LO 7-9 Outline diagnostic techniques used for evaluating the condition of a piston ring-to-cylinder wall seal

1. Cylinder Component Service

Cylinder Component Service

Cylinder component servicing consists of testing, removing, disassembling, inspecting, and replacing pistons, piston rings, and connecting rods. A worn engine may be indicated when oil consumption is increased, engine power is reduced, fuel consumption is increased, or the engine runs rough, misfires, produces excessive exhaust emissions or has excessive crankcase blowby. While time and distance are the typical wear factors, there are many causes for premature wear and failure of cylinder components, including problems with oil quality, the air induction system leaking dirt into the engine, internal coolant and fuel leaks, or failures in the EGR system that allow excess recirculation of exhaust gas into the cylinders. Inspecting and diagnosing worn or damaged cylinder components should begin with evaluating the cylinder wear and ring-to-cylinder wall sealing. This can be performed non-intrusively, without disassembling the engine. Inspection of cylinder components can also take place after disassembly.

Most cylinder wear takes place at the top of the cylinder and tapers toward the bottom Figure 58. Lack of lubrication oil above the oil control ring and intermediate ring is the primary reason for most wear taking place in the ring turnaround area. This means cylinder wear is usually tapered, with the widest dimension of the cylinder near the top in a worn-out engine. Major side thrust force is responsible for wear in the middle of the cylinder wall where the angle between the connecting rod and the piston, combined with high cylinder pressures during the power stroke, force the piston against the cylinder wall Figure 59.

FIGURE 07-58: A view of a cylinder from the crankcase. Most wear takes place at the top and tapers toward the bottom. Major side thrust force causes wear in the middle of the cylinder wall.

FIGURE 07-59: Cylinders wear in a tapered shape, with the top of the cylinder wearing away fastest.

1.1. Video: Installing a Connecting Rod

1.2. Video: Removing the Pistons

1.3. Piston Service

Piston Service

A piston has a micro-finish on the skirt that allows it to retain oil, which improves wear characteristics. It is important to remember that this finish should not be damaged during servicing. Do not clean pistons with abrasive cleaners, wire wheels, or other items that can damage the finish of the piston skirt. The best method for cleaning aluminum pistons is to soak them in a mild aluminum-compatible cleaner followed by steam cleaning. Hard carbon deposits on the crown may be scraped away with a plastic scraper.

Visual inspection of pistons should be made to find evidence of cracking, overheating, and wear Figure 60. The presence of vertical or horizontal cracks in the piston bowl or piston pin bore gives important clues when performing failure analysis. Horizontal cracks usually indicate a bonding problem between the piston skirt and the crown. Vertical cracks in the bowl often indicate high cylinder temperatures. Cracking between the crown and the piston pin bore indicate excessive cylinder pressure Figure 61.

FIGURE 07-60: Inspection points for pistons.

FIGURE 07-61: Piston cracks between the piston pin bore and the crown indicate abnormally sharp spikes in cylinder pressure caused by conditions such as the improper use of ether as a starting aid or hydrostatic locks.

Worn ring grooves will cause excessive oil consumption. Wear in the piston’s ring grooves is typically checked with specialized OEM gauge pins Figure 62. Specialized OEM gauge pins fit snuggly into the compression piston ring groove. A pair of springs holds two gauge pins in the grooves, and the distance between the gauge pins is measured with a micrometer. If the piston ring groove wear is excessive, the pins will sit farther into the groove and the dimension between the pins will become smaller. Comparing actual, measured dimensions with manufacturer specifications will determine whether the piston can be reused. Another way to measure wear in the piston’s ring grooves is with a go-no-go gauge Figure 63. To measure piston ring grooves and cylinder wear, follow the guidelines in Skill Drill 7-1.

FIGURE 07-62: Measuring piston ring groove wear with gauge pins and a micrometer.

FIGURE 07-63: Ring groove wear can be checked with a go-no-go gauge. If the gauge touches the piston, the ring groove is excessively worn.

The clearances between a piston and a cylinder wall are best measured with a long feeler blade Figure 64. This is done by placing the piston in the cylinder. The thickest possible feeler blade is then slid in between the piston and the cylinder wall 90 degrees to the wrist pin. The blade should not be forced into the clearance, but a 5–8 lb (2.3–3.6 kg) drag should be felt when pulling the blade out of the clearance. Generally, clearances of between 0.003–0.006" (0.0076–0.152 mm) are encountered in cylinders that fit well. Less clearance than this prevents the piston from properly expanding while more clearance will create noise, cause high oil consumption, and indicates excessive wear or damage to the piston and cylinder wall Figure 65. To measure the clearance between the piston and the cylinder wall, follow the guidelines in Skill Drill 7-2.

FIGURE 07-64: Measuring piston-to-cylinder wall clearance is performed using a long feeler blade between the piston and the cylinder wall.

