ASE · Engine Repair (A1) · 8 min read
Cylinder Heads and Valve Trains
- Identify cylinder head inspection and machining requirements, including crack detection, warpage measurement, and resurfacing limits.
- Evaluate valve train components such as valves, guides, springs, lifters, and camshafts using proper precision measurement procedures.
- Apply diagnostic and service procedures for timing drives, variable valve systems, and cylinder head reassembly following manufacturer service information.
Cylinder Head Disassembly, Inspection, and Surface Preparation
Proper cylinder head service begins with careful disassembly procedures. An aluminum cylinder head must be allowed to cool completely before removal as directed by the manufacturer. Removing an aluminum head while it is still hot significantly increases the risk of distortion and warpage. Loosen cylinder head bolts in the sequence and stages specified by the manufacturer; when no removal sequence is specified, reverse tightening order in stages is a common practice. This step-by-step relief of clamping force helps prevent cylinder head warpage during teardown.
After cleaning, inspect the casting thoroughly for cracks and structural flaws before spending time on precision measurements. Select a crack-detection method based on the material, suspected defect, component geometry, and manufacturer’s service information; pressure testing may be needed to find leaks that a surface inspection misses. Magnetic particle inspection is used to detect surface cracks in ferrous, cast iron engine components. Because aluminum is nonferrous, dye penetrant inspection is used to detect surface-breaking cracks in aluminum cylinder heads and other nonporous parts. In addition, pressure-testing a cylinder head with its coolant passages sealed and the casting submerged in water can reveal internal and external leaks as escaping air bubbles. Technicians must follow the manufacturer's specific test procedure and pressure specification.
Check cylinder head flatness using a precision straightedge and a feeler gauge across multiple specified directions along the deck face. If warpage exceeds the limit, resurfacing may be necessary, but this process must be approached with caution. Excessive resurfacing can reduce chamber volume and raise compression ratio on some engine designs, and may affect valve-to-piston clearance; check the manufacturer’s minimum head height, surface-finish requirements, and related clearances before machining. Furthermore, modern multi-layer steel (MLS) head gaskets typically require a significantly smoother sealing surface than traditional composite gaskets. Always consult the gasket and engine manufacturer's surface-finish specifications before reconditioning the head deck.
| Inspection Method | Component Material Type | Operating Principle |
|---|---|---|
| Magnetic Particle Inspection | Ferrous (Cast Iron) | Uses magnetic fields and iron powder to reveal surface fractures. |
| Dye Penetrant Inspection | Nonferrous (Aluminum) and nonporous components | Uses liquid penetrant and developer to highlight surface-breaking cracks. |
| Submerged Pressure Testing | Cast iron or aluminum castings | Pressurizes sealed coolant passages to reveal leaks via air bubbles. |
Common mistakes
- Assuming Plastigage can identify an out-of-round camshaft journal or bearing; it can only determine assembled oil clearance, whereas micrometers and bore gauges are required to detect out-of-round.
- Applying spray or paste sealant to a multi-layer steel (MLS) head gasket when the manufacturer specifies a clean, dry installation.
- Failing to clean oil or coolant from blind cylinder head bolt holes, resulting in hydraulic lock that causes false torque readings or cracked castings.
- Assuming all engines time at cylinder No. 1 top dead center (TDC) rather than referencing the manufacturer's designated alignment marks and positions.
Key points
- Allow aluminum heads to cool as directed, and loosen head bolts in the manufacturer's specified sequence and stages; if none is specified, loosening in reverse tightening order in stages is a common practice.
- Magnetic particle inspection is suitable for ferrous heads, while dye penetrant can reveal surface-breaking cracks in aluminum; select additional tests, including pressure testing where appropriate, using the manufacturer’s service information, and check flatness as specified.
- Excessive head resurfacing can reduce combustion chamber volume on some engine designs and can affect other critical dimensions; over-refacing a valve can thin its margin and promote burning or pre-ignition.
- Measure valve guide clearance, seat runout, spring squareness, and installed spring height to ensure proper sealing and valvetrain geometry.
- Hydraulic lifters normally maintain near-zero lash and may tick if collapsed; excessively tight lash on a mechanical valve train can hold a valve open, potentially causing misfires and valve burning.
- Oil-actuated VVT phasers depend on proper oil condition and pressure; sludge or low pressure can impair their operation and may set timing-related trouble codes.
- Torque-to-yield head bolts stretch into their plastic range and generally require replacement; tighten them using the manufacturer's specified sequence, torque values, and angle steps where required.
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