Engine Servicing, Tune-up and Overhaul
Servicing versus overhaul
Service = scheduled adjustments and replacements (oil, filters, plugs, clearances, chain, fluids). Tune-up = restoring correct mixture, ignition timing and idle to specification. Overhaul = dismantling, measuring wear, replacing out-of-limit parts, and reassembling to specification.
The tune-up sequence
Work in a logical order so earlier steps are not invalidated: valve clearance first (engine cold), then compression check, then ignition (plug condition and gap, timing), then fuel/air (air filter, carburettor balance and idle mixture), then idle speed. A compression test should be done with the throttle wide open and a fully charged battery; a wet test (a little oil in the cylinder) that raises compression points to rings, while no change points to valves or a head gasket.
Overhaul essentials
- Measure before condemning: bore taper and ovality, piston-to-bore clearance, ring end gap in the bore, crankpin and big-end clearance with Plastigauge, cam lobe height, valve stem and guide clearance, valve seat width and concentricity.
- Ring end gap is checked by squaring the ring in the bore with the piston crown.
- Torque sequences: usually centre-outward in stages; stretch bolts (many cylinder heads) are often torque-to-yield and must be replaced.
- Break-in: limited load and rpm, early oil change to remove assembly debris.
Traps
Testing compression with a closed throttle; setting valve clearance on a hot engine when the spec is cold; reusing a head gasket or stretch bolts; mixing up taper and ovality; forgetting to prime the oil system before first start.
Rule of thumb
Adjust, measure, compare to spec, then replace. If a measurement is within the service limit, adjustment is enough; if it is beyond the wear limit, replacement is mandatory.