FIGURE 07-65: Wear points on a worn piston. Worn pistons tilt and scrape oil up into the combustion chamber.

1.4. SKILL DRILL 7-1  Measuring Piston Ring End Gap and Ring Groove Wear

1.5. SKILL DRILL 7-2  Measuring Piston-to-Cylinder Wall Clearance

1.6. Piston Ring Installation

Piston Ring Installation Care must be exercised when installing rings onto a piston and into the cylinder because rings are fragile. A common cause of ring breakage is overstretching the ring during installation. Ring expanders are tools that stretch a ring just far enough for it to be removed but not too far to cause it to bend or break. To prevent loss of compression and damage to the cylinder wall, ring end gaps must be staggered 120 degrees apart in a three-ring pack configuration. End gaps should not be aligned near the wrist pins Figure 66. If rings are reused, such as after performing warranty repairs on engines having accumulated low travel distances, extra caution should be exercised when removing pistons from cylinders to prevent ring damage. The wear step or cylinder ridge found above the ring travel area in the cylinder should first be removed to prevent damage to the rings and the piston when both the piston and the rings are pushed up over the ridge during removal. Failure to remove the cylinder ridge, by cutting or grinding it away, will score the piston and even break the rings.

FIGURE 07-66: Piston ring end gaps must be staggered 120 degrees apart to prevent gases from easily leaking past the rings.

Use a ring compressor to install a piston into the cylinder and prevent damage to the rings Figure 67. A number of designs are available to ease ring installation. Both the cylinder wall and the compressor should be well lubricated during the installation procedure. A soft hammer handle can be used to gently tap a piston into the cylinder bore.

FIGURE 07-67: Various types of piston ring compressors are used to squeeze rings on the piston while inserting the piston into the cylinder.

FIGURE 07-68: Honing stones used to apply a crosshatch finish to a cylinder wall.

When new rings are installed into the cylinder, a new crosshatch finish needs to be made on the cylinder wall. As long as taper wear is within specifications, a ball hone or straight-sided honing stone can be used to put crosshatch finish into the cylinder Figure 68. About one stroke per second is required at 60 rpm to establish the correct angle on the crosshatch finish. Cylinders are rarely honed dry; usually diesel fuel or special cutting oil is used to lubricate the stone. Coarse-, medium-, and fine-grit stones are available, depending on whether a new crosshatch is needed to help seat the new rings and retain oil or to remove some taper wear. If excessive scoring or taper wear is present in a cylinder, the cylinder may require boring out to an oversize dimension. The final dimension depends on the availability of oversize pistons and rings. Oversize pistons in sizes 0.01" (0.25 mm), 0.02" (0.5 mm), 0.03" (0.76 mm), and 0.04" (1.0 mm) are typically available for popular engines constructed without cylinder sleeves.

1.7. Ring Break-In

Ring Break-In
When piston rings are initially installed with a new crosshatch finish on the cylinder walls, the rings will need to seat, or slightly wear, into the cylinder wall and conform to wall irregularities. A time period, called the break-in period , is needed to seat rings as well as other engine parts, including bearings, valve guides, piston pins, cam bushings, and other moving parts. During the break-in period, crosshatch is slightly worn away to conform to the shape of the piston rings, enabling the rings to seat better on the cylinder wall. It is important to operate the engine under heavy load for a short time after an engine is overhauled to seat rings and prevent formation of cylinder glaze in the engine. Under load, the cylinder gas pressure will push the rings against the cylinder wall hardest, which best helps rings to seat. This procedure is important, especially during the initial break-in period. Manufacturers will often provide a schedule in their service manuals that outlines specific break-in procedures, including what percentage of load to place the engine under for a prescribed amount of time.

1.8. Ring-to-Cylinder Wall Seal Testing

Ring-to-Cylinder Wall Seal Testing Evaluating the piston ring-to-cylinder wall seal is an important indicator of engine life. Poor sealing means the engine is worn out or has internal damage. Three methods can be used to evaluate the condition of piston ring-to-cylinder wall seal: compression testing cylinder leak-down testing crankcase pressure testing Cylinder Compression Testing Compression testing is a measure of the maximum pressure of the engine cylinders when the engine is cranking Figure 69. It is rarely used on diesel engines due to the small clearance volume of the cylinders and the prospect of hydraulic locking of the piston with oil when wet testing. Wet testing is a procedure used to check for worn rings. It is performed by squirting oil into the cylinders. If compression pressures significantly increase after adding more cylinder wall lubricant, it usually indicates worn rings. In addition to potentially locking the engine, the other problem with this method is that oil may also ignite during compression testing.

FIGURE 07-69: Diesel compression testing gauge with a variety of glow plug and injector adapters.

