| Home | Fundamentals Shams + Scams | Glossary |
PISTON AND PISTON RINGS The piston forms the lower portion of the combustion chamber. The pressures from combustion are exerted against the top of the piston, called the head or dome. A piston must be strong enough to face this pressure; however, it should also be as light as possible. This is why most pistons are made of aluminum or aluminum alloys. Aluminum pistons alloyed with copper, magnesium, nickel, and silicon are common. Silicon is the most common element mixed with aluminum to make pistons. Silicon makes the piston more resistive to corrosion and improves its strength, hardness, and wear resistance. It also helps to reduce the piston's weight. There are three basic types of aluminum silicon alloys used in pistons: hypoeutectic, eutectic, and hypereutectic. Hypoeutectic pistons, common in earlier engines, have about 9% silicon. Most eutectic pistons have 11% to 12% silicon. Eutectic alloys provide good strength and are economical to make. Hypereutectic pistons have a silicon con tent above 12%. They offer low thermal expansion rates, improved groove wear, good resistance to high temperatures, and greater strength and scuff and seizure resistance. The head of the piston can be flat, concave, convex, crowned, raised and relieved for valves, or notched for valves. Newer pistons are typically flat, flat with valve notches, or have a slightly dished crown. The dished crown concentrates the pressure of combustion at the thickest part of the piston head, right above the top of the piston pin boss. The piston pin boss is a built-up area around the bore for the piston pin, sometimes called the wrist pin (Fgr__56). The pin bore is not always centered in the piston. It can be offset toward the major thrust side of the piston, which is the side that will contact the cylinder wall during the power stroke. === Fgr__56 The features and terminology used to describe a piston. By MAHLE International GmbH. Piston skirt; Wrist pin boss; Ring lands; Ring headland; Crown Valve recess Toroidal recess; Combustion area; Top compression ring; Oil control ring ;Lubrication channel; Second compression ring; Snap ring groove === Piston heads are often coated with hard anodizing, ceramics, or electroplating. These coatings increase the hardness and resistance to corrosion, cracking, wear, and scratching. New ceramic coatings offer nearly three times the surface hardness of traditional hard-anodized coatings. Ceramic coatings also help protect against spontaneous detonation. Just below the dome, around the sides of the piston, is a series of grooves. The grooves are used to hold the piston rings. The high parts between the grooves are called ring lands. Some pistons have a ceramic coating in the top ring groove to prevent the ring from being "welded" inside the groove. Normally there are three grooves: two compression and one oil control. The compression grooves are located toward the top of the piston. The depth of grooves varies with the size of the piston and the type of rings used. The oil control groove is the lowest groove on the piston. They are normally wider than the compression ring grooves and have holes or slots to allow oil to drain. The positions of the ring grooves vary with engine design. Many newer engines have the top compression ring as close as possible to the piston head. This reduces the amount of fuel that can drop down the sides of the piston before combustion. This hidden fuel is not involved in the combustion process but leaves as unburned hydrocarbons during the exhaust stroke. In this design, all rings are placed close together. On a few pistons, the piston pin bore is very close to the piston head, behind the groove for the lower oil control ring. == Fgr__57 Always make sure that the markings on the piston and connecting rod are in the correct relation ship to each other and they face the correct direction. Rubber hose Connecting rod oil hole must face the rear of the engine The arrow must face the timing belt side of the engine === Fgr__55 The effects of rod misalignment on its bearings. By AE Clevite Engine Parts Fgr__54 A piston and connecting rod assembly. By BMW of North America, LLC. Major misalignment A sign of a bent and twisted rod Rubbing Pressure areas === The area below the piston pin is called the piston skirt. The area from just below the bottom ring groove to the tip of the skirt is the piston thrust surface. There are two basic types of piston skirts: the slipper and full skirt. The full skirt is used primarily in truck and commercial engines. The slipper type is used in automobile engines and allows the piston enough thrust surface for normal operation. A slipper skirt also allows the piston to be lighter and reduces piston expansion because there is less