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Marine Engine Major Overhaul Schedule for Offshore 4-Stroke Main Engines

A marine engine major overhaul (MOH) is a planned maintenance activity carried out to restore engine reliability, performance, and operational safety. Offshore vessels operating with high- and medium-speed 4-stroke main engines require periodic major overhauls to inspect critical components, replace wear parts, and verify engine condition.
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The following marine engine MOH schedule outlines the scope of work typically performed during a main engine major overhaul, covering inspections, measurements, component replacements, overhauls, testing, and commissioning activities.
Table of Contents

Main Engine Major Overhaul Schedule

System / Component Scope of Work
Engine Performance Record engine performance before overhaul and record crankshaft deflection.
Filters Inspection and cleaning of filters. Overhaul self-cleaning filter, especially the self-cleaning device (if equipped).
Lube Oil System Lube oil replacement and sump cleaning.
Engine Foundation Check tightening of engine fixing bolts.
Starting Air Distributor Replace gasket.
Starting Air Valve Overhaul starting air valve.
Piston Replace pistons. Measure diameter of piston pin. Replace piston rings.
Turbocharger Overhaul turbocharger, measure clearances, and replace bearings.
Air Cooler Clean air side and cooling water side.
Lube Oil & Fuel Oil Filters Inspection and servicing as required.
Cooling Water Pump Replace mechanical seal and oil seal. Replace ball bearings. Check pinion.
Cooling Water & Lube Oil Coolers Overhaul and clean inside of tubes. Replace zincs, O-rings, and gaskets. Pressure test coolers.
Fuel Injection Union Check seat and inner surface. Replace O-rings.
Crankshaft Measure crankshaft deflection. Conduct visual inspection of crank journals. Measure crank pin diameter. Replace main bearings and thrust bearings.
Camshaft Carry out visual inspection of camshaft and take readings.
Crankcase Check for water condensation and corrosion. Inspect inside and outside condition of crankcase and paint condition.
Gear Train Inspect timing gears. Overhaul all gears. Conduct dye penetration testing on gear cranks. Inspect and replace bearings.
Connecting Rod Calibrate piston pin bush and replace if required. Replace crank pin bolts and connecting rod bearings. Check and record connecting rod ovality.
Cylinder Liner Remove and clean cylinder liner. Clean and replace O-rings. Conduct dye penetration test on upper flange for cracks.
Cylinder Head Replace rocker arm bushings. Replace valve springs. Replace cotters, intake and exhaust valve guides. Replace valve rotators. Measure rocker arm shaft clearance. Conduct hydro pressure test and dye penetration test on cylinder head face.
Exhaust Manifold Threads Clean all threads and replace bolts where required. Replace all gaskets and O-rings.
Valve Gear Overhaul tappets and shafts.
Fuel Injection Pump Overhaul fuel injection pump. Inspect plunger and barrel and replace as required.
Governor Driving Device Check gears and measure backlash. Replace oil seals and ball bearings.
Fuel Feed Pump Replace coupling, oil seals, gaskets, and ball bearings.
Lubricating Oil Pump Replace bushings and O-rings.
Exhaust Manifold Inspect expansion joints for leakage.
Fuel Injector Replace fuel injection nozzle, spring, push rod, and pin. Conduct injection pressure test and spray pattern check. Replace gasket and O-rings.
Main Layshaft / Governor Layshaft Inspect layshaft. Replace bearings and bushings.
Safety Checks & Sea Trial Check engine safeties under supervision. Set tappets and conduct engine sea trial.

External Services Required (Shipyard Scope)

External Service Scope
Turbocharger Overhaul Complete turbocharger overhaul.
Cooler Cleaning Cleaning of all coolers and associated cooling equipment.
Governor Overhaul Complete governor overhaul.
Liner Honing Cylinder liner honing.
A well-planned marine engine overhaul schedule helps ensure that all critical engine components are inspected, measured, overhauled, and replaced where necessary during a maintenance period. Following a structured main engine MOH schedule allows offshore vessel operators to systematically complete overhaul activities and verify engine condition through testing and sea trials before returning the engine to service.

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