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 Leaking head gasket 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: Ways to detect cracks 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 Cracks in iron heads 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