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Chapter 9

Chapter 9. Camshafts, Lifters, Timing Belts, and Chains. Objectives. Analyze theories of camshaft and related parts Describe operation of hydraulic lifters and lash adjusters. Objectives. Install single and dual overhead camshafts with correct valve timing

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Chapter 9

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  1. Chapter 9 Camshafts, Lifters, Timing Belts, and Chains

  2. Objectives • Analyze theories of camshaft and related parts • Describe operation of hydraulic lifters and lash adjusters

  3. Objectives • Install single and dual overhead camshafts with correct valve timing • Verify valve timing and be able to correctly position a camshaft in relation to TDC

  4. Camshaft • Located in block or on cylinder head • Cam in block design is typically called a pushrod engine • Made from hardened cast iron or steel • Cam has one lobe for each valve • Some engines have multiple cams

  5. Controlling Camshaft End Thrust • On some engines • Resistance from driving the oil pump causes cam to be forced backwards • Rearward movement of cam is called end thrust • Could affect ignition timing in a distributor type engine

  6. Controlling Camshaft End Thrust • Thrust plates and machined areas on the back of the timing chain gear • Used to control end thrust

  7. Controlling Camshaft End Thrust

  8. Valve Lash (OHC) • OHC engines • Valve clearance adjusted hydraulically while others have provisions for adjustment

  9. Valve Lash (OHC) • Some OHC engines have cam lobes that act directly on the valves • On these type engines, lash is adjusted by adding or removing shims

  10. Valve Lash (OHC)

  11. Valve Lifters • Mechanical or hydraulic • Mechanical are also referred to as solid lifters or tappets • Two types • Flat tappet • Roller

  12. Hydraulic Lifter Operation • When clearance occurs in valvetrain • Spring between plunger and lifter body causes lifter to expand • Oil fills cavity created under plunger • Excessive oil in lifter leaks down between plunger and lifter body

  13. Hydraulic Lifter Failure • Causes of hydraulic lifter failure • Dirt lodged in check valve • Too much wear between plunger and lifter body causing excessive leak down • Insufficient oil pressure • Varnish between plunger and body

  14. Cam Lobe Shape • Base circle • Opening and closing ramp • Lift • Duration

  15. Cam Lobe Shape

  16. Valve Timing • Intake valve opens before TDC • Allows cylinder to fill with air-fuel mixture • Intake valve remains open after BDC • Exhaust valve opens considerably before end of power stroke • Exhaust valve remains open past TDC, partly into intake stroke

  17. Valve Timing

  18. Roller Cam and Lifters • Accept higher rate of movement without wear to lifter or cam lobe • Valvetrain friction cut in half • Increase in power and fuel economy • Roller lifters use brackets to keep them from spinning

  19. Roller Cam and Lifters

  20. Freewheeling and Interference Engines • Interference engines experience piston-to-valve contact if timing belt or chain breaks • Damage may occur to pushrods, pistons, and valves • Freewheeling engines will not damage valvetrain in the event of a timing belt-chain failure

  21. Timing Chains • Two types of chains: • Roller chain • Silent chain • Roller chains have single or double rows • Timing chains have drive side and a side where slack accumulates

  22. Timing Chains

  23. Checking Timing Chain for Stretch • Observe dwell section of ignition waveform on a scope • While watching distributor rotor, turn vibration damper back and forth Movement should be less than 5º • Measure chain slack with timing cover removed

  24. Timing Chain Tensioners • Used on OHC engines with long timing chains • Should be replaced when chain is replaced

  25. Chain Drive Timing Marks • Some manufactures specify a certain number of chain links between marks • Some have colored links that must be aligned with marks on sprockets

  26. Chain Drive Timing Marks • Some have a mark on cam sprocket that lines up with a mark on cylinder head • Some OHC engine have a drive chain and one or two auxiliary chains

  27. Timing Belts • Drive camshaft on OHC engines • Compared to timing chains • Quieter • Require no lubrication • More efficient • Very strong and durable • Sometimes drives coolant pump and oil pump

  28. Timing Belt Materials • Neoprene • High-temperature neoprene • Highly saturated nitrile

  29. Timing Belt Inspection • Look for fraying, cracks at base of teeth or loose fibers • Rotate engine by hand to inspect entire belt

  30. Timing Belt Inspection • Wear on one side of tooth indicates misalignment problem • Check for oil leaks which could damage belt

  31. Timing Belt Inspection

  32. Timing Belt Replacement • Replacements will be done on a preventative maintenance basis, or when they fail. • Use a non-petroleum based solvent to clean sprockets • Inspect sprockets for wear • Inspect or replace front camshaft oil seal • Cogs can strip off the belt, resulting in serious engine damage

  33. Timing Belt Replacement • Adjust belt tension according to manufacturer specification • Place sticker on valve cover or door post listing mileage when belt was replaced

  34. Automatic Belt Tensioners • Some are spring loaded and must be unloaded before belt can be installed • Some tensioners have a specific clearance measurement that is adjustable and measured with a feeler gauge

  35. Automatic Belt Tensioners • Some require belt tension gauge to set proper timing tension

  36. Automatic Belt Tensioners

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