Aviation Engines Part 31

valve head truing tool must have some simple means of centering the valve stem in order to insure concentric machining of the valve head. A valve head truer which employs an ingenious method of guiding the valve stem is shown at Fig. 183, E. The device consists of a body portion, B, provided with an external thread at the top on which the cutter head, A, is screwed. A number of steel b.a.l.l.s, C, are carried in the grooves which may be altered in size by the adjustment nut, F, which screws in the bottom of the body portion, B. As the nut F is screwed in against the s.p.a.cer member E, the V-grooves are reduced in size and the steel b.a.l.l.s, C, are pressed out in contact with the valve stem. As the circle or annulus is filled with b.a.l.l.s in both upper and lower portions the stem may be readily turned because it is virtually supported by ball bearing guides. When a larger valve stem is to be supported, the adjusting nut F, is screwed out which increases the size of the grooves and permits the b.a.l.l.s, C, to spread out and allow the larger stem to be inserted.

VALVE GRINDING PROCESSES

Mention has been previously made of the importance of truing both valve head and seat before attempt is made to refit the parts by grinding.

After smoothing the valve seat the next step is to find some way of turning the valve. Valve heads are usually provided with a screw-driver slot pa.s.sing through the boss at the top of the valve or with two drilled holes to take a forked grinding tool. A combination grinding tool has been devised which may be used when either the two drilled holes or the slotted head form of valve is to be rotated. This consists of a special form of screw driver having an enlarged boss just above the blade, this boss serving to support a U-shape piece which can be securely held in operative position by the clamp screw or which can be turned out of the way if the screw driver blade is to be used.

As it is desirable to turn the valve through a portion of a revolution and back again rather than turning it always in the same direction, a number of special tools has been designed to make this oscillating motion possible without trouble. A simple valve grinding tool is shown at Fig. 184, C. This consists of a screw-driver blade mounted in a handle in such a way that the end may turn freely in the handle. A pinion is securely fastened to the screw-driver blade shank, and is adapted to fit a race provided with a wood handle and guided by a bent bearing member securely fastened to the screw-driver handle. As the rack is pushed back and forth the pinion must be turned first in one direction and then in the other.

[Ill.u.s.tration: Fig. 184.--Tools and Processes Utilized in Valve Grinding.]

A valve grinding tool patterned largely after a breast drill is shown at Fig. 184, D. This is worked in such a manner that a continuous rotation of the operating crank will result in an oscillating movement of the chuck carrying the screw-driver blade. The bevel pinions which are used to turn the chuck are normally free unless clutched to the chuck stem by the sliding sleeve which must turn with the chuck stem and which carries clutching members at each end to engage similar members on the bevel pinions and lock these to the chuck stem, one at a time. The bevel gear carries a cam-piece which moves the clutch sleeve back and forth as it revolves. This means that the pinion giving forward motion of the chuck is clutched to the chuck spindle for a portion of a revolution of the gear and clutch sleeve is moved back by the cam and clutched to the pinion giving a reverse motion of the chuck during the remainder of the main drive gear revolution.

It sometimes happens that the adjusting screw on the valve lift plunger or the valve lift plunger itself when L head cylinders are used does not permit the valve head to rest against the seat. It will be apparent that unless a definite s.p.a.ce exists between the end of the valve stem and the valve lift plunger that grinding will be of little avail because the valve head will not bear properly against the abrasive material smeared on the valve seat.

The usual methods of valve grinding are clearly outlined at Fig. 184.

The view at the left shows the method of turning the valve by an ordinary screw driver and also shows a valve head at A, having both the drilled holes and the screw-driver slot for turning the member and two special forms of fork-end valve grinding tools. In the sectional view shown at the right, the use of the light spring between the valve head and the bottom of the valve chamber to lift the valve head from the seat whenever pressure on the grinding tool is released is clearly indicated.

