The cylinder compression test determines if the valves, piston rings, and pistons are adequately sealing the combustion chamber. If pressure leakage is excessive, the cylinder cannot develop its full power. The purpose of testing cylinder compression is to determine whether cylinder replacement is necessary. The detection and replacement of defective cylinders prevents a complete engine change because of cylinder failure. It is essential that cylinder compression tests be made periodically. Low compression, for the most part, can be traced to leaky valves.

Conditions that affect engine compression are: 1. Incorrect valve clearances 2. Worn, scuffed, or damaged piston 3. Excessive wear of piston rings and cylinder walls 4. Burned or warped valves 5. Carbon particles between the face and the seat of the valve or valves 6. Early or late valve timing.

Perform a compression test as soon as possible after the engine is shut down so that piston rings, cylinder walls, and other parts are still freshly lubricated. However, it is not necessary to operate the engine prior to accomplishing compression checks during engine buildup or on individually replaced cylinders. In such cases, before making the test, spray a small quantity of lubricating oil into the cylinder(s), and turn the engine over several times to seal the piston and rings in the cylinder barrel.

Be sure that the ignition switch is in the OFF position so that there is no accidental firing of the engine. Remove necessary cowling and the most accessible spark plug from each cylinder. When removing the spark plugs, identify them to coincide with the cylinder. Close examination of the plugs aid in diagnosing problems within the cylinder. Review the maintenance records of the engine being tested. Records of previous compression checks help in determining progressive wear conditions and in establishing the necessary maintenance actions.

Differential Pressure Tester 

The differential pressure tester checks the compression of aircraft engines by measuring the leakage through the cylinders. The design of this compression tester is such that minute valve leakages can be detected, making possible the replacement of cylinders where valve burning is starting. 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 the orifice results.

As the airflow and pressure changes, pressure varies accordingly in the same direction. If air is supplied under pressure to the cylinder with both intake and exhaust valves closed, the amount of air that leaks by the valves or piston rings indicates their condition; the perfect cylinder would have no leakage. The differential pressure tester requires the application of air pressure to the cylinder being tested with the piston at top-center compression stroke.

Guidelines for performing a differential compression test are: 

1. Perform the compression test as soon as possible after engine shutdown to provide uniform lubrication of cylinder walls and rings.

2. Remove the most accessible spark plug from the cylinder, or cylinders, and install a spark plug adapter in the spark plug insert. 

3. Connect the compression tester assembly to a 100 to 150 psi compressed air supply. With the shutoff valve on the compression tester closed, adjust the regulator of the regulated pressure gauge compression tester to obtain 80 psi.

4. Open the shutoff valve and attach the air hose quickconnect fitting to the spark plug adapter. The shutoff valve, when open, automatically maintains a pressure in the cylinder of 15 to 20 psi when both the intake and exhaust valves are closed.

5. By hand, turn the engine over in the direction of rotation until the piston in the cylinder being tested comes up on the compression stroke against the 15 psi. Continue turning the propeller slowly in the direction of rotation until the piston reaches top dead center. Top dead center can be detected by a decrease in force required to move the propeller. If the engine is rotated past top dead center, the 15 to 20 psi tends to move the propeller in the direction of rotation. If this occurs, back the propeller up at least one blade prior to turning the propeller again in the direction of rotation. This backing up is necessary to eliminate the effect of backlash in the valve-operating mechanism and to keep the piston rings seated on the lower ring lands. 

6. Close the shutoff valve in the compression tester and re-check the regulated pressure to see that it is 80 psi with air flowing into the cylinder. If the regulated pressure is more or less than 80 psi, readjust the regulator in the test unit to obtain 80 psi. When closing the shutoff valve, make sure that the propeller path is clear of all objects. There is sufficient air pressure in the combustion chamber to rotate the propeller if the piston is not on top dead center.

7. With regulated pressure adjusted to 80 psi, if the cylinder pressure reading indicated on the cylinder pressure gauge is below the minimum specified for the engine being tested, move the propeller in the direction of rotation to seat the piston rings in the grooves. Check all the cylinders and record the readings. 

If low compression is obtained on any cylinder, turn the engine through with the starter, or re-start, and run the engine to takeoff power and re-check the cylinder, or cylinders, having low compression.

If the low compression is not corrected, remove the rockerbox cover and check the valve clearance to determine if the difficulty is caused by inadequate valve clearance. If the low compression is not caused by inadequate valve clearance, place a fiber drift on the rocker arm immediately over the valve stem and tap the drift several times with a 1 to 2 pound hammer to dislodge any foreign material that may be lodged between the valve and valve seat.

After staking the valve in this manner, rotate the engine with the starter and re-check the compression. Do not make a compression check after staking a valve until the crankshaft has been rotated either with the starter or by hand to re-seat the valve in normal manner. The higher seating velocity obtained when staking the valve will indicate valve seating, even though valve seats are slightly egged or eccentric. This procedure should only be performed if approved by the manufacturer.