material to hold heat. Late-model engines that are capable of running to fairly high rpm use lighter pistons. These pistons have skirts only on the thrust sides. Often the skirts are coated with molybdenum to prevent cylinder wall scuffing. To ensure that the piston is installed correctly and has the correct offset, the top of the piston will have a mark. The most common mark is a notch that is machined into the top edge of the piston. Always check with the service manual for the correct direction and position of the mark. The front of the piston must match the front of the connecting rod. When an engine is designed, piston expansion determines how much piston clearance is required in the cylinder bore. Too little clearance will cause the piston to bind at operating temperatures. Too much will cause piston slap. The normal piston clearance for an engine is about 0.001 to 0.002 inch (0.0254 to 0.0508 mm). This clearance is measured between the piston skirt and the cylinder wall. Advances in piston technology have allowed manufacturers to build engines with about half that clearance. This leads to increased efficiency and lower emissions. Piston Terminology Many different terms are used to describe the design of a piston; these include: ¦ Compression distance (or height) - The distance from the center of the piston pin bore to the top of the piston. ¦ Ring belt - The area between the top of the piston and the pin bore where the piston rings are installed. ¦ Heat dam - A narrow groove cut in some pistons to reduce heat flow to the top ring groove. During engine operation, the groove fills with carbon and absorbs the heat of combustion. ¦ Land diameter - The diameter of the ring land. On some pistons, the land diameter will be same for each ring; on others it will increase from top to bottom. ¦ Land clearance - The difference between the diameters of the land and the cylinder. ¦ Groove root diameter - The diameter of a piston measured at the bottom of a ring groove. The root diameter of each groove may vary with the type of ring used. ¦ Groove protector - A steel or cast-iron insert placed into the top groove of an aluminum piston to extend the life of the top compression ring. ¦ Top groove spacer - A steel spacer installed above the ring in a reconditioned groove to bring the ring's side clearance within specifications. ¦ Piston pin bushing - Found primarily in cast-iron pistons, this bushing serves as a bearing for the piston pin. It’s inserted into the piston pin bore. ¦ Major thrust face - The part of the piston skirt that has the greatest thrust load. This is typically the right side when looking at the engine from the fly wheel end. ¦ Minor thrust face - The part of the piston skirt that is opposite the major thrust face. ¦ Skirt clearance - The difference between the diameter of the piston skirt diameter and the diameter of the cylinder. ¦ Piston skirt taper - The difference between the diameter of the piston at the top and bottom of the skirt. ¦ Piston cam - The shape of the piston skirt area, which provides correct cylinder wall contact and clearances. Inspection-- Each piston should be carefully checked for damage and cracks. Pay attention to the ring lands and the pin boss area. Look for scuffing on the sides of the piston (Fgr__58). Light up and down scuffing is normal. Excessive, irregular, or diagonal scuff marks indicate lubrication, cooling system, or combustion problems. Scuffing may also be caused by a bent connecting rod, seized piston pin, or inadequate piston-to-wall clearance. If any damage is evident, the piston should be replaced. === Scuff marks Fgr__58 Each piston should be carefully checked for scuffing on the sides of the piston. Fgr__59 Ring side clearance should be checked on each piston. 16.0 mm. (0.63 in.) Skirt diameter Fgr__60 The diameter of the piston is measured across specific points on the skirt. === Remove the piston rings. A ring expander should be used to remove the compression rings. Normally the oil control ring can be rolled off by hand. Remove the carbon from the top of the piston with a gasket scraper. Carbon and oil build up in the back part of the groove. This buildup must be removed. The dirt will prevent the rings from seating properly. Clean the grooves of the piston with a groove cleaning tool or a broken piston ring. When doing this, make sure no metal is scraped off. The oil control ring groove has slots or holes. These should also be cleaned. Use a drill bit or small brush. Once the grooves are clean, use a brush and solvent to thoroughly clean the piston. Don’t use a wire brush. Ring side clearance should be measured. Side clearance is the difference between the thickness of the ring and the width of its groove. To measure this, place a new ring in its groove and, with a feeler