engine diagnosis and service: cylinder head and valvetrain
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Engine Diagnosis and Service: Cylinder Head and Valvetrain. Chapter 52. Objectives. Disassemble a cylinder head in the correct manner Clean and inspect a cylinder head for cracks and warpage Diagnose cylinder head and valve train wear problems and determine the correct repair procedure - PowerPoint PPT PresentationTRANSCRIPT
© © 2012 Delmar, Cengage Learning
Engine Diagnosis and Service: Cylinder Head and
Valvetrain
Chapter 52
© © 2012 Delmar, Cengage Learning
Objectives• Disassemble a cylinder head in the correct
manner• Clean and inspect a cylinder head for cracks
and warpage• Diagnose cylinder head and valve train wear
problems and determine the correct repair procedure
• Understand machine shop repair processes for cylinder heads
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Objectives (cont’d.)• Reassemble a cylinder head• Understand camshaft and cam drive service
procedures
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Introduction• Valve job
– Cylinder head is removed for valve refinishing
• Leaking head gasket – Removed for resurfacing and gasket
replacement
• Timing chain or timing belt service– Important maintenance procedures on today’s
long-life engines
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Head Disassembly and Carbon Removal
• Cylinder heads: easier to work on if clean– OHC heads with removable cam caps: verify
caps are correctly numbered
• Removing valve springs: wear face protection – Keep valves in order
– Measure and record valve stem and spring height
• Carbon removal– Most OHC heads have oil galleries
– Carbon can be removed from necks of valves
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Cylinder Head Inspection• Cylinder heads sometimes warp
– Warped heads are resurfaced
• Clean head before checking for flatness– Rock the straightedge so one edge of it rests
against the opposite side of the head
– A round, straight bar is also available for checking straightness
• Warpage– Cast iron head warpage
– Aluminium head warpage
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Resurfacing by Grinding, Cutting, or Sanding
• Resurfacing methods– Fly-cutting
– Grinding the head
• Correct surface finish is very important– Multilayered steel (MLS) gaskets
• Require a very smooth surface finish
• Head resurfacing – Can increase compression
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Straightening Cylinder Heads• Warped aluminum OHC heads
– Commonly straightened
– Several methods for straightening cylinder heads• Best: heating oven
– Straighten the head prior to surfacing• Combustion chamber volumes will remain equal
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Crack Inspection• Cracks are sometimes found:
– In combustion chambers
– Between adjacent combustion chambers
– On the valve spring side of the head
• Ways to detect cracks– Magnetic crack inspection
– Dye penetrant
– Pressure testing
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Crack Repair• Cracks are sometimes repairable
– Only practical if the cost of a bare head is more than twice the cost of the crack repair
• Cracks in iron heads – Repaired with tapered, threaded plugs
• Welding heads– Common method of repairing aluminum head
cracks
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Checking Valve Springs• Springs are
tested for:– Tension
– Squareness
– Height
• Specifications are available in the service manual
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Checking Valve Stems• Valves wear: oil consumption results
– Measure the valve stem with a micrometer
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Valve Guide Service• Valve guides are checked for wear
– Wear in a bellmouth fashion• Can result in oil consumption
• Valve seat has worn and is wider than usual– Look for a worn valve guide as the cause
• Checking valve stem-to-guide clearance– Split ball gauge and micrometer
– Dial indicator
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Guide Repair• Guides are repaired in several ways
– Worn integral guide bored out to accept a pressed-fit insert guide
– Worn insert guide pressed out and replaced with a new one
– Knurling
– Thinwall insert
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Grinding Valves• Valves are refinished on face angle using a
valve grinder– Stem tip is reground flat
– Grinding wheel is dressed with an industrial diamond
– Some machinists grind an interference angle
– Very little metal is removed from surface of the valve face
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Grinding Valve Seats• Valve guides must be refinished before
refinishing valve seats– Valve seats are refinished with a grinding stone
or a seat cutter• 45-degree seat angle that mates with valve face is
machined until seat area is clean and free of pits• 60-degree angle in the bottom of the seat (i.e.,
throat angle) is cut very lightly
– Head must be thoroughly cleaned of all grit before beginning assembly
