Camshafts, Lifters, Timing Belts, and Chains 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 Verify valve timing and be able to correctly position a camshaft in relation to TDC

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

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

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

Controlling Camshaft End Thrust

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

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

Valve Lash (OHC)

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

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

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

Cam Lobe Shape Base circle Opening and closing ramp Lift Duration

Cam Lobe Shape

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

Valve Timing

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

Roller Cam and Lifters

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

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

Timing Chains

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

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

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

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

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

Timing Belt Materials Neoprene High-temperature neoprene Highly saturated nitrile

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

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

Timing Belt Inspection

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

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

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

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

Automatic Belt Tensioners