gauge, measure the clearance between the ring and the top of the groove (Fgr__59). If the clearance is not within the specified range, the piston should be replaced. The diameter of the piston should be measured. This measurement is normally taken across specific points on the skirt (Fgr__60). If the diameter is not within specifications, the piston should be replaced. Some engine rebuilders will knurl the skirts if the diameter is slightly less than specifications. Piston Pins Piston pins are basically thick-walled hollow tubes. Like the rest of the piston and connecting rod assembly, it’s built to be strong and light. Most are made from alloy steel and are plated with chrome, carburized, and/or heat-treated to provide good wear resistance. Piston pins are lubricated by oil fed through passages in the connecting rods, oil splashing in the crankcase, or spray nozzles in the rods or pistons. A piston pin fits through the small end of the connecting rod and the piston's pin bore. The way the pin is retained is used to describe it. ¦ A stationary pin is pressed into the piston. The connecting rod swivels on the pin. ¦ A semi-floating pin is pressed into the connecting rod. The piston swivels on the pin. ¦ A full-floating pin is able to move or rotate in the piston and connecting rod. The pins are retained by caps, plugs, snap-rings, or spring clips inserted in the piston at the ends of the pin. Full-floating pins are the most commonly used. Inspect the pin boss area on the piston for signs of pin wobble. Then remove the pin to inspect it. With full-floating pins, the retaining clips are removed and the pin pushed out. A pin press is used to remove and install pressed fit pins. When installing a piston pin, make sure the piston is facing the correct direction in regard to the connecting rod. Inspect the pin closely for signs of wear. Full floating pins should have an even wear pattern. Carefully inspect the pin bore in the piston. Since a piston is made from softer material than the pin, the piston will wear before the pin. If there are signs of uneven wear, suspect a lubrication or connecting rod problem. Check the fit of the pin. It should move freely through the bores. Also attempt to move the pin up and down in its bores. Any movement means the piston bore or pin is worn. To determine if the bore or pin is worn, measure the diameter of the pin bore. If the bore is not within specifications, replace the piston. Then measure the diameter of the pin. If the pin is not within specifications, replace it. If the piston bore and pin meet specifications, measure the small end bore of the connecting rod. If the diameter is not within specifications, replace the connecting rod. Some manufacturers recommend a check of the pin's oil clearance. To do this, subtract the diameter of the pin from the diameter of the piston pin bore. If the oil clearance exceeds specifications, replace the piston and pin. Now subtract the diameter of the pin from the diameter of the connecting rod's small end. If the oil clearance exceeds specifications, replace the connecting rod and/or the pin. Connecting rods may have a piston pin bushing. Measure the inside diameter of the bushings and compare the reading to specifications. If the bushing is worn or damaged, it should be replaced. The bushing is pressed out of the rod with a pin press. Installing new bushings is also done with a press; some technicians heat the rods and freeze the pins before pressing them in. This makes installation easier. Before applying pressure on the pin, make sure it’s set squarely above the bore. === Fgr__61 The piston pin is measured at a variety of spots and its diameter compared to the ID of the piston's pin bore and the small end of the connecting rod. === Piston Rings Piston rings are used to fill the gap between the piston and cylinder wall. The piston rings seal the combustion chamber at the piston. Piston rings must also remove oil from the cylinder walls to prevent oil from entering into the combustion chamber. They also carry heat from the piston to the cylinder walls to help cool the piston. In most engines, pistons are fitted with two compression rings and one oil control ring. The compression rings are found in the two upper grooves closest to the piston head. The oil ring is fitted to the groove just above the wrist pin. There are many different designs of rings; each has a specific application. Compression Rings--Compression rings are designed to use combustion pressure to force them against the cylinder wall. During the power stroke, the pressure caused by the expanding air-fuel mixture is applied between the inside of the ring and the piston ring groove. This forces the ring into full contact with the cylinder walls. The same force is applied to the top of the ring, forcing