Part 30 (1/2)
49 Main bearing sc.r.a.per For sc.r.a.ping in bearings.
50 Cylinder carbon sc.r.a.per For removing carbon from heads of cylinders.
51 Valve seating tool For seating valves in cylinder heads.
52 Sc.r.a.per, small For general bearing use.
53 Sc.r.a.per, large For general bearing use.
54 Crank-shaft f.l.a.n.g.e puller For pulling crank-shaft f.l.a.n.g.e from crank-shaft.
55 Piston and connecting rod racks.
56 Main bearing stud nuts and s.h.i.+m rack.
57 Main bearing board rack.
58 Rocker arm and cover rack.
The special tools and fixtures recommended by the Hall-Scott Company for work on their engines are clearly shown at Fig. 180. All tools are numbered and their uses may be clearly understood by reference to the ill.u.s.tration and explanatory list given on pages 410 and 411.
OVERHAULING AIRPLANE ENGINES
After an airplane engine has been in use for a period ranging from 60 to 80 hours, depending upon the type, it is necessary to give it a thorough overhauling before it is returned to service. To do this properly, the engine is removed from the fuselage and placed on a special supporting stand, such as shown at Fig. 181, so it can be placed in any position and completely dismantled. With a stand of this kind it is as easy to work on the bottom of the engine as on the top and every part can be instantly reached. The crank-case shown in place in ill.u.s.tration is in a very convenient position for sc.r.a.ping in the crank-shaft bearings.
[Ill.u.s.tration: Fig. 180.--Special Tools and Appliances to Facilitate Overhauling Work on Hall-Scott Airplane Engines.]
In order to look over the parts of an engine and to restore the worn or defective components it is necessary to take the engine entirely apart, as it is only when the power plant is thoroughly dismantled that the parts can be inspected or measured to determine defects or wear. If one is not familiar with the engine to be inspected, even though the work is done by a repairman of experience, it will be found of value to take certain precautions when dismantling the engine in order to insure that all parts will be replaced in the same position they occupied before removal. There are a number of ways of identifying the parts, one of the simplest and surest being to mark them with steel numbers or letters or with a series of center punch marks in order to retain the proper relation when rea.s.sembling. This is of special importance in connection with dismantling multiple cylinder engines as it is vital that pistons, piston rings, connecting rods, valves, and other cylinder parts be always replaced in the same cylinder from which they were removed, because it is uncommon to find equal depreciation in all cylinders. Some repairmen use small s.h.i.+pping tags to identify the pieces. This can be criticised because the tags may become detached and lost and the ident.i.ty of the piece mistaken. If the repairing is being done in a shop where other engines of the same make are being worked on, the repairman should be provided with a large chest fitted with a lock and key in which all of the smaller parts, such as rods, bolts and nuts, valves, gears, valve springs, cam-shafts, etc., may be stored to prevent the possibility of confusion with similar members of other engines. All parts should be thoroughly cleaned with gasoline or in the potash kettle as removed, and wiped clean and dry. This is necessary to show wear which will be evidenced by easily identified indications in cases where the machine has been used for a time, but in others, the deterioration can only be detected by delicate measuring instruments.
[Ill.u.s.tration: Fig. 181.--Special Stand to Make Motor Overhauling Work Easier.]
In taking down a motor the smaller parts and fittings such as spark-plugs, manifolds and wiring should be removed first. Then the more important members such as cylinders may be removed from the crank-case to give access to the interior and make possible the examination of the pistons, rings and connecting rods. After the cylinders are removed the next operation is to disconnect the connecting rods from the crank-shaft and to remove them and the pistons attached as a unit. Then the crank-case is dismembered, in most cases by removing the bottom half or oil sump, thus exposing the main bearings and crank-shaft. The first operation is the removal of the inlet and exhaust manifolds. In some cases the manifolds are cored integral with the cylinder head casting and it is merely necessary to remove a short pipe leading from the carburetor to one inlet opening and the exhaust pipe from the outlet opening common to all cylinders. In order to remove the carburetor it is necessary to shut off the gasoline supply at the tank and to remove the pipe coupling at the float chamber. It is also necessary to disconnect the throttle operating rod. After the cylinders are removed and before taking the crank-case apart it is well to remove the water pump and magneto. The wiring on most engines of modern development is carried in conduits and usually releasing two or three minor fastenings will permit one to take off the plug wiring as a unit. The wire should be disconnected from both spark-plugs and magneto distributor before its removal. When the cylinders are removed, the pistons, piston rings, and connecting rods are clearly exposed and their condition may be readily noticed.
