DIAGNOSING LOCKRING FAILURES
Diagnosing and Correcting the Cause of Failures
Introduction
In recent years the great majority of automobile engines have been produced with piston pins pressed in the connecting rod. Because of this, many rebuilders see relatively few applications in which lockrings are used. However, the passenger car replacement field and the heavy duty field, where full-floating pins retained by lockrings are used almost exclusively, will continue of offer opportunities for analysis of lockring failures. These failures are often not recognized as such, and even when recognized, the cause of the trouble is rarely understood.
The purpose of this paper is to describe and illustrate typical failures so they may be readily identified, and to explain the causes so corrective action can be taken to prevent their recurrence.
It should be emphasized that most causes of trouble which are attributed to lockring failures are in fact caused by some factor other than the lockring. To which falls the role of symptom. By proper diagnosis of the symptom it is generally possible to determine the true cause of the trouble and thereby effect a remedy.
Identifying Damage
Damage resulting from broken lockrings can be readily identified. The action of inertia forces during reciprocation of the piston causes the broken particles of the lockring to be thrown violently up and down and to beat out the sides of the piston adjacent to the pin hole and to enlarge the portion of the pin hole exposed beyond the end of the pin. This eating away of sides of the piston starts at the pin hole and progresses both upward and downward, but predominantly upward from the pin hole. The peening action of the broken bits of lockring rounds off all projecting corners and produces flowing curves with a peculiar satin-like finish, a typical example of which is shown in Fig. 1.
 Figure 1 - Result of Broken Lockring
The broken particles also work their way through the hole in the piston pin and begin eating away at the opposite side of the piston. If the other lockring remains in place, as in Fig. 2, it acts as a shield to protect its groove from the up and down beating action of particles long after other portions of the pin hole have been hammered away, thus producing the effect seen in Fig. 3.
 Figure 2 - Opposite Lockring May Remain in Place
Figure - 3 Groove Shielded by Lockring
Sometimes when one lockring breaks up the pin will slide out and rub against the cylinder wall which will generate a spherical radius on the end of the pin, as in Fig. 4,
 Figure 4 - Pin Has Contacted Cylinder Wall
and wear a keyway in the cylinder wall. When the piston pin moves out in this manner it often protects the point of origin from further erosive action of the pieces of broken lockring: however, those pieces which have found their way through the hole in the piston pin to the other side continue their beating action, with the result that the side of the piston opposite to where the trouble originated will show much more extensive damage.
The erosive action of small lockring particles is truly amazing. It is not uncommon for them to work their way entirely through a complete set of iron and steel piston rings and emerge out through the top land, as shown in Fig. 5,
 Figure 5 - Lockring Particles Work Their Way Through Piston Rings
to bounce around in the combustion chamber where they eventually foul out the spark plug or blow out through the exhaust port. Although the particles hammer away at areas both above and below the pin hole, most of the erosion takes place upward because the piston changes its direction more rapidly in the top half of its stroke and hence the inertia forces are greatest in an upward direction. This accounts for the fact the skirt ring of the piston in Fig. 6 was able to contain the erosion whereas the upper rings of the pistons in Figs. 5, 6 and 7 were not.
 Figure 6 - Skirt Ring Contains Erosion Where Upper Rings Do Not
 Figure 7 - Typical Lockring Failure
Lockrings should always be installed with their gaps down to take advantage of the largest bearing area to withstand the greater upward inertia forces.
Improper Installation
In the few instances where failure is directly traceable to the lockring, it is more often than not the result of improper installation; as failure of the lockring per se, if it occurs at all, is an extreme rarity. If a lockring is improperly heat treated it will usually be detected during installation either by taking a set or breaking, hence there is little chance of using a defective ring.
On the other hand, careless installations where the lockring was not actually snapped into the groove or where it was compressed too much when being inserted into the pin hole have been the cause of an untold number of engine teardowns. The lockring should be compressed no more than is necessary to allow it to be inserted in the pin hole. Closing the lockring too tightly will stretch it beyond its elastic limit and cause it to lose tension so it will not remain seated firmly in the groove.
Unless the lockring is seated firmly so that it cannot move, inertia forces will cause it to rattle in the groove. The comparatively soft piston alloy gives way under the repeated impacts of the lockring and permits the groove to enlarge to the point where it is no longer able to restrain the lockring. As soon as it is able to bounce around freely it breaks into small pieces and initiates the chain of events previously described which leads to the ultimate destruction of the piston. A failure of this type generally develops after a very short period of engine operation; and the end of the pin does not usually display any circular wear marks because it has not been bearing against the lockring.