it against the bottom of the ring groove. These two actions help to form a tight ring seal. Common compression rings are made of cast iron, cast iron coated with molybdenum (moly), and cast iron coated with chrome (Fgr__62). Cast iron offers a durable wear surface and costs less than a moly- or chrome-faced ring. These rings are ideal for normal driving. Moly coatings are quite porous and can hold oil. As a result, moly rings have a very high resistance to scuffing. These rings are used in engines that are run at continuous high speeds or severe load conditions. Chrome also has good resistance to scuffing but does not have the oil retention capabilities of moly. Chrome rings are recommended when driving conditions include frequent travel on dusty or unpaved roads. Chrome is very dense and hard and will push away any dirt that enters the cylinder on the intake stroke. Moly coatings, due to their porosity, will allow dirt to become embedded in a ring's face. Normally, a moly ring is used in the top ring groove with a cast iron or chrome ring in the second groove. === Fgr__62 (A) A moly-coated compression ring. (B) A chrome-faced compression ring. Moly-filled groove. Cast iron (A) Chrome facing === Other face coatings include ceramics, graphite, phosphate, and iron oxide. All coatings are designed to help in the wear-in process. Wear-in is the time required for the rings to conform to the shape and surface of the cylinder wall. Oil Control Rings--Oil is constantly being applied to the cylinder walls. The oil lubricates and cleans the cylinder wall and helps cool the piston. Controlling this oil is the primary purpose of the oil ring. The two common types of oil rings are the segmented oil ring and the cast-iron oil ring. Both are slotted so that excess oil from the cylinder wall can pass through the ring. The piston's oil ring groove is also slotted. Oil passes through the ring and slots in the piston and returns to the oil sump. Segmented oil rings have an upper and a lower scraper rail and an expander. The scraper rings are often chrome rings. The expander pushes the two scrapers out against the cylinder wall. During installation, the end gaps of the three pieces must be staggered to prevent oil from escaping into the cylinder. Installing the Rings Some engines use low-tension piston rings; make sure the new rings are the correct ones for the engine. Before installing the rings onto a piston, check the rings' end gap. Place a compression ring into a cylinder. Use an upside-down piston to square the ring in the bore. Measure the gap between the ends of the ring with a feeler gauge. Compare the reading to specifications. If the gap exceeds limits, oversized rings should be used. If the gap is less than specifications, the ends of the ring can be filed with a special tool. The preceding procedure for checking ring gap assumes that all taper and imperfections in the bore have been corrected. If the bore has some taper, the end gap should be checked in the cylinder at the lowest point of piston travel. Piston ring gap is critical. Excessive gap will allow combustion gases to leak into the crankcase. This is commonly called blowby. Too little clearance can score the cylinder walls as the ends of the rings come in contact with each other as the engine heats up. The gap of the top compression ring allows some combustion pressure to leak onto the second compression ring. This helps the second ring seal. Apply a light coat of oil on the rings. The oil control ring is installed first. Insert the expander; position the ends above a pin boss but don’t allow them to overlap. Then install the rails. Stagger the ends of the three parts. The oil control ring assembly can be installed by hand. If the piston pin is set up into the oil ring groove, an oil ring support must be inserted in the ring groove. The support gives the oil rings a place to sit at the points of the groove where there is no piston material beneath the rings. The support has a dimple to prevent it from rotating around the piston. This keeps the ring gap at the desired spot at all times. Use a piston expander to install the top and second compression rings. Install the second ring first. Make sure the rings are installed in the right position. This includes making sure that the correct side of the ring is facing up. Rings have some sort of mark to show which side should be up. Check the instructions of the ring manufacturer. INSTALLING PISTONS AND CONNECTING RODS The piston and connecting rod assemblies are installed next. Check the marks on the connecting rod caps and the connecting rods to make sure they are a match. The bearings for the connecting rods must be the correct size. If the crankshaft has been machined undersize, matching rod bearing inserts must be installed. The