Pistons
Large diesel engines often employ cast-iron or steel pistons; smaller, high-speed engines generally use aluminum castings. That such pistons survive combustion temperatures that can reach 4500°F and cylinder pressures that, highly supercharged engines, can exceed 2000 psi, is a triumph of engineering over materials. Aluminum has a melting point of 1220°F and rapidly loses strength as this temperature is approached.
Construction
The heavy construction of these pistons—diesel pistons typically weigh half as much again as equivalent SI engine pistons-provides mechanical strength and the heat conductivity necessary to keep the piston crown at about 500°F. The standard practice is to direct a stream of oil to the underside of the crown, usually by means of spray jets. Some designers go a step further, and insulate the crown, which is relatively easy to cool, from the ring belt and skirt. Turn ahead to Fig. 8-48 for an illustration of a heat dam. The cavity above the piston pin slows heat transfer by reducing piston wall thickness. Another approach is to lengthen the thermal path by grooving the area above the ring belt (Fig. 8-40). Yet another approach is to apply a thermal coating to the upper side of the piston crown, thus confining heat to the combustion chamber (Fig. 8-41).
The skirts to aluminum pistons run hotter than cast iron and have a coefficient of expansion that is about twice that of iron. Consequently, light-metal pistons are assembled with fairly generous bore clearances to compensate for thermal expansion, and might be noisy upon starting. Semi-exotic alloys, such as Lo-Ex, or cast-in steel struts, help control expansion and knocking. It is interesting that one of the first experimenters with aluminum pistons, Harvey Marmon, found it necessary to sheathe the piston skirt in an iron “sock.”
Most alloy pistons are cam-ground; when cold the skirts are ovoid, with the long dimension across the thrust faces. As the piston heats and expands, it becomes circular, filling the bore. Other ways of coping with thermal expansion are progressively to reduce piston diameter above the pin and to relieve, or cut back, the skirts in the pin area (also shown in Fig. 8-41). The heavy struts that support the pin bosses transfer heat from the underside of the crown to the skirts.
Critical wearing points include the thrust faces and the sides of the ring grooves. With the exception of certain two-cycle applications, rings are designed to rotate in their grooves and, according to one researcher, reach speeds of about 100 rpm. Rotation is the primary defense against varnish buildup and consequent ring sticking. But it also wears “steps” into the grooves. Piston thrust faces are also subject to rapid wear.
Some manufacturers run the compression ring against a steel insert, cast integrally with the piston. Another approach is to substitute a long-wearing eutectic alloy for the ASE 334 or 335 usually specified. Eutectic alloys consist of clusters of hard silicon crystals distributed throughout an aluminum matrix. As the aluminum wears, silicon— one of the hardest materials known—emerges as the bearing surface. The same mechanism rapidly dulls cutting tools, which is why the cost of eutectic pistons approaches the cost of forgings.
Forged pistons are an aftermarket item, used as a last resort in highly super- charged engines when castings have failed. Forging eliminates voids in the metal and compacts the grain structure at the crown, pin bosses, and ring lands (Fig. 8-42). These pistons have superior hot strength characteristics but require generous running clearances. An engine with forged pistons will be heard from during cold starts.
A two-piece piston consists of a piston dome, or ring carrier, element and a skirt element (Fig. 8-43). These parts pivot on the piston pin. Although other manufacturers use this form of the piston, the GM version first appeared on Electromotive railroad engines and, in 1971, replaced conventional trunktype pistons on turbocharged DDA Series 71 engines. It eventually found its way into several other DDA engines, includ- ing the Series 60.
Detroit Diesel describes this design as a “crosshead” piston. As the term is usually applied, it refers to a kind of articulated piston used on very large engines. A pivoted extension bar separates the upper piston element and the skirt, which rides against the engine frame. The crosshead isolates the cylinder bores from crankshaft-induced side forces. The DDA design does not relieve the bore of side forces, but it increases the bearing area of the pin and centralizes the load more directly on the conn rod for a reduction in bending forces. The connecting rod, illustrated in the following section, bolts to the underside of the pin, making the upper half of the pin available to sup- port piston-dome thrust.
