Engine Diagnosis and Service: Cylinder Head and Valvetrain

© © 2012 Delmar, Cengage Learning

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

• Understand machine shop repair processes for cylinder heads


Objectives (cont’d.)• Reassemble a cylinder head• Understand camshaft and cam drive service

procedures


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


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


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


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


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


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


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


Checking Valve Springs• Springs are

tested for:– Tension

– Squareness

– Height

• Specifications are available in the service manual


Checking Valve Stems• Valves wear: oil consumption results

– Measure the valve stem with a micrometer


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


Documents

Engine Diagnosis and Service: Cylinder Head and Valvetrain