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CASE STUDYWhile traveling, the engine in a family's late model car overheats. Water is added to the radiator. Then the car is taken to a service station to be diagnosed and repaired. Simple tests reveal that a heater hose has split and is leaking coolant. The hose is replaced and new coolant added to the system. With the car repaired, the family continues on their trip. After driving only a short time, smoke is noticed coming out of the tailpipe. The engine shakes violently whenever it’s placed in a load condition. The driver turns the car around and heads back to the shop that fixed the leak. Upon arrival, the driver says the technician must have knocked something loose or broken something while replacing the hose. The technician doubts this but agrees to take another look. A cylinder power balance test would verify this, but it cannot be conducted because of the erratic idle. Instead, the spark plugs are removed and inspected. All look normal except for plug #2, which looks newer than the rest. A compression test is taken and cylinder #2 has very low dry and wet readings. Then a cylinder leakage test is con ducted. Cylinder #2 has excessive leakage and air can be felt leaving the exhaust pipe. These test results lead to the conclusion that the engine is running on one less cylinder because an exhaust valve is not sealing. The customer still feels the technician did something wrong but agrees to allow the technician to remove the cylinder head for further diagnosis. As soon as the head is off the block, the problem is identified. The exhaust valve seat for cylinder #2 came loose from the head and is preventing the valve from closing. The technician then explains that this undoubtedly happened when the engine overheated, due to the expansion of the metal. The customer is satisfied with the explanation and allows the technician to correct the problem. The family is then able to continue their trip. === Plastigage strip; Plastigage measure Fgr__82 OHC camshaft bearing clearances can be checked with Plastigage. Fgr__83 Coating the camshaft with lubricant. Dana Corporation === Service to the camshaft bearings can now be done. Full-round insert bearings are pressed into the bores in the cylinder head. Special tools are designed to make this job easier. Never use a standard camshaft bearing driver and hammer to install these bearings. The hammering can easily break or damage the bearing supports. After each bearing is fully seated in its bore, double-check the alignment of the bearing's oil hole with the oil hole in the head. Some OHCs don’t have bearing inserts. The bores in the aluminum casting serve as a bearing surface. These surfaces can be cleaned up and/or align bored if needed. Most late-model OHC engines use split bearings and bearing caps or have a separate housing for the camshaft. Working with split camshaft bearings is like working with crankshaft main bearings. This includes checking bearing clearances with Plastigage (82). Before installing the camshaft(s), install the timing gear. Make sure the alignment pin or key is aligned. Tighten the gear-to-shaft bolt to specifications. If the head has two camshafts, the manufacturer will give specific instructions for the alignment of the timing gears and chain. Normally one camshaft is installed with the chain wrapped around its gear. Once the camshaft shaft and gear are aligned with the timing reference marks, the bearing caps are installed and tightened in a specified order and torque. Then the other camshaft is installed by slipping the gear into the chain and then setting it into the bearing journals. After the timing marks are aligned on both shafts, the bearing caps for the second camshaft are tightened. When both camshafts are in place, the chain tensioner is installed. Prior to installation, wipe off each cam bearing with a lint-free cloth, then thoroughly coat the cam shaft lobes and bearing journals with the lube recommended by the manufacturer (83). Install the camshaft. Install new O-rings and gaskets, as required, while installing the camshaft. Some camshafts have an end thrust plate or shims to limit shaft end play. Make sure the originals are installed. After the camshaft is in place, check the end play and change the thickness of the shims as needed. If the assembly has bearing caps, place them in their correct positions and tighten them according to specifications. Once tightened, the shaft should be able to rotate smoothly. On some engines, turning the camshaft causes the valves to open and close. If this is the case, stand the cylinder head on its end to prevent the valves from hitting the bench while you turn the camshaft. If the cam does not turn, binding might be the cause. Binding is the