Bent Connecting Rod
By far the greatest majority of lockring failures are related to the connecting rod, either through misalignment of the rod or through improper fit of the pin in the rod bushing.
When a bent connecting rod is installed in an engine, one end of the piston pin will be higher than the other causing the piston to ride in a titled position in the cylinder as shown in Fig. 8.
 Figure 8 - Bent Connecting Rod
The existence of this condition can easily be verified by the wear pattern on the piston. When looking at the thrust face of such a piston the wear at the top of the skirt will be to one side of center, whereas at the bottom it will be to the other side producing a diagonal wear pattern, as shown in Fig. 9.
 Figure 9 - Diagonal Wear Pattern Resulting From Bent Rod
The slant of the wear pattern on the opposite thrust face will be reversed.
Because of the low ratio of length to diameter in the rod bushing compared to the piston, the pin binds in the rod whereas the piston can slide on the pin. Compression and firing pressures overcome upward inertia forces so that during three-fourths of the cycle the piston tends to slide down the pin until the lockring on the high side of the piston contacts the end of the pin. This causes the piston to slide back and forth on the pin hammering the lockring on the high side with the greatest force, thus upsetting the outer edge of the groove on the high side of the piston, Fig.10.
 Figure 10 - Upset Lockring Groove
If the rod is bent enough, upsetting will progress to the point where the lockring comes out of the groove and breaks up in the typical manner previously described.
Failures of this origin are easily identified by:1) A diagonal wear pattern. 2)Upset outer edge of lockring groove, usually on the high side of the piston only. 3) Wear on high end of pin where it has rubbed against lockring and very little on opposite end. 4) Frequently, though not always, the pin will have moved out to contact the cylinder wall.
Generally when a piston with a tilted wear pattern is examined, the outer edge of the lockring groove on the high side of the piston will be found to have been upset.
Twisted Connecting Rod
A less common type of failure resulting from connecting rod misalignment is due to rod twist, where the large and small bores of the rod are parallel in a horizontal plane but not in a vertical plane, as shown in Fig. 11.
 Figure 11 - Twisted Conntecting Rod
In such a case one end of the pin would be high during one half an engine revolution, but during the other half the opposite end would be high. In this instance both lockrings would hammer alternately against opposite ends of the pin thereby upsetting the outer edge of the lockring grooves on both sides of the piston. However, since the amount of tilt imparted to the pin is so small in proportion to the amount of rod twist, and because the wear is evenly distributed between two lockrings, rod twist would not be expected to be as troublesome as rod bend. Experience has proven that it is not, and it is only in cases of extreme twist that trouble is likely to be encountered.
Such cases can be recognized by evidence of piston rocking in a fore-and-aft direction as indicated by wear spots on the piston above and below each pin hole and by upsetting of the outer edge of both lockring grooves. Rod twist may very often be found in conjunction with rod bend, but because of the geometry of the system its influence is so much less on the tilt of the pin that the effect of bend will override it.
Pin-Fit in Rod Bushing
Responsible for as many cases of lockring trouble as the bent rod is the malpractice of fitting the pin too tightly in the connecting rod bushing. In a full-floating pin installation there must be enough clearance between the pin and rod bushing to permit maintenance of an oil film so that oscillation takes place between the rod and pin rather than between the pin and piston where lubrication, if it exits at all, is marginal. Moreover, there must be additional clearance to allow for thermal growth of the piston pin which receives heat from the piston and therefore expands more than the rod bushing, thus causing a fit which might be snug during assembly to become even tighter during engine operation.
If the connecting rod is assembled to the piston pin with insufficient clearance, the rod is not free to slide on the pin and find its own center when it is bolted to the crankshaft. Now if the effective center of the crankthrow does not coincide exactly with the centerline of the cylinder, the rod exerts a continuous axial pressure on the pin forcing it to bear against only one of the lockrings. Rotation of the pin against only one lockring wears both the end of the pin and inside edge of the lockring. In such cases the evidence points primarily to wear rather than pounding, although the outer edge of the lockring groove may still be upset.
This condition as a source of lockring trouble may be recognized even though the connecting rod and its bushing may not be available for examination. Fig. 12 shows how in such cases there will be heavy wear on the end of the pin which rubs against the lockring and severe wear on one of the two lockrings – often enough to wear through and cut off the closure tabs.