Large variations between compression pressure measurements in an engine’s cylinders at cranking speed can be the result of keystone ring design, which requires high cylinder pressure to help rings seal effectively. Furthermore, it is cumbersome and time consuming to remove an injector to install a compression pressure gauge into a cylinder. Smaller, light-duty (LD) diesel OEMs may recommend the removal of glow plugs to measure cylinder compression pressure. Other methods to evaluate cylinder sealing are more effective. Dynamic cylinder balance testing to identify weak, underperforming cylinders is a better place to start this type of diagnostic work.

1.9. Cylinder Leak-Down Testing

Cylinder Leak-Down Testing
Cylinder leakage is the percentage of gas leakage past the piston rings. High leakage rates are usually due to worn cylinders and rings. Cylinder leakage testing is an accurate and meaningful measurement of the cylinders’ sealing capabilities. The leak-down test can be performed separately or together with compression testing. Cylinder leak-down testing is more practical for diesel engines than compression testing. During cylinder leak-down testing, air is fed into the cylinder at pressures of 80–120 psi (552–827 kPa), and the engine should be warm to minimize the clearance between the cylinder wall and the piston. With the valves closed, the piston is first positioned at TDC and then the engine is locked there. The cylinder leakage tester is connected to the cylinder and pressurized with regulated shop air. A second gauge on the leakage tester will measure actual cylinder pressure, which varies with the amount of cylinder leakage. A worn cylinder will allow more air leakage past the rings, and the gauge will read a lower pressure. Leakage is calculated as a percentage of measured cylinder pressure compared with regulated air pressure Figure 70. The rate of leakage is compared against the manufacturer’s specification to determine whether the amount of leakage is acceptable. Engine problems can be detected by listening for large volumes of air leaking out of the turbocharger, the intake or exhaust system, or even the oil filler tube. Excess air leakage in these areas may indicate burned or broken valves, damaged pistons, or simply worn cylinders. An engine in good condition should generally show only 5–10% leakage of the pressure of the air supplied to the cylinder after 1 minute. Some engines, in acceptable condition, may have leakage as high as 20%; more than 30% leakage indicates a severe problem Figure 71.

FIGURE 07-70: A leak-down tester supplies regulated air pressure to a cylinder and measures the percentage of regulated air pressure maintained in the cylinder. The percentage of leak-down is an indicator of cylinder and ring condition.

FIGURE 07-71: A cylinder leakage test reports the amount of air pressure retained in a cylinder, which corresponds to cylinder condition. © Snap-On Incorporated.

1.10. Crankcase Pressure Testing

Crankcase Pressure Testing
One of the best practices for evaluating the condition of piston rings in a diesel engine is a crankcase pressure test. This is a measurement of the amount of cylinder blowby, which is proportional to the sealing ability of the piston rings and cylinder walls. Because blowby gas pressure in the crankcase is a function of the effectiveness of the piston ring-to-cylinder wall seal, the quantity of blowby and crankcase pressure is a good measure of piston ring and cylinder wall condition. Excessive blowby and crankcase pressure or blowby volume means there is poor sealing and the cylinders are worn out. Crankcase blowby pressure is usually very low—just a few inches (centimetres) of water column pressure when the engine is under full load and the rings are seated well by high cylinder pressure. When the piston rings and cylinder walls are worn, blowby volume increases, driving up the crankcase pressure. Blowby testing is a fast, simple, and nonintrusive technique to evaluate the condition of piston ring-to-cylinder wall sealing Figure 72. Pressure is measured at high-idle, no-load operation. When piston rings seal well, little blowby gas enters the crankcase. When cylinder pressures are higher, such as during high idle rpm operation, the rings seal much better than they would at low engine rpm. Crankcase pressure is typically higher at idle than at high idle in good engines; however, glazed cylinders are indicated when crankcase pressure is significantly higher at idle and drops as engine speed increases.

FIGURE 07-72: Measuring crankcase blowby pressure involves the use of a restrictor orifice and a Magnahelic gauge measuring inches of water column pressure (or kilopascals). Blowby is measured on a warm engine operated at high-idle rpm.

FIGURE 07-73: A Magnahelic gauge measures pressure in inches of water column (or kilopascals). These gauges are much more sensitive to pressure changes than measuring using with mercury.

FIGURE 07-74: Various types and diameters of restriction orifices for measuring crankcase pressure.

Pressure in the crankcase is measured in inches (centimetres) of water column using a water manometer or Magnehelic gauge Figure 73. A calibrated restriction fitting is placed over the crankcase breather tube Figure 74. If equipped, the crankcase depression regulator, a device that limits how low crankcase pressure can fall, is blocked during the test. After the engine coolant and oil are warmed to operating temperature, the engine is put to full throttle, no load, and crankcase pressure is measured. Usually only a few inches (centimetres) of pressure are measured in an engine in good condition. Small-bore diesels will typically have a maximum limit of 10" (25 cm) of water column pressure indicating 0% service life. Larger engines may be allowed as much as 15–18" (38–46 cm) of water column pressure TABLE 7-3.