It will be noted also that a ball of waste or cloth is interposed in the pa.s.sage between the valve chamber and the cylinder interior to prevent the abrasive material from pa.s.sing into the cylinder from the valve chamber. When a bitstock is used, instead of being given a true rotary motion the chuck is merely oscillated through the greater part of the circle and back again. It is necessary to lift the valve from its seat frequently as the grinding operation continues; this is to provide an even distribution of the abrasive material placed between the valve head and its seat. Only sufficient pressure is given to the bitstock to overcome the uplift of the spring and to insure that the valve will be held against the seat. Where the spring is not used it is possible to raise the valve from time to time with the hand which is placed under the valve stem to raise it as the grinding is carried on. It is not always possible to lift the valve in this manner when the cylinders are in place on the engine base owing to the s.p.a.ce between the valve lift plunger and the end of the valve stem. In this event the use of the spring as shown in sectional view will be desirable.

The abrasive generally used is a paste made of medium or fine emery and lard oil or kerosene. This is used until the surfaces are comparatively smooth, after which the final polish or finish is given with a paste of flour emery, grindstone dust, crocus, or ground gla.s.s and oil. An erroneous impression prevails in some quarters that the valve head surface and the seating must have a mirror-like polish. While this is not necessary it is essential that the seat in the cylinder and the bevel surface of the head be smooth and free from pits or scratches at the completion of the operation. All traces of the emery and oil should be thoroughly washed out of the valve chamber with gasoline before the valve mechanism is a.s.sembled and in fact it is advisable to remove the old grinding compound at regular intervals, wash the seat thoroughly and supply fresh material as the process is in progress.

The truth of seatings may be tested by taking some Prussian blue pigment and spreading a thin film of it over the valve seat. The valve is dropped in place and is given about one-eighth turn with a little pressure on the tool. If the seating is good both valve head and seat will be covered uniformly with color. If high spots exist, the heavy deposit of color will show these while the low spots will be made evident because of the lack of pigment. The grinding process should be continued until the test shows an even bearing of the valve head at all points of the cylinder seating. When the valves are held in cages it is possible to catch the cage in a vise and to turn the valve in any of the ways indicated. It is much easier to clean off the emery and oil and there is absolutely no danger of getting the abrasive material in the cylinder if the construction is such that the valve cage or cylinder head member carrying the valve can be removed from the cylinder. When valves are held in cages, the tightness of the seat may be tested by partially filling the cage with gasoline and noticing how much liquid oozes out around the valve head. The degree of moisture present indicates the efficacy of the grinding process.

The valves of Curtiss OX-2 cylinders are easily ground in by using a simple fixture or tool and working from the top of the cylinder instead of from the inside. A tube having a bore just large enough to go over the valve stem is provided with a wooden handle or taped at one end and a hole of the same size as that drilled through the valve stem is put in at the other. To use, the open end of the tube is pushed over the valve stem and a split pin pushed through the tube and stem. The valve may be easily manipulated and ground in place by oscillating in the customary manner.

DEPRECIATION IN VALVE OPERATING SYSTEMS

There are a number of points to be watched in the valve operating system because valve timing may be seriously interfered with if there is much lost motion at the various bearing points in the valve lift mechanism.

The two conventional methods of opening valves are shown at Fig. 185.

That at A is the type employed when the valve cages are mounted directly in the head, while the form at B is the system used when the valves are located in a pocket or extension of the cylinder casting as is the case if an L, or T-head cylinder is used. It will be evident that there are several points where depreciation may take place. The simplest form is that shown at B, and even on this there are five points where lost motion may be noted. The periphery of the valve opening cam or roller may be worn, though this is not likely unless the roller or cam has been inadvertently left soft. The pin which acts as a bearing for the roller may become worn, this occurring quite often. Looseness may materialize between the bearing surfaces of the valve lift plunger and the plunger guide casting, and there may also be excessive clearance between the top of the plunger and the valve stem.