result of a damaged bearing, a nick on the cam's journal, or a slight misalignment of the block journals. The problem should be identified and corrected. Gather the rocker arms, lash adjusters, pushrods, lifters, and other parts that transfer the motion of the camshaft to the valve stem. Coat all of these parts with clean engine oil. These parts should be installed according to the order given in the service manual. Some are installed before the camshaft is installed; others are installed after. Make sure all parts fit securely. Normally the rocker arm assemblies are installed by turning the cam until the lobe for that valve faces away from a valve stem. The rocker arms can be slipped into position by depressing the valve spring slightly. Make sure the rocker arm mounting bolts are tightened to specifications. Follow the same procedure for all of the valves. The manufacturer may recommend lubricating some of the bolts, so check the manual. Mount all manifolds and other parts that were on the head when it was removed from the block. Use new gaskets or the correct sealant where necessary. Adjust the valve lash (clearance) and mount the head onto the block. TERMSBroaching CC-ing Cross-flow ports Damper spring Guide plate Hemi-head Hemispherical combustion chamber Knurling Margin Milling Pentroof Phaser Poppet Porosity Positive seal Pumping loss Quench area Quenching Reaming Shrouding Siamese ports Stainless steel Stellite Throating Topping Tungsten inert gas (TIG) Turbulence Umbrella-type seal Valve face Valve guide Valve keeper Valve stem; Wedge-type combustion chamber SUMMARY¦ Pushrods are the connecting link between the rocker arm and the valve lifter. ¦ The rocker arm converts the upward movement of the valve lifter into a downward motion to open the valve. ¦ Aluminum cylinder heads are used on late-model engines because of their light weight. The thermal expansion characteristics of aluminum can lead to problems such as leaking and cracking. ¦ Every cylinder of a four-stroke engine contains at least one intake valve and one exhaust valve. ¦ Multivalve engines feature either three, four, or five valves per cylinder, which means better combustion and reduced misfire and detonation. These benefits are offset to some extent by a more complicated camshaft arrangement. ¦ The means of resurfacing the deck of a cylinder head include grinding, milling, belt surfacing, and broaching. ¦ The amount of stock removed from the cylinder head gasket surface must be limited. Excessive surfacing can create problems with the engine's compression ratio, not to mention piston/valve interference and misalignment. ¦ The two surfaces of a valve reconditioned by grinding are the face and the tip. Valves can be refaced on grinding or cutting machines. ¦ One of the fastest ways for restoring the inside diameter of a worn valve guide is knurling. Reaming increases the guide bore to take an oversize valve stem or restores the guide to its original dimension after knurling or installing inserts. ¦ Pressing out an old valve guide to install a new one can be difficult on some aluminum heads where the interference fit is considerable. ¦ To ensure proper seating of a valve, the seat must be the correct width, in the correct location on the valve face, and concentric with the guide. ¦ When grinding a valve seat, choosing the correct size pilot and stone is important. ¦ Valve stem seals are used to control the amount of oil between the valve stem and guide. Too much oil produces deposits, while insufficient lubrication leads to excessive wear. ¦ The valve spring closes the valve and maintains valve train contact during the opening and closing of the valve. To determine if a spring needs to be replaced, three tests are valuable: freestanding height, spring squareness, and open/close spring pressure. ¦ Two critical measurements that must be made before a cylinder head is reassembled are installed stem height and installed spring height. The first of these is determined by measuring the distance between the spring seat and the stem tip. The latter is measured by the spring seat to the underside of the retainer when it’s assembled with keepers and held in place. QUIZ1. What happens when spring tension is too low? 2. Define valve margin. 3. What usually causes warpage in an aluminum cylinder head? 4. What are the two ways pushrods can be checked for straightness? 5. What welding method is preferred for repairing aluminum heads? 6. Why do some technicians not consider knurling a long-term repair? 7. Which of the following is not true of knurling? a. It’s one of the fastest techniques for restoring the ID dimensions of a worn valve guide. b. It reduces the amount of work necessary to reseat the valve. c. It’s useful for restoring badly worn guides to their original condition. d. None of the above. 