Engine Diagnosis and Service: Cylinder Head and Valvetrain

1. Engine Diagnosis and Service: Cylinder Head and Valvetrain Chapter 52

2. 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

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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. Checking Valve Springs • Springs are tested for: • Tension • Squareness • Height • Specifications are available in the service manual

12. Checking Valve Stems • Valves wear: oil consumption results • Measure the valve stem with a micrometer

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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