[Ill.u.s.tration: Fig. 185.--Outlining Points in Valve Operating Mechanism Where Depreciation is Apt to Exist.]

On the form shown at A, there are several parts added to those indicated at B. A walking beam or rocker lever is necessary to transform the upward motion of the tappet rod to a downward motion of the valve stem.

The pin on which this member fulcrums may wear as will also the other pin acting as a hinge or bearing for the yoke end of the tappet rod. It will be apparent that if slight play existed at each of the points mentioned it might result in a serious diminution of valve opening.

Suppose, for example, that there were .005-inch lost motion at each of three bearing points, the total lost motion would be .015-inch or sufficient to produce noisy action of the valve mechanism. When valve plungers of the adjustable form, such as shown at B, are used, the hardened bolt head in contact with the end of the valve stem may become hollowed out on account of the hammering action at that point. It is imperative that the top of this member be ground off true and the clearance between the valve stem and plunger properly adjusted. If the plunger is a non-adjustable type it will be necessary to lengthen the valve stem by some means in order to reduce the excessive clearance. The only remedy for wear at the various hinges and bearing pins is to bore the holes out slightly larger and to fit new hardened steel pins of larger diameter. Depreciation between the valve plunger guide and the valve plunger is usually remedied by fitting new plunger guides in place of the worn ones. If there is sufficient stock in the plunger guide casting as is sometimes the case when these members are not separable from the cylinder casting, the guide may be bored out and bushed with a light bronze bus.h.i.+ng.

A common cause of irregular engine operation is due to a sticking valve.

This may be owing to a bent valve stem, a weak or broken valve spring or an acc.u.mulation of burnt or gummed oil between the valve stem and the valve stem guide. In order to prevent this the valve stem must be smoothed with fine emery cloth and no burrs or shoulders allowed to remain on it, and the stem must also be straight and at right angles to the valve head. If the spring is weak it may be strengthened in some cases by stretching it out after annealing so that a larger s.p.a.ce will exist between the coils and re-hardening. Obviously if a spring is broken the only remedy is replacement of the defective member.

Mention has been made of wear in the valve stem guide and its influence on engine action. When these members are an integral part of the cylinder the only method of compensating for this wear is to drill the guide out and fit a bus.h.i.+ng, which may be made of steel tube.

In some engines, especially those of recent development, the valve stem guide is driven or screwed into the cylinder casting and is a separate member which may be removed when worn and replaced with a new one. When the guides become enlarged to such a point that considerable play exists between them and the valve stems, they may be easily knocked out or unscrewed.

PISTON TROUBLES

If an engine has been entirely dismantled it is very easy to examine the pistons for deterioration. While it is important that the piston be a good fit in the cylinder it is mainly upon the piston rings that compression depends. The piston should fit the cylinder with but little looseness, the usual practice being to have the piston about .001-inch smaller than the bore for each inch of piston diameter at the point where the least heat is present or at the bottom of the piston. It is necessary to allow more than this at the top of the piston owing to its expansion due to the direct heat of the explosion. The clearance is usually graduated and a piston that would be .005-inch smaller than the cylinder bore at the bottom would be about .0065-inch at the middle and .0075-inch at the top. If much more play than this is evidenced the piston will "slap" in the cylinder and the piston will be worn at the ends more than in the center. Aluminum or alloy pistons require more clearance than cast iron ones do, usually 1.50 times as much. Pistons sometimes warp out of shape and are not truly cylindrical. This results in the high spots rubbing on the cylinder while the low spots will be blackened where a certain amount of gas has leaked by.

Mention has been previously made of the necessity of reboring or regrinding a cylinder that has become scored or scratched and which allows the gas to leak by the piston rings. When the cylinder is ground out, it is necessary to use a larger piston to conform to the enlarged cylinder bore. Most manufacturers are prepared to furnish over-size pistons, there being four standard over-size dimensions adopted by the S. A. E. for rebored cylinders. These are .010-inch, .020-inch, .030-inch, and .040-inch larger than the original bore.

