manufacturer. (This usually differs from cold clearance and the two should not be confused.) The clearances, are set with the valves fully closed and with the piston on top center following the compression stroke.

(2) Install the timing disk or fixture on the engine and a top center indicator in the front spark-plug hole of No. 1 cylinder.

(3) Rotate the propeller shaft slowly in the proper direction of rotation until No. 1 piston is at exact top center.

(4) Install the timing fixture pointer so that it registers exactly with the top center mark on the timing disk.

(5) Place a 0.0015-inch thickness gage blade between the exhaust valve rocker arm and the valve stem in No. 1 cylinder and continue to turn the crankshaft until the thickness gage blade no longer has clearance, indicating that the exhaust valve in No. 1 cylinder is just at the point of opening.

(6) If the engine is in time the pointer will register with the point on the timing disk marked  "exhaust valve opens." Should this mark not correspond to the indication of the pointer, the engine must be retimed.

(7) As a further check, the 0.0015-inch thickness gage blade should next he inserted under the intake valve rocker arm of No. 1 cylinder and the above operation repeated. At the point that the thickness gage no longer has clearance, the pointer should indicate "intake valve opens'' on the timing fixture. A slight variation between the timing of the exhaust and intake valves is allowable, due to manufacturing tolerances and normal wear. Any excessive variation would, however, indicate an error in timing or mechanical trouble in the valve operating mechanism.

(8) In the event that the timing does not correspond to the specifications, it may be changed by resetting the timing gear or serrations.

(9) After the valve timing has been checked all valve tappets should be adjusted to the specified cold clearance.

44. Checking ignition timing. - The exact procedure of checking the ignition timing varies somewhat for each type of engine; therefore, only general instructions will be given here. The following general procedure may be applied to the average engine:

a. Install the timing fixture on the propeller shaft as when checking the valve timing.

b. Adjust the breaker points to the correct clearance (pivot type).

c. Connect a test light circuit in series with ignition breaker points or place a very thin gage between the breaker points (spark fully advanced).

d. Turn the propeller shaft in the direction of rotation until No. 1 piston starts up on the compression stroke, then proceed very slowly until the breaker points just break (this will be indicated by the light going out or the gage between the breaker points being released).

e. The pointer should now indicate the specified number of degrees before top center on the timing disk for the firing of No. 1 cylinder. Also the marks on the magneto distributor segment should line up with corresponding marks on the magneto housing indicating that the rotor is lined up with No. 1 distributor segment.

f. Where two magnetos or a double magneto is employed on an engine requiring "staggered" ignition timing, each unit or distributor must be checked separately. In case the ignition units are "synchronized" the timing of both systems may be accomplished at the same time.