size of the bearing inserts is printed on their box and is stamped on the backs of the bearings or color coded. Snap the bearing inserts into the connecting rods and rod caps. Make sure the tang on the bearing fits snugly into the matching notch. Check the clearance of the connecting rod bearings with Plastigage or a micrometer. Follow the same procedure used to check main bearing clearance. If the clearance is not within specifications, the connecting rod may need to be machined or replaced, or undersized bearings should be installed. Wipe a coat of clean oil on the cylinder walls. Then install the piston and rod according to the procedure shown in Photo Sequence 8. Remember that connecting rods are numbered for proper assembly. Also make sure that the end gaps of the piston rings are staggered according to the manufacturer's recommendations prior to installing the piston assembly. Coat the crankshaft with clean lubricant or engine oil. After each piston is installed, rotate the crankshaft and check its freedom of movement. If the crankshaft is hard to rotate after a piston has been installed, remove it and look for signs of binding. === Fgr__64 Install the piston rings onto the piston with a ring expander. Also make sure that the ring end gaps are arranged according to specifications. By Ford Motor Company Fgr__65 A wrinkle band ring compressor. Reproduced under license from Snap-on Incorporated. All of the marks are marks of their owners. === HOW-TO … Installing a Piston and Rod Assembly ___1 Insert a new piston ring into the cylinder. Use the head of the piston to position the ring so that it’s square with the cylinder wall. Use a feeler gauge to check the end gap. Compare end-gap specifications with the measured gap. Correct as needed. Normally, the gaps of the piston rings are staggered to prevent them from being in line with each other. Piston rings are installed easily with a ring expander. ___2 Before attempting to install the piston and rod assembly into the cylinder bore, place rubber or aluminum protectors or boots over the threaded section of the rod bolts. This will help prevent bore and crankpin damage. ___3 Lightly coat the piston, rings, rod bearings, cylinder wall, and crankpin with an approved assembly lubricant or a light engine oil. Some technicians submerge the piston in a large can of clean engine oil before it’s installed. ___4 Stagger the ring end gaps and compress the rings with the ring compressor. This tool is expanded to fit around the piston rings. It’s tightened to compress the piston rings. When the rings are fully compressed, the tool won’t compress any further. The piston will fit snugly but not tightly. ___6 Lightly tap on the head of the piston with a mallet handle or block of wood until the piston enters the cylinder bore. Push the piston down the bore while making sure that the connecting rod fits into place on the crankpin. Remove the protective covering from the rod bolts. ___5 Rotate the crankshaft until the crankpin is at its lowest level (BDC). Then place the piston/rod assembly into the cylinder bore until the ring compressor contacts the cylinder block deck. Make sure that the piston reference mark is in correct relation to the front of the engine. Also, when installing the assembly, make certain that the rod threads don’t touch or damage the crankpin. ___7 Position the matching connecting rod cap and finger tighten the rod nuts. Make sure the connecting rod blade and cap markings are on the same side. Gently tap each cap with a plastic mallet as it’s being installed to properly position and seat it. Torque the rod nuts to the specifications given in the service manual. Repeat the piston/rod assembly procedure for each assembly. === SHOP TALK -- Many technicians find it easier to use a wrinkle band ring compressor (Fgr__ 65) to install pistons. These push the rings into their grooves but don’t contact the entire outside of the piston. This makes it easier to push the piston out of the tool. Fgr__63 Use a ring expander to install the compression rings. === After the pistons and rods have been installed, check the connecting rod side clearances (Fgr__ 66). Side clearance is the distance between the crankshaft and the side of the connecting rod. It’s measured with a feeler gauge. If the clearance is not correct, the rods may need to be machined or replaced. === Fgr__66 Measuring connecting rod side clearance. Feeler gauge. Fgr__67 A worn lifter and camshaft. Federal-Mogul Corporation Fgr__68 The possible wear patterns of a lifter that does not spin in its bore. By TRW, Inc. Fgr__69 Cam bearings are normally press-fit into the block or head using a bushing driver and hammer. === Next: Camshaft and Related Parts Prev.: Installing Main Bearings and Crankshaft Home Article Index top of page |