The piston rings should be taken out of the piston grooves and all carbon deposits removed from the inside of the ring and the bottom of the groove. It is important to take this deposit out because it prevents the rings from performing their proper functions by reducing the ring elasticity, and if the deposit is allowed to acc.u.mulate it may eventually result in sticking and binding of the ring, this producing excessive friction or loss of compression. When the rings are removed they should be tested to see if they retain their elasticity and it is also well to see that the small pins in some pistons which keep the rings from turning around so the joints will not come in line are still in place. If no pins are found there is no cause for alarm because these dowels are not always used. When fitted, they are utilized with rings having a b.u.t.t joint or diagonal cut as the superior gas retaining qualities of the lap or step joint render the pins unnecessary.

If gas has been blowing by the ring or if these members have not been fitting the cylinder properly the points where the gas pa.s.sed will be evidenced by burnt, brown or roughened portions of the polished surface of the pistons and rings. The point where this discoloration will be noticed more often is at the thin end of an eccentric ring, the discoloration being present for about 1/2-inch or 3/4-inch each side of the slot. It may be possible that the rings were not true when first put in. This made it possible for the gas to leak by in small amounts initially which increased due to continued pressure until quite a large area for gas escape had been created.

PISTON RING MANIPULATION

Removing piston rings without breaking them is a difficult operation if the proper means are not taken, but is a comparatively simple one when the trick is known. The tools required are very simple, being three strips of thin steel about one-quarter inch wide and four or five inches long and a pair of spreading tongs made up of one-quarter inch diameter keystock tied in the center with a copper wire to form a hinge. The construction is such that when the hand is closed and the handles brought together the other end of the expander spreads out, an action just opposite to that of the conventional pliers. The method of using the tongs and the metal strips is clearly indicated at Fig. 186. At A the ring expander is shown spreading the ends of the rings sufficiently to insert the pieces of sheet metal between one of the rings and the piston. Grasp the ring as shown at B, pressing with the thumbs on the top of the piston and the ring will slide off easily, the thin metal strips acting as guide members to prevent the ring from catching in the other piston grooves. Usually no difficulty is experienced in removing the top or bottom rings, as these members may be easily expanded and worked off directly without the use of a metal strip. When removing the intermediate rings, however, the metal strips will be found very useful.

These are usually made by the repairman by grinding the teeth from old hacksaw blades and rounding the edges and corners in order to reduce the liability of cutting the fingers. By the use of the three metal strips a ring is removed without breaking or distorting it and practically no time is consumed in the operation.

FITTING PISTON RINGS

Before installing new rings, they should be carefully fitted to the grooves to which they are applied. The tools required are a large piece of fine emery cloth, a thin, flat file, a small vise with copper or leaden jaw clips, and a smooth hard surface such as that afforded by the top of a surface plate or a well planed piece of hard wood. After making sure that all deposits of burnt oil and carbon have been removed from the piston grooves, three rings are selected, one for each groove. The ring is turned all around its circ.u.mference into the groove it is to fit, which can be done without springing it over the piston as the outside edge of the ring may be used to test the width of the groove just as well as the inside edge. The ring should be a fair fit and while free to move circ.u.mferentially there should be no appreciable up and down motion. If the ring is a tight fit it should be laid edge down upon the piece of emery cloth which is placed on the surface plate and carefully rubbed down until it fits the groove it is to occupy. It is advisable to fit each piston ring individually and to mark them in some way to insure that they will be placed in the groove to which they are fitted.