Failure modes
Piston failure is usually quite obvious. Wear should not be a serious consideration in low-hour engines, because the skirt areas are subject to relatively small forces and have the benefit of surplus lubrication. Excessive wear can be traced to dirty or improperly blended lube oil or inadequate air filtration. Poor cylinder finishing might also contribute to it. Piston collapse or shrinkage is usually due to over- heating. If the problem shows itself in one or two cylinders, expect water-jacket stoppage or loose liners.
Combustion roughness or detonation damage begins by eroding the crown, usually near the edge. The erosion spreads and grows deeper until the piston “holes.” Typically the piston will look as if it were struck by a high-velocity projec- tile. Scuffing and scoring (a scuff is a light score) might be confined to the thrust side of the piston. If this is the case, look for the following:
• Oil pump problems such as clogged screen, excessive internal clearances.
• Insufficient rod bearing clearances, which reduce throw-off, robbing the cylinders of oil.
• Lugging.
The probable causes of damage to both sides of the skirt include the ones just mentioned, plus the following:
• Low or dirty oil.
• Detonation.
• Overheating caused by cooling system failure.
• Coolant leakage into the cylinder.
• Inadequate piston clearance.
Scuffs or scores fanning out 45° on either side of the pinhole mean one of the following conditions:
• Pin fit problems such as too tight in the small end of the rod or in the piston bosses.
• Pinhole damage (see below for installation procedures). Ring land breakage can be caused by the following:
• Excessive use of starting fluid.
• Detonation.
• Improper ring installation during overhaul.
• Excessive side clearance between the ring and groove.
• Water in cylinder.
Free-floating pins sometimes float right past their lock rings and contact the cylinder walls. Several causes (listed below) have been isolated.
• Improper installation: Some mechanics force the lock rings beyond the elas- tic limit of the material. In a number of cases, it is possible to install lock rings by finger pressure alone.
• Improper piston alignment: This might be caused by a bent rod or inaccura- cies at the crankshaft journal. Throws that are tapered or out of parallel with the main journals will give the piston a rocking motion that can dislodge the lock ring. Pounding becomes more serious if the small-end bushing is tight.
• Excessive crankshaft end play: Fore-and-aft play is transmitted to the lock rings and can pound the grooves open. Again, a too-tight fit at the connecting rod’s small end will hasten piston failure.
Servicing
Used pistons can give reliable service in an otherwise rebuilt engine, but only after the most exhaustive scrutiny. Scrape and wire-brush carbon accumulations from the crown, but do not brush the piston flanks. Carbon above the compression ring and on the underside of the crown should be removed chemically. A notch, letter, arrow, or other symbol identifies the forward edge of the piston. These marks are usually stamped on the crown, on the pin boss relief (illustrated in Fig. 8-41), or hidden under the skirt. Make note of the relationship between the leading edge of the piston and the numbered side of the connecting rod.
Lay out the rings in sequence, topsides up, on the bench. Spend some time “reading” the rings—the history of the upper engine is written on them, just as the crank bearings testify to events below. The orientation code—the word Top (T) or Ober. (O)—will be found on the upper sides of the compression and scraper rings, adjacent to the ring ends.
You might wonder why attention is given to these codes for piston assemblies that, at this stage, are of unknown quality, and for rings that will, in any event, be discarded. The purpose is to become familiar with the concept of orientation as it applies to the particular engine being serviced. New parts might not carry identical codes, but the relationship between coded parts will not change.
While still attached to the rod and before investing any more time in it, inspect the piston for obvious defects. Reject if the piston is fractured, deeply pitted, scored, or if it exhibits ring land damage (Fig. 8-44). Cracks tend to develop where abrupt changes in cross section act as stress risers. Deep pits usually develop at the edges of the piston; scoring is most likely to develop on the thrust faces. Contact with the cylinder bore occurs at two areas, 90° from the piston-pin centerline. The major thrust face lies in the direction of the crankshaft rotation and is normally the first to score. (Viewed from the front of a clockwise-rotation engine, the major thrust face is on the right.) Machine marks, the light cross-hatching left by the cutting tool, should remain visible over most of the contact area. The next section describes the relationship of wear patterns to crankshaft and rod alignment.