 Figure 12 - Result of Tight Rod Bushing
It also shows how the circumferential lacquer bands which form on the pin between the rod bushing and piston pin bosses may be of unequal width indicating a displacement of the rod to the front or rear of the engine. This offset of the rod in turn forces the piston to the front or rear of the cylinder and often is reflected in a corresponding shift of the wear pattern on the piston skirt. If the pin is fit too tightly in the rod bushing, the oscillation is forced to take place in the piston pin hole rather than in the rod bushing; and this may sometimes result in circumferential wear marks on the portions of the pin which fit in the piston and lacquer marks on the center part of the pin indicating that it has not moved in the bushing.
In cases of this sort the pin, being fit so tightly in the rod, is not free to slide. Therefore if there is any evidence to indicate that the pin has moved out to contact the cylinder wall it precludes a tight pin fit in the rod bushing as the source of the trouble. This is not necessarily true, however, if broken ring or lockring particles are found to have brinelled or imbedded in the piston head, as the resultant hammering may have beat out the rod bushing and freed the pin.
Faulty Crankshaft
If a crankshaft is improperly machined so that the axis of the rod journals is not parallel to the axis of the main journals, a rocking motion is imparted to the connecting rod that causes the rod to tilt towards the front of the engine during one half a crankshaft revolution and to the rear during the other half, Fig. 13.
 Figure 13 - Rod Journals Not Parallel to Mains
This drags the pin back and forth, pounding it against the lockrings and upsetting the outer edge of the lockring grooves.
The same situation develops if the crankshaft rod journals are tapered, Fig. 14.
 Figure 14 - Tapered Rod Journals
During the upper half of the exhaust and intake strokes the inertia force of the piston and rod assembly is acting upward and causes the rod to conform to the underside of the rod journal and tilt in one direction, but during the lower half of the stroke the inertia force is in the opposite direction so that the rod conforms to the top side of the rod journal and tilts in the other direction.
In either of these cases the effect on the piston is similar to that produced by a twisted rod, but may be distinguished from it by the fact a defective crankshaft will produce the same result on all the pistons in a set whereas a twisted rod affects only the piston to which it is assembled. Notice how in Fig 15 all six of the pistons removed from an engine with faulty crankshaft show wear on the portion of the top land over the pin hole.
 Figure 15 - Result of Faulty Crankshaft
Furthermore, the back and forth movement of the rod eye in some cases will be of sufficient magnitude to hammer the rod bushing against the inside faces of both pin bosses, thus producing a wear mark, Fig. 16 which offers conclusive evidence as to the nature of the trouble.
 Figure 16 - Rod Bushing Contacting Pin Boss
Other Causes
There are other less common and less easily identified causes of lockring trouble. If the piston appears to have been operating in a tilted position in the cylinder but the connecting rod is free of excessive bend, it may be that the cylinder is bored out-of square to the crankshaft. While such a condition is rare in a new engine, it can be produced in a rebored block by careless or improper use of a boring bar. Excessive connecting rod side clearance can also be a cause of lockring trouble. If all the pistons in a set develop trouble, yet none display a slanted wear pattern, the cause may be excessive end play of the crankshaft. Lacking evidence from the appearance of the piston assembly, an investigation should be undertaken to determine the existence of any misalignment which would cause the pin to slide back and forth, as any such condition my be regarded as a potential source of lockring trouble.
Occasionally lockring trouble is incorrectly attributed to a piston pin being either too short or too long. The fact the lockrings can be installed in their respective grooves with the pin in place is an indication in itself that the pin is not too long. It should be understood that under operating conditions the clearance between the end of the pin and lockring can never be less than it was during installation because the thermal growth of the piston, by virtue of both its greater coefficient of thermal expansion and its higher temperature, is much greater than that of the pin. On the other hand, a short pin should not have any adverse effect because in a properly assembled engine there is nothing to impart axial motion to the pin. If conditions are such as to cause the pin to slide back and forth, excessive end clearance will permit the lockrings to hammer out sooner; however, in such a case reducing the end clearance merely serves to delay but not prevent the inevitable pounding out of the lockrings. The lockrings must not be relied upon to restrain axial vibration imparted to the pin by misalignment.
Unfortunately it sometimes happens that lockring trouble results from a combination of more than one of the foregoing causes. While this complicates the task of the analyst, it by no means makes it insolvable. An understanding of the basic effects of the various individual conditions described will aid him in interpreting observed results by permitting him to deduct how they might interrelate in various combinations.
Summary
Though the lockring is often blamed, it by itself is rarely the true cause of a failure. For the most part, lockring trouble represents the effect of improper installation of either the lockring or related engine components. A bent connecting rod or a pin fit too tightly in the connecting rod are the two most common sources of trouble, and both are readily identifiable by the appearance of the piston assembly. Any other condition which will impart an axial oscillation to the piston pin will, if not corrected, eventually beat out the lockrings and result in failure.
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