The repairman next turns his attention to fitting the ring in the cylinder itself. The ring should be pushed into the cylinder at least two inches up from the bottom and endeavor should be made to have the lower edge of the ring parallel with the bottom of the cylinder. If the ring is not of correct diameter, but is slightly larger than the cylinder bore, this condition will be evident by the angular slots of the rings being out of line or by difficulty in inserting the ring if it is a lap joint form. If such is the case the ring is removed from the cylinder and placed in the vise between soft metal jaw clips. Sufficient metal is removed with a fine file from the edges of the ring at the slot until the edges come into line and a slight s.p.a.ce exists between them when the ring is placed into the cylinder. It is important that this s.p.a.ce be left between the ends, for if this is not done when the ring becomes heated the expansion of metal may cause the ends to abut and the ring to jam in the cylinder.

[Ill.u.s.tration: Fig. 186.--Method of Removing Piston Rings, and Simple Clamp to Facilitate Insertion of Rings in Cylinder.]

It is necessary to use more than ordinary caution in replacing the rings on the piston because they are usually made of cast iron, a metal that is very fragile and liable to break because of its brittleness. Special care should be taken in replacing new rings as these members are more apt to break than old ones. This is probably accounted for by the heating action on used rings which tends to anneal the metal as well as making it less springy. The bottom ring should be placed in position first which is easily accomplished by springing the ring open enough to pa.s.s on the piston and then sliding it into place in the lower groove which on some types of engines is below the wrist pin, whereas in others all grooves are above that member. The other members are put in by a reversal of the process outlined at Fig. 186, A and B. It is not always necessary to use the guiding strips of metal when replacing rings as it is often possible, by putting the rings on the piston a little askew and maneuvering them to pa.s.s the grooves without springing the ring into them. The top ring should be the last one placed in position.

Before placing pistons in the cylinder one should make sure that the slots in the piston rings are s.p.a.ced equidistant on the piston, and if pins are used to keep the ring from turning one should be careful to make sure that these pins fit into their holes in the ring and that they are not under the ring at any point. Practically all cylinders are chamfered at the lower end to make insertion of piston rings easier. The operation of putting on a cylinder casting over a piston really requires two pairs of hands, one to manipulate the cylinder, the other person to close the rings as they enter the cylinder. This may be done very easily by a simple clamp member made of sheet bra.s.s or iron and used to close the ring as indicated at Fig. 186, C. It is apparent that the clamp must be adjusted to each individual ring and that the split portion of the clamp must coincide with the split portion of the ring. The cylinder should be well oiled before any attempt is made to install the pistons.

The engine should be run with more than the ordinary amount of lubricant for several hours after new piston rings have been inserted. On first starting the engine, one may be disappointed in that the compression is even less than that obtained with the old rings. This condition will soon be remedied as the rings become polished and adapt themselves to the contour of the cylinder.

WRIST PIN WEAR

While wrist pins are usually made of very tough steel, case hardened with the object of wearing out an easily renewable bronze bus.h.i.+ng in the upper end of the connecting rod rather than the wrist pin it sometimes happens that these members will be worn so that even the replacement of a new bus.h.i.+ng in the connecting rod will not reduce the lost motion and attendant noise due to a loose wrist pin. The only remedy is to fit new wrist pins to the piston. Where the connecting rod is clamped to the wrist pin and that member oscillates in the piston bosses the wear will usually be indicated on bronze bus.h.i.+ngs which are pressed into the piston bosses. These are easily renewed and after running a reamer through them of the proper size no difficulty should be experienced in replacing either the old or a new wrist pin depending upon the condition of that member. If no bus.h.i.+ngs are provided, as in alloy pistons, the bosses can sometimes be bored out and thin bus.h.i.+ngs inserted, though this is not always possible. The alternative is to ream out the bosses and upper end of rod a trifle larger after holes are trued up and fit oversize wrist pins.

