In addition to the camshaft and bearing troubles already mentioned, the cams of a camshaft may be damaged as a result of improper valve tappet adjustment, worn or stuck cam followers, or failure of the camshaft gear. Cams are likely to be damaged when a loose valve tappet adjustment or a broken tappet screw causes the valve to jam against the cylinder head, and the push rods to jam against their cams. This will result in scoring or breaking of the cams and followers, as well as severe damage to the piston and the cylinder.

Valves must be timed correctly at all times, not only for the proper operation of the engine but also to prevent possible damage to the engine parts. You should inspect frequently the valve actuating linkage during operation to determine if it is operating properly. Such inspections should include taking tappet clearances and adjusting, if necessary; checking for broken, chipped, or improperly seated valve springs; inspecting push rod end fittings for proper seating; and inspect-ing cam follower surfaces for grooves or scoring.

JOURNAL BEARINGS

Engine journal bearing failure and their causes may vary to some degree, depending upon the type of bearing. The following discussion of the causes of bearing failure applies to most bearingsmain bearings as well as crank pin bear-ings. The most common journal bearing failures may be due to one or to a combination of the following causes:

1. Corrosion of bearing materials caused by chemical action of oxidized lubricating oils. Oxidation of oil may be minimized by changing oil at the designated intervals, and by keeping engine temperatures within recommended limits. Bearing failures due to corrosion may be identified by very small pits covering the surfaces. In most instances, corrosion occurs over small bearings areas in which high localized pressures and temperatures exist. Since the small pits caused by corrosion are so closely spaced that they form channels, the oil film is not continuous and the load-carrying area of the bearing is reduced below the point of safe operation.

2. Surface pitting of bearings due to high localized temperatures that cause the lead to melt. This is generally the result of very close oil clearances and the use of an oil having a viscos-ity higher than recommended. Early stages of the loss of lead, due to melting, will be evidenced by very small streaks of lead on the bearing surface.

3. Inadequate bond between the bearing metal and the bearing shell. A poor bond may be caused by fatigue resulting from cyclic loads, or it may be the result of defective manufacturing. A failure due to inadequate bond is shown in figure 3-20. In such failures, the bearing shell shows through the bearing surface clearly.

4. Out-of-round journals due to excessive bearing wear. As the bearings wear, excessive clearance is created; this leads to engine pounding, oil leakage from the bearing, reduced flow of oil to other bearings, and overheating, with the con-sequent melting of bearing material. To prevent bearing wear, the journals should be checked for out-of-roundness. Manufacturers require crank pins to be reground when the out-of-roundness exceeds a specified amount, but the amount varies with manufacturers. Always check the engine manual for this type of data.

5. Rough spots. Burrs or ridges may cause grooves in the bearings and lead to bearing failure. Removal of rough spots is done with a fine oil stone and a piece of crocus cloth. Be sure to place a clean cloth beneath the journal to catch all par-ticles. Apply a coat of clean lubricating oil to the journal and to the bearing before a bearing is installed.

6. Misalignment of parts. Misalignment of the main bearings can be caused by a warped or bent crankshaft. Such misalignment imposes heavy

Figure 3-20.Bearing failure due to inadequate bond.

loads on the main bearings because of the force that is necessary to retain correct alignment be-tween the bearing and the journal.

A bent or misaligned connecting rod can be the cause of a ruined crank-pin bearing. Misalign-ment between the connecting rod bore and the piston pin bushing bore is indicated by the crack-ing of the bearing material at the opposite ends of the upper and lower-bearing shell. An indica-tion of a bent connecting rod is heavy wear or scoring on the piston surface.

7. Faulty installation, due to negligence or lack of experience. The paramount factor is inattention to cleanliness. Hard particles lodge between the bearing shell and the connecting rod bore, and create an air space. This space retards the normal flow of heat and causes localized high temperatures. Such condition may be further ag-gravated if the bearing surface is forced out into the oil clearance spaces and creates a high spot in the bearing surface. The result of a bearing failure is illustrated in figure 3-21. Foreign par-ticles, excessive clearance, or rough surface may cause poor contact between a bearing shell and a connecting rod. Poor contact is indicated by the formation of a gumlike deposit (sometimes re-ferred to as lacquer or varnish) on the back of the shell.

Bearing failures may result from improper fit of the shell to the connecting rod. If the locking lip of a bearing does not fit properly into the recess of the bearing housing, distortion of the shell and failure of the bearing results.

Figure 3-21.Bearing failure resulting from wiping and excessive temperatures.

Another source of trouble during installation is due to the interchanging of the upper and lower shells. The installation of a plain upper shell in place of a lower shell, which contains an oil groove, completely stops the oil flow and leads to early bearing failure. The resulting damage not only may ruin the bearing but may also extend to other parts, such as the crankshaft connecting rod, piston, and wrist pin.

8. Failure to follow recommended procedures in the care of lubricating oil. Lack of proper amount of lubricating oil will cause the overheating of a bearing, causing its failure (see figure 3-22). In large engines, the volume of the lubricating oil passages is so great that the time required to fill them when starting an engine could be sufficient to permit damage to the bearings. To prevent this, separately driven lubricating oil priming pumps are installed, and by their action, the oil is circulated to the bearings before an engine is started. Priming pumps should be secured prior to starting the engine when the prescribed pressure has been obtained.

Maintenance of recommended oil pressures is essential to ensure an adequate supply of oil at all bearing surfaces. Refer to the oil pressure gage as it is the best source of operational information to indicate satisfactory performance.

Use Navy-approved, low-corrosive lubricating oils at recommended oil temperatures. Recom-mended temperatures have been determined by extensive tests in laboratory and in service. They are sufficiently high to assure satisfactory

Figure 3-22.Overheated bearing.

circulation, and sufficiently low to prevent excessive oxidation of the lubricating oil. Nor-mally, the manufacturers technical manual should be followed as to the correct lubricating oil temperature to maintain. However, if no manual is available, the temperature of the oil leaving the engine should be maintained between 160 and 200F. When possible, oil must be analyzed at recommended intervals to determine its suitability for further use. In addition, regular service of oil filters and strainers must be main-tained, and oil samples must periodically be drawn from the lowest point in the sump to determine the presence of abrasive materials or water. The lube oil purifier should be used in accordance with required procedures. Strict adherence to recom-mended practices will reduce the failure of bear-ings and other parts because of the contaminated oil or insufficient supply of clean oil.