Cylinders having compression below the minimum specified should be further checked to determine whether leakage is past the exhaust valve, intake valve, or piston. Excessive leakage can be detected (during the compression check): 1. At the exhaust valve by listening for air leakage at the exhaust outlet; 2. At the intake valve by escaping air at the air intake; and 3. Past the piston rings by escaping air at the engine breather outlets.

Next to valve blow-by, the most frequent cause of compression leakage is excessive leakage past the piston. This leakage may occur because of lack of oil. To check this possibility, apply engine oil into the cylinder and around the piston. Then, re-check the compression. If this procedure raises compression to or above the minimum required, continue the cylinder in service. If the cylinder pressure readings still do not meet the minimum requirement, replace the cylinder. When it is necessary to replace a cylinder as a result of low compression, record the cylinder number and the compression value of the newly installed cylinder on the compression check sheet.

Cylinder Replacement 

Reciprocating engine cylinders are designed to operate for a specified time before normal wear requires their overhaul. If the engine is operated as recommended and proficient maintenance is performed, the cylinders normally last until the engine has reached its TBO. It is known from experience that materials fail, and engines are abused through incorrect operation; this has a serious effect on cylinder life. Another reason for premature cylinder change is poor maintenance. Therefore, exert special care to ensure that all the correct maintenance procedures are adhered to when working on the engine. Some of the reasons for cylinder replacement are: 1. Low compression 2. High oil consumption in one or more cylinders 3. Excessive valve guide clearance 4. Loose intake pipe flanges 5. Loose or defective spark plug inserts 6. External damage, such as cracks.

The cylinder is always replaced as a complete assembly, which includes piston, rings, valves, and valve springs. Obtain the cylinder by ordering the cylinder assembly under the part number specified in the engine parts catalog. Parts, such as valve springs, rocker arms, and rocker box covers, may be replaced individually.

Normally, all the cylinders in an engine are similar, all are standard size or all a certain oversize, and all are steel bore or all are chrome-plated. The size of the cylinder is indicated by a color code around the barrel between the attaching flange and the lower barrel cooling fin. In some instances, air-cooled engines are equipped with chrome-plated cylinders. Chromeplated cylinders are usually identified by a paint band around the barrel between the attaching flange and the lower barrel cooling fin. This color band is usually international orange.

When installing a chrome-plated cylinder, do not use chrome-plated piston rings. The matched assembly includes the correct piston rings. However, if a piston ring is broken during cylinder installation, check the cylinder marking to determine what ring, chrome-plated or otherwise, is correct for replacement. Similar precautions must be taken to be sure that the correct size rings are installed.

Correct procedures and care are important when replacing cylinders. Careless work or the use of incorrect tools can damage the replacement cylinder or its parts. Incorrect procedures in installing rocker-box covers may result in troublesome oil leaks. Improper torque on cylinder hold down nuts or cap-screws can easily result in a cylinder malfunction and subsequent engine failure. 

Cylinder Removal 

Since these instructions are meant to cover all air-cooled engines, they are of a very general nature. The applicable manufacturer’s maintenance manual should be consulted for torque values and special precautions applying to a particular aircraft and engine. However, always practice neatness and cleanliness, and always protect openings so that nuts, washers, tools, and miscellaneous items do not enter the engine’s internal sections. 

Assuming that all obstructing cowling and brackets have been removed, first remove the intake pipe and exhaust pipes. Plug or cover openings in the intake or diffuser section. Then, remove cylinder deflectors and any attaching brackets that would obstruct cylinder removal. Loosen the spark plugs and remove the spark plug lead clamps. Do not remove the spark plugs until ready to pull the cylinder off. Remove the rocker box covers. First, remove the nuts and then tap the cover lightly with a rawhide mallet or plastic hammer. Never pry the cover off with a screwdriver or similar tool.

Loosen the pushrod packing gland nuts or hose clamps, top and bottom. Pushrods are removed by depressing the rocker arms with a special tool, or by removing the rocker arm. Before removing the pushrods, turn the crankshaft until the piston is at top dead center on the compression stroke. This relieves the pressure on both intake and exhaust rocker arms. It is also wise to back off the adjusting nut as far as possible, because this allows maximum clearance for pushrod removal when the rocker arms are depressed.

On some model engines, or if the engine is rotated, tappets and springs of lower cylinders can fall out. Provision must be made to catch them as the pushrod and housing are removed. After removing the pushrods, examine them for markings or mark them so that they may be replaced in the same location as they were before removal. The ball ends are usually worn to fit the sockets in which they have been operating. Furthermore, on some engines, pushrods are not all of the same length. A good procedure is to mark the pushrods near the valve tappet ends No. 1 IN, No. 1 EX, No. 2 IN, No. 2 EX., etc. On fuel injection engines, disconnect the fuel injection line and any line clamps that interfere with cylinder removal.

The next step in removing the cylinder is to cut the lock wire or remove the cotter pin, and pry off the locking device from the cylinder-attaching cap-screws or nuts. Remove all the screws or nuts except two located 180° apart. Use the wrench specified for this purpose in the special tools section of the applicable manual.