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Checking Valve Stem Installed Height
• Seat and valve are reground– Stem moves further into the cylinder head
• Results in increased valve stem tip height and valve spring installed height
• After grinding the valve and seat– Check installed height
– Shims may be installed under the springs when a head is reassembled
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Solvent Testing the Valve and Seat
• After the valve and seat have been ground:– Install the spark plugs in their holes
– Turn head over so combustion chamber faces up
– Place head on head stands and put it on a shelf in solvent tank
– Install valves in the ports
– Fill the combustion chambers with solvent and check for leaks
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Reassembling the Head and Valve Guide Seal Installation
• Clean head before reassembly– Thoroughly clean the guides
– Lubricate all valve stems
• Valve guide seal installation– Install guide seals before installing springs on all
but O-ring seals
– Check instructions in gasket set regarding placement of seals
– Lubricate seals before installing them
– Positive seals: often supplied with a plastic sheath
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Install the Valve and Spring Assembly
• Some springs have coils more closely spaced at one end than at the other– End more tightly coiled IS positioned against the
cylinder head
• Compress spring just enough to install keepers– Inspect each keeper for wear
– Use grease to help hold keepers in place
• Newer engines may use bee hive-shaped springs– One end of coils smaller in diameter
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Pushrod Engine Rocker Arm Service
• Stud-mounted rocker arms – Not serviceable
– Replaced when worn
• Cast rocker arms that are shaft-mounted – Can be reground
• Thoroughly lubricate rocker arms – Before installing
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Inspect Pushrods and OHC Camshaft
• Inspect pushrod ends and surface of socket where it pivots on rocker arm– Look for pitting or other unusual wear
– Roll pushrods on a bench to see if they are bent
• Overhead camshafts often have oil galleries and holes drilled in cam lobes for direct lubrication– Small oil holes are prone to plugging
– Check that oil holes are clear before installing
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Reassembling OHC Heads• Important steps
– Reinstall camshaft in the head
– Check to see camshaft cap alignment bushings are installed and positioned correctly
– Bucket-type OHC heads• Lubricate buckets and install them in the head
prior to installing the cam
– Adjust the valve clearance before installing the head on the engine
– Valve lash must be enough to allow heat to dissipate from valve to valve seat
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Camshaft Service• Camshaft is inspected for wear
– Comparison measurement is made by measuring each lobe
• Visual check of cam lobes for wear is standard
• Lobes on roller cams are polished to fine matte finish– During engine break-in, the lifter burnishes the
lobe to a smooth mirror finish
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Lifter Service and Cam and Lifter Break-in
• Hydraulic lifters fail for several reasons– Dirt lodged in the check valve
– Oil pressure problem
– Varnish accumulates between plunger and body
• Hydraulic lifters are not rebuilt– Relatively inexpensive
• Worn mechanical lifters can be reground• Lubrication and break-in are critical
– Cam that survives the first half hour of use without wear should last the life of the vehicle
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Timing Chain and Belt Service• Camshafts on modern engines
– Driven by belt and chain• Some older engines used two gears between the
crankshaft and camshaft
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Timing Belt Service• Increased timing belt fabric wear causes
– Poor alignment
– Incorrect tension
– Worn sprockets
• Inspect condition– Twist belt gently
• Belt life – Affected by contact with foreign materials
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Timing Belt Replacement• Follow manufacturer's recommendations for belt
replacement interval– Most American cars are free-wheeling
– Most replacements take three- to four-hours
– Install new belt and adjust belt tension until snug• Do not adjust timing belt tension on a hot engine
– Affix a sticker to fender that tells the mileage when timing belt service was done
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Timing Chain Service• Excessive chain stretch
– Checked in different ways• Long chains like those used on OHC engines
always use chain tensioners
• Ways to time the cam to the crank– Some timing sprockets are properly timed when
the marks face each other
– Some require a certain number of chain links between marks
– Some have colored links that must be aligned
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