INSPECTION AND REFITTING OF ENGINE BEARINGS

While the engine is dismantled one has an excellent opportunity to examine the various bearing points in the engine crank-case to ascertain if any looseness exists due to depreciation of the bearing surfaces. As will be evident, both main crank-shaft bearings and the lower end of the connecting rods may be easily examined for deterioration. With the rods in place, it is not difficult to feel the amount of lost motion by grasping the connecting rod firmly with the hand and moving it up and down. After the connecting rods have been removed and the propeller hub taken off the crank-shaft to permit of ready handling, any looseness in the main bearing may be detected by lifting up on either the front or rear end of the crank-shaft and observing if there is any lost motion between the shaft journal and the main bearing caps. It is not necessary to take an engine entirely apart to examine the main bearings, as in most forms these may be readily reached by removing the sump. The symptoms of worn main bearings are not hard to identify. If an engine knocks regardless of speed or spark-lever position, and the trouble is not due to carbon deposits in the combustion chamber, one may reasonably surmise that the main bearings have become loose or that lost motion may exist at the connecting rod big ends, and possibly at the wrist pins.

The main journals of any well resigned engine are usually proportioned with ample surface and will not wear unduly unless lubrication has been neglected. The connecting rod bearings wear quicker than the main bearings owing to being subjected to a greater unit stress, and it may be necessary to take these up.

ADJUSTING MAIN BEARINGS

[Ill.u.s.tration: Fig. 187.--Tools and Processes Used in Refitting Engine Bearings.]

When the bearings are not worn enough to require refitting the lost motion can often be eliminated by removing one or more of the thin s.h.i.+ms or liners ordinarily used to separate the bearing caps from the seat.

These are shown at Fig. 187, A. Care must be taken that an even number of s.h.i.+ms of the same thickness are removed from each side of the journal. If there is considerable lost motion after one or two s.h.i.+ms have been removed, it will be advisable to take out more s.h.i.+ms and to sc.r.a.pe the bearing to a fit before the bearing cap is tightened up. It may be necessary to clean up the crank-shaft journals as these may be scored due to not having received clean oil or having had bearings seize upon them. It is not difficult to true up the crank-pins or main journals if the score marks are not deep. A fine file and emery cloth may be used, or a lapping tool such as depicted at Fig. 187, B. The latter is preferable because the file and emery cloth will only tend to smooth the surface while the lap will have the effect of restoring the crank to proper contour.

A lapping tool may be easily made, as shown at B, the blocks being of lead or hard wood. As the width of these are about half that of the crank-pin the tool may be worked from side to side as it is rotated. An abrasive paste composed of fine emery powder and oil is placed between the blocks, and the blocks are firmly clamped to the crank-pin. As the lead blocks bed down, the wing nut should be tightened to insure that the abrasive will be held with some degree of pressure against the shaft. A liberal supply of new abrading material is placed between the lapping blocks and crank-shaft from time to time and the old mixture cleaned off with gasoline. It is necessary to maintain a side to side movement of the lapping tool in order to have the process affect the whole width of the crank-pin equally. The lapping is continued until a smooth surface is obtained. If a crank-pin is worn out of true to any extent the only method of restoring it is to have it ground down to proper circular form by a competent mechanic having the necessary machine tools to carry on the work accurately. A crank-pin truing tool that may be worked by hand is shown at Fig. 187, K.

After the crank-shaft is trued the next operation is to fit it to the main bearings or rather to sc.r.a.pe these members to fit the shaft journal. In order to bring the bra.s.ses closer together, it may be necessary to remove a little metal from the edges of the caps to compensate for the lost motion. A very simple way of doing this is shown at Fig. 187, D. A piece of medium emery cloth is rested on the surface plate and the box or bra.s.s is pushed back and forth over that member by hand, the amount of pressure and rapidity of movement being determined by the amount of metal it is necessary to remove. This is better than filing, because the edges will be flat and there will be no tendency for the bearing caps to rock when placed against the bearing seat. It is important to take enough off the edges of the boxes to insure that they will grip the crank tightly. The outer diameter must be checked with a pair of calipers during this operation to make sure that the surfaces remain parallel. Otherwise, the bearing bra.s.ses will only grip at one end and with such insufficient support they will quickly work loose, both in the bearing seat and bearing cap.