Service and Repair Engine Components (Cylinder Head, Piston, Crankshaft, Valve Train)
Service and Repair Engine Components
This domain tests measurement-driven overhaul of the cylinder head, piston, crankshaft, and valve train. Every decision should rest on a measured value compared with the service limit.
Core rules
- Cylinder head: decarbonize the chamber, then check warpage with a straightedge and feeler gauge in several directions; around 0.05 mm is a common limit. Lap valves with paste until a uniform grey band shows, then check seat width and valve stem-to-guide clearance.
- Piston and rings: square the ring in the bore with the piston crown and measure end gap with a feeler gauge. Check ring side clearance in the groove, piston-to-bore clearance, and piston pin fit. Install rings with the marked side up and stagger the end gaps about 120° apart.
- Crankshaft: check runout between centres or on V-blocks with a dial indicator (limits are often 0.03–0.05 mm); check connecting-rod big-end side clearance and bearing play.
- Valve train: measure cam lobe height, rocker arm pad wear, valve spring free length, and cam chain stretch. Align timing marks exactly on reassembly.
- Clean every part, oil moving surfaces on assembly, fit new gaskets and seals, and torque in sequence.
Worked example
A 150 cc engine smokes and has low compression. The bore measures 57.45 mm against a 57.50 mm limit, so the cylinder is serviceable. The top ring end gap measures 0.65 mm against a 0.50 mm limit. The technician files a new ring's gap to 0.25 mm, checking it in the bore after every few strokes. Reasoning: the ring gap, not the bore, caused the failure; gap must be measured in the actual bore, and filing is gradual because material cannot be added back.
Traps
Filing rings without checking them in the bore; installing rings upside down; misaligning cam timing by one tooth; reusing a head gasket; torqueing head bolts in a circle instead of crisscross stages.