Finally, while supporting the cylinder, remove the two remaining nuts and gently pull the cylinder away from the crankcase. Two technicians working together during this step, as well as during the remaining procedure for cylinder replacement, helps prevent damage or dropping of the cylinder. After the cylinder skirt has cleared the crankcase, but before the piston protrudes from the skirt, provide some means (usually a shop cloth) for preventing pieces of broken rings from falling into the crankcase. After the piston has been removed, remove the cloths and carefully check that all pieces were prevented from falling into the crankcase.

Place a support on the cylinder mounting pad and secure it with two cap-screws or nuts. Then, remove the piston and ring assembly from the connecting rod. A pin pusher or puller tool can be used when varnish makes it hard to remove the pin. If the special tool is not available and a drift is used to remove the piston pin, the connecting rod should be supported so that it does not have to take the shock of the blows. If this is not done, the rod may be damaged.

After the removal of a cylinder and piston, the connecting rod must be supported to prevent damage to the rod and crankcase. This can be done by supporting each connecting rod with the removed cylinder base oil seal ring looped around the rod and cylinder base studs.

Using a wire brush, clean the studs or cap-screws and examine them for cracks, damaged threads, or any other visible defects. If one cap-screw is found loose or broken at the time of cylinder removal, all the cap-screws for the cylinder should be discarded, since the remaining cap-screws may have been seriously weakened. A cylinder hold down stud failure places the adjacent studs under a greater operating pressure, and they are likely to be stretched beyond their elastic limit. The engine manufacturer’s instruction must be followed for the number of studs that have to be replaced after a stud failure. When removing a broken stud, take proper precautions to prevent metal chips from entering the engine crankcase section. In all cases, both faces of the washers and the seating faces of stud nuts or cap-screws must be cleaned and any roughness or burrs removed.

Cylinder Installation 

See that all preservative oil accumulation on the cylinder and piston assembly is washed off with solvent and thoroughly dried with compressed air. Install the piston and ring assembly on the connecting rod. Be sure that the piston faces in the right direction. The piston number stamped on the bottom of the piston head should face toward the front of the engine. Lubricate the piston pin before inserting it. It should fit with a push fit. If a drift must be used, follow the same precaution that was taken during pin removal. 

Oil the exterior of the piston assembly generously, forcing oil around the piston rings and in the space between the rings and grooves. Stagger the ring gaps around the piston and check to see that rings are in the correct grooves, and whether they are positioned correctly, as some are used as oil scrapers, others as pumper rings. The number, type, and arrangement of the compression and oil-control rings vary with the make and model of engine.

Perform any and all visual, structural, and dimensional inspection checks before installing the cylinder. Check the flange to see that the mating surface is smooth and clean. Coat the inside of the cylinder barrel generously with oil. Be sure that the cylinder oil-seal ring is in place and that only one seal ring is used.

Using a ring compressor, compress the rings to a diameter equal to that of the piston. With the piston at TDC, start the cylinder assembly down over the piston, making certain that the cylinder and piston plane remain the same. Ease the cylinder over the piston with a straight, even movement that moves the ring compressor as the cylinder slips on. Do not rock the cylinder while slipping it on the piston, since any rocking is apt to release a piston ring or a part of a ring from the ring compressor prior to the ring’s entrance into the cylinder bore. A ring released in this manner expands and prevents the piston from entering the cylinder. Any attempt to force the cylinder onto the piston is apt to cause cracking or chipping of the ring or damage to the ring lands.

After the cylinder has slipped on the piston, so that all piston rings are in the cylinder bore, remove the ring compressor and the connecting rod guide. Then, slide the cylinder into place on the mounting pad. If cap-screws are used, rotate the cylinder to align the holes. While still supporting the cylinder, install two cap-screws or stud nuts 180° apart.

Install the remaining nuts or cap-screws and tighten them until they are snug. The hold down nuts, or cap-screws, must now be torqued to the value specified in the table of torque values in the engine manufacturer’s service or overhaul manual. Apply the torque with a slow, steady motion until the prescribed value is reached. Hold the tension on the wrench for a sufficient length of time to ensure that the nut or cap-screw tightens no more at the prescribed torque value. In many cases, additional turning of the cap-screw, or nut, as much as one-quarter turn can be done by maintaining the prescribed torque on the nut for a short period of time. After the stud nuts, or cap-screws, have been torqued to the prescribed value, safety them in the manner recommended in the engine manufacturer’s service manual.

Reinstall the push rods, push rod housings, rocker arms, barrel deflectors, intake pipes, ignition harness lead clamps and brackets, fuel injection line clamps and fuel injection nozzles (if removed), exhaust stack, cylinder head deflectors, and spark plugs. Remember that the push rods must be installed in their original locations and must not be turned end to end. Make sure that the push rod ball end seats properly in the tappet. If it rests on the edge or shoulder of the tappet during valve clearance adjustment and later drops into place, valve clearance is off.

Furthermore, rotating the crankshaft with the push rod resting on the edge of the tappet may bend the push rod. After installing the push rods and rocker arms, set the valve clearance. Before installing the rocker-box covers, lubricate the rocker arm bearings and valve stems. Check the rocker-box covers for flatness; re-surface them if necessary. After installing the gaskets and covers, tighten the rocker-box cover nuts to the specified torque. Always follow the recommended safety procedures.