Cylinders, pistons, rings, valves and valve train
Cylinders, pistons, rings, valves and valve train
This section covers the components that seal combustion, transfer heat and control gas flow. The exam tests fits, clearances, ring function and valve-train adjustment.
Core rules and concepts
- Air-cooled cylinder barrels are finned and honed; the head carries valve seats, guides and spark-plug bosses. Deep cooling fins and tight baffling are needed because cooling depends on airflow.
- Pistons are cam-ground and may have offset pins to reduce slap. The crown shape controls mixture motion; the skirt guides the piston; the pin transfers force to the rod.
- Compression rings seal gas and conduct heat to the barrel; oil-control rings meter oil from the skirt. Ring end gap, side clearance and groove wear all affect sealing.
- Valve train: camshaft, lifters/tappets, pushrods, rocker arms, valves and springs. Valve clearance must be set at the specified point, usually with the cylinder on its compression stroke or per the manufacturer’s sequence.
- Valves must seat concentrically and seal; guides control stem wear; springs must hold the valve closed at high speed.
Worked example
A valve clearance is specified 0.010–0.014 in cold. A feeler gauge shows 0.006 in. The clearance is below minimum, so the valve may not fully seat and can burn. Adjust the rocker or tappet to bring the feeler gauge to a light drag within the range, then recheck after rotation.
Traps
Do not set clearance hot unless the manual says so; do not confuse ring end gap with side clearance; and do not diagnose low compression as valve trouble before checking rings and head sealing.