If the piston passes this initial examination, make a micrometric measurement of skirt diameter across the thrust faces. Depending on the supplier, the measurement is made at the lower edge of the skirt, at the pin centerline, or at an arbitrary dis- tance between these points (Fig. 8-45). Piston OD subtracted from cylinder bore ID equals running clearance.
All traces of carbon in the ring grooves and oil spill ports must be removed. Whenever possible, this tedious task should be consigned to the machinist and thought about no more. Figure 8-46 illustrates a factory-supplied plug gauge used to
measure groove width; the next drawing, Fig. 8-47, shows an alternate method using a new ring and a feeler gauge. The latter method is accurate, so long as the gauge bottoms against the back of the groove.
Most engines of this class employ “full-floating” piston pins that, at running tem- peratures, are free to oscillate on both the rod and piston (Fig. 8-48). Pins secure lat- erally with one or another variety of snap rings, some of which are flattened on the inboard (or wearing) sides.
Protecting your eyes with safety glasses, disengage and withdraw the snap rings. Although mechanics generally press out (and sometimes hammer out) piston pins, these practices should be discouraged. Instead, take the time to heat the pistons, either with a heat gun or by immersion in warm (160°F) oil. Pins will almost fall out.
While the piston is still warm, check for bore integrity. Insert the pin from each side. If the pin binds at the center, the bore might be tapered; if the bore is misaligned, the pin will click or bind as it enters the far boss (Fig. 8-49).
Other critical areas are illustrated in Fig. 8-45. Measurements are to be made at room temperature. Damage to retainer rings or ring grooves suggests excessive crankshaft end play, connecting-rod misalignment, or crankpin taper. Slide forces generated by a badly worn crankpin or severely bent rod react against the snap rings. Rod or crankpin misalignment might also appear as localized pin and pin- bearing wear, a subject discussed in the following section.
To install, warm the piston, oil the pin and pin bores, and, with the rod in its original orientation, slip the pin home with light thumb or palm pressure. Use new snap rings, compressing them no more than necessary. Verify that snap rings seat around their full diameters in the grooves.
A few engines use pressed-in pins, which make a lock on the piston with an interference fit. Support the piston on a padded V-block and press the pin in two
stages: stop at the point of entry to the lower boss, relieve press force to allow the piston to regain shape, and press the pin home. If the pin is installed in one pressing, the lower boss might be shaved.
What remains is to establish the running clearance. Piston-to-bore clearance is fundamentally a matter of specification, but specifications are never so rigid that they cannot be bent. For example, one John Deere piston/liner combination requires a running clearance of 0.0034–0.0053 in., as measured at the bottom of the piston skirt. A high-volume rebuilder typically goes toward the outside limit, building clearance here and in the crankshaft bearings. A “loose” engine will be more likely to tolerate severe loads as delivered and without the benefit of a break-in period. A custom machinist, who works on one or two engines at a time, often goes in the other direction, aiming at the tightest clearance the factory allows. Besides being aesthetically more satisfying, “tight” engines tend to live long, quiet lives. Of course, such an engine must be carefully run in during the first hours of operation.
In an attempt to stabilize the maintenance process, factory manuals include, wear limits for critical components. The concept of permissible wear is a value judgment, made in an engineering office remote from the world of mechanics, back-ordered parts, and budgetary restraints. Permissible piston-to-liner clearance for the Deere engine is 0.0060 in. Suppose that the running clearance is found to be 0.0055 in.—only 0.0002 in. over the allowable 0.0053 with 0.0030 in. to go before the wear limit is reached. Assuming that the wear is equally distributed over both parts, should the piston or liner or both be replaced? This is only one cylinder of six, none of them worn by identical amounts. Questions like this go beyond mechanics and depend for their answers on the politics of the situation, interpreted in light of the philosophy of maintenance—formal or informal—that characterizes the operation.