Before disturbing the arrangement of the timing gears, it is important that these be marked so that they will be replaced in exactly the same relation as intended by the engine designer. If the gears are properly marked the valve timing and magneto setting will be undisturbed when the parts are replaced after overhauling. With the cylinders off, it is possible to ascertain if there is any undue wear present in the connecting rod bearings at either the wrist pin or crank-pin ends and also to form some idea of the amount of carbon deposits on the piston top and back of the piston rings. Any wear of the timing gears can also be determined. The removal of the bottom plate of the engine enables the repairman to see if the main bearings are worn unduly. Often bearings may be taken up sufficiently to eliminate all looseness. In other cases they may be worn enough so that careful refitting will be necessary.
Where the crank-case is divided horizontally into two portions, the upper one serving as an engine base to which the cylinders and in fact all important working parts are attached, the lower portion performs the functions of an oil container and cover for the internal mechanism. This is the construction generally followed.
DEFECTS IN CYLINDERS
After the cylinders have been removed and stripped of all fittings, they should be thoroughly cleaned and then carefully examined for defects.
The interior or bore should be looked at with a view of finding score marks, grooves, cuts or scratches in the interior, because there are many faults that may be ascribed to depreciation at this point. The cylinder bore may be worn out of round, which can only be determined by measuring with an internal caliper or dial indicator even if the cylinder bore shows no sign of wear. The f.l.a.n.g.e at the bottom of the cylinder by which it is held to the engine base may be cracked. The water jacket wall may have opened up due to freezing of the jacket water at some time or other or it may be filled with scale and sediment due to the use of impure cooling water. The valve seat may be scored or pitted, while the threads holding the valve chamber cap may be worn so that the cap will not be a tight fit. The detachable head construction makes it possible to remove that member and obtain ready access to the piston tops for sc.r.a.ping out carbon without taking the main cylinder portion from the crank-case. When the valves need grinding the head may be removed and carried to the bench where the work may be performed with absolute a.s.surance that none of the valve grinding compound will penetrate into the interior of the cylinder as is sometimes unavoidable with the I-head cylinder. If the cylinder should be scored, the water jacket and combustion head may be saved and a new cylinder casting purchased at considerably less cost than that of the complete unit cylinder.
The detachable head construction has only recently been applied on airplane engines, though it was one of the earliest forms of automobile engine construction. In the early days it was difficult to procure gaskets or packings that would be both gas and water tight. The sheet asbestos commonly used was too soft and blew out readily. Besides a new gasket had to be made every time the cylinder head was removed. Woven wire and asbestos packings impregnated with rubber, red lead, graphite and other filling materials were more satisfactory than the soft sheet asbestos, but were p.r.o.ne to burn out if the water supply became low.
Materials such as sheet copper or bra.s.s proved to be too hard to form a sufficiently yielding packing medium that would allow for the inevitable slight inaccuracies in machining the cylinder head and cylinder. The invention of the copper-asbestos gasket, which is composed of two sheets of very thin, soft copper bound together by a thin edging of the same material and having a piece of sheet asbestos interposed solved this problem. Copper-asbestos packings form an effective seal against leakage of water and a positive retention means for keeping the explosion pressure in the cylinder. The great advantage of the detachable head is that it permits of very easy inspection of the piston tops and combustion chamber and ready removal of carbon deposits.
CARBON DEPOSITS, THEIR CAUSE AND PREVENTION
Most authorities agree that carbon is the result of imperfect combustion of the fuel and air mixture as well as the use of lubricating oils of improper flash point. Lubricating oils that work by the piston rings may become decomposed by the great heat in the combustion chamber, but at the same time one cannot blame the lubricating oil for all of the carbon deposits. There is little reason to suspect that pure petroleum oil of proper body will deposit excessive amounts of carbon, though if the oil is mixed with castor oil, which is of vegetable origin, there would be much carbon left in the interior of the combustion chamber. Fuel mixtures that are too rich in gasoline also produce these undesirable acc.u.mulations.
A very interesting chemical a.n.a.lysis of a sample of carbon sc.r.a.ped from the interior of a motor vehicle engine shows that ordinarily the lubricant is not as much to blame as is commonly supposed. The a.n.a.lysis was as follows:
Oil 14.3% Other combustible matter 17.9 Sand, clay, etc. 24.8 Iron oxide 24.5 Carbonate of lime 8.9 Other const.i.tuents 9.6