8. How does resurfacing the cylinder head of an OHC engine affect valve timing and what should be done when the head needs resurfacing? 9. To ensure proper seating of the valve, the valve seat must be . a. the correct width b. in the correct location on the valve face c. concentric with the guide d. all of the above 10. When grinding valve seats …. a. a pilot shaft is inserted into the valve guide b. a hard stone should be used on a hard seat c. a soft stone should be used on a soft seat d. all of the above 11. If the valve face and valve seat don’t contact each other evenly after reconditioning. a. regrind with the same stone b. regrind with stones of different angles c. discard the cylinder head d. none of the above 12. Many engines with VVT have a phaser mounted to the end of one or more camshafts. Which of the following statements is not true? a. A phaser is used to alter valve timing. b. Most phasers are controlled by oil pressure. c. Phasers are used to change valve lift. d. Phasers can be used to change valve overlap. 13. Multiple valve engines tend to be more efficient than two-valve-per-cylinder engines because they . a. allow for increased port areas b. have smaller valves c. provide less restriction to the airflow d. all of the above 14. Which type of surfacing machine uses underside rotary cutters? a. milling c. belt b. broaching d. grinding 15. Which of the following statements is not true? a. Normally the desired valve face-to-seat con tact area for intake valves is 1/16 inch. b. Normally the desired valve face-to-seat con tact area for exhaust valves is 1/16 inch. c. The average valve seat width is 0.060 inch and the average seat begins 0.030 inch from the valve margin. d. A properly reconditioned seat has the correct seat width and sealing position on the valve face. ASE QUESTIONS1. While discussing the reasons for resurfacing a cylinder head: Technician A says that it should be resurfaced to make it flat and very smooth so the head gasket can seal properly during engine assembly. Technician B says that it can be resurfaced to raise the compression. Who is correct? a. Technician A c. Both A and B b. Technician B d. Neither A nor B 2. Technician A says that bronze valve guides retain oil better than cast-iron ones. Technician B says that cast-iron valve guides are easier to machine than bronze guides. Who is correct? a. Technician A c. Both A and B b. Technician B d. Neither A nor B 3. Technician A says that positive valve stem seals fit tightly around the top of the stem. Technician B says that positive stem seals scrape oil off the valve as it moves up and down. Who is correct? a. Technician A c. Both A and B b. Technician B d. Neither A nor B 4. When fitting a freshly ground valve into a freshly ground seat: Technician A says that the seat should be ground more with the same stone if the margin is too small. Technician B says that the seat should be ground more with the same stone if the valve seat is too high. Who is correct? a. Technician A c. Both A and B b. Technician B d. Neither A nor B 5. Technician A says that low spring pressure may allow a valve to float. Technician B says that excessive spring tension may cause premature valve train wear. Who is correct? a. Technician A c. Both A and B b. Technician B d. Neither A nor B 6. Technician A says that it’s not necessary to mea sure valve stem height unless the valves have been replaced. Technician B says that the valve stem height can be adjusted with shims. Who is correct? a. Technician A c. Both A and B b. Technician B d. Neither A nor B 7. Technician A uses a 30-degree stone for topping a 45-degree valve seat. Technician B uses a 60 degree stone to lower a 45-degree valve seat. Who is correct? a. Technician A c. Both A and B b. Technician B d. Neither A nor B 8. While discussing the Valvetronic system: Technician A says that the system has no need for a throttle plate. Technician B says that the system alters the duration and the lift of the intake valves. Who is correct? a. Technician A c. Both A and B b. Technician B d. Neither A nor B 9. While checking a hydraulic phaser after the cam shaft has been removed from the engine: Technician A attempts to rotate the phaser's timing gear by hand and replaces it if it does move. Technician B applies air pressure to the advance and retard ports and replaces it if the gear does not move. Who is correct? a. Technician A c. Both A and B b. Technician B d. Neither A nor B 10. While servicing a warped aluminum cylinder head: Technician A checks the thickness of the head and the specifications to be sure that material can be safely removed from the surface. Technician B replaces the head if the warpage exceeds === Prev. | Next Home Article Index top of page |