To perform a crankcase blowby test, follow the guidelines in Skill Drill 7-3.

1.11. SKILL DRILL 7-3  Performing a Crankcase Blowby Test

STEP 1 Before starting the crankcase pressure test, it is important to run the engine until it reaches normal operating temperature. A cold engine will produce an abnormally high pressure reading. STEP 2 Install the crankcase orifice restrictor tool in a road draft tube or a crankcase pressure test adapter in the oil fill hole. Use the manufacturer’s prescribed tool and restriction diameters. STEP 3 Ensure that the oil dipstick tube seal and any other gaskets and seals on the engine are sealing properly. STEP 4 Connect the Magnehelic gauge with plastic tubing to the restriction tool. There are two ports on the gauge, pressure and vacuum; connect the orifice port to the gauge pressure port. STEP 5 Make sure that the gauge tubing is clean and dry so that the gauge does not become damaged. It is good practice to blow clean, dry compressed air through the tubing before each use. STEP 6 Zero the Magnehelic gauge. STEP 7 Chock the wheels of the vehicle and apply the brakes. Place the transmission in neutral or park. STEP 8 Start the vehicle’s engine and run it at wide-open throttle under no load. Maintain this engine speed for at least 30 seconds to observe and record a stabilized reading. Make sure that the hole in the top of the crankcase orifice restrictor tool is never blocked when the engine is running. STEP 9 Record your results and compare them against specifications. Any reading of more than 4" (102 mm) of water during the crankcase pressure test indicates a worn engine. It is likely that base engine mechanical concerns exist. STEP 10 Make a service recommendation to overhaul the engine if engine hours and distance traveled are high.

1.12. Inspecting for Cylinder Wear

Inspecting for Cylinder Wear

Several methods are used to measure engine wear when an engine is partially or fully disassembled. One effective method to visually evaluate cylinder wear is to examine the ring turnaround area Figure 75. Using a piece of paper to reflect light directly into the crosshatch helps make the cylinder wear more visible. If 20% or more of the crosshatch is missing, 0% service life remains. If 10% of the crosshatch is missing, 50% service life remains. An engine with a 5" (13 cm) stroke and 0.25" (6 mm) of polish has 75% service life remaining.

FIGURE 07-75: You can visually evaluate cylinder wear in the ring turnaround area.

FIGURE 07-76: The ring end gap method is used to check cylinder taper. Any differences when measuring end gap at the top, middle, and bottom of the cylinder identifies dimensional differences.

A second method is to measure the piston ring end gap at three points in the cylinder swept by the piston rings: top, middle, and bottom Figure 76. After squaring a used compression ring in the cylinder, a feeler blade is placed in the end gap of the ring. The maximum clearance in the end gap is measured at three points. Because the circumference of a circle is found by multiplying π (3.14) and the circle’s diameter, the end gap differences are divided by 3.1 (which is rounded from π, which is 3.14). That means a difference of 0.003" (0.076 mm) between the widest and the smallest end gap equals 0.001" (0.025 mm) of taper wear. A general rule for engine wear is that no more than 0.001" (0.025 mm) of taper wear is permissible for every 1" (25 mm) of cylinder bore up to 0.005" (0.127 mm). If wear exceeds this amount, two problems will occur. The first is that the cylinder wall will not be able to retain an oil film for lubricating and sealing the rings because the crosshatch is worn from the cylinder wall. The second is that the rings will open and close too far during each stroke; they will eventually become fatigued from the continual flexing and break.

The dimensions of the cylinder can also be measured using a telescoping gauge set. This is done in three places as well: the top, middle, and bottom of the cylinder, where it is swept by the piston rings. An experienced and skilled technician can measure the bore diameter with a telescoping gauge and an outside micrometer; however, due to the difficulty of developing the skills to accurately use these types of measurement tools, they are seldom used.

The method most commonly used by machinists when reconditioning cylinder bores involves the use of a dial bore gauge, which is a precision measuring tool used to measure taper wear in a cylinder. This tool has a dial-like indicator measuring mechanism. After the gauge is adjusted to a dimension just slightly more than the bore diameter, it is placed in the cylinder and rocked back and forth to sweep the gauge through a perpendicular point or at a right angle to the cylinder wall. The smallest dimension of the bore gauge indicates when the gauge has found the narrowest point in the cylinder. The gauge is moved to several more points in the cylinder to determine what the narrowest dimensions are and then a comparison is made between the measurements. Once again, the maximum permissible variation between the smallest and the largest measurements should be less than 0.005" (0.127 mm), which indicates 0% remaining service life.