10 Important Tips for Diesel Generator Major Maintenance

A diesel generator is a critical investment for any business or homeowner, providing reliable backup power during electrical outages. To ensure that your diesel generator continues to operate smoothly, it’s essential to perform regular maintenance, including major maintenance tasks such as engine overhauls, turbocharger replacements, and alternator inspections.

In this blog post, we’ll provide important tips for diesel generator major maintenance to help you ensure optimal performance and longevity of your generator. We’ll discuss how to identify potential problems early and carry out major maintenance tasks efficiently and safely.

We’ll cover essential areas such as engine maintenance, electrical system maintenance, cooling system maintenance, and fuel system maintenance. We’ll also provide guidance on creating a comprehensive maintenance schedule that ensures your diesel generator is always ready to provide backup power when you need it.

Whether you’re a business owner or a homeowner, this post will provide you with valuable insights into the major maintenance tasks that should be carried out on diesel generators and how to keep your investment in top condition.

Diesel Generator Major Maintenance Important Tests

Major Overhauling of a Diesel Generator of a Ship is a critical and complex task
requiring specialized skill, knowledge, and experience. As the lifeline of
ships, generators require regular maintenance, including regular and major

As a marine engineer, you must overhaul generators during routine maintenance and
emergencies. A thorough knowledge of the Diesel Generator Overhauling process
is therefore essential for any level of marine engineering.

Various tests are performed on various tools and parts of the generator before and
during the overhaul. Below are 10 important tests performed during a ship
generator overhaul?

Bearing cap serration test 

Bearing cap serration testing is an essential part of a diesel generator overhaul. This test ensures that the serrations on the bearing housing hold two caps tightly with the connecting rod bolt. This test helps reduce damage from excessive vibration during operation.

The serration test includes a detailed visual inspection of all bearing caps and related parts. Engineers look for signs of wear and tear that can affect generator performance. If defective parts are identified, replace them immediately to extend service life. By performing this test regularly during general overhaul, the diesel generator can be operated safely and reliably for an extended period.

Hydraulic jack test

Hydraulic jacks are essential for various tasks when overhauling diesel generator sets. These tools will help you open the main engine cover cylinder head and bottom bolts and other components such as fuel pumps, valves, etc. Testing these hydraulic jacks is essential because these jobs require immense strength and precision. Before using. A hydraulic jack test ensures that all generator components are installed and functioning correctly before servicing. This way, you can ensure that your diesel generator will perform optimally throughout its life cycle.

Cylinder head Hydro

The cylinder head is a significant component of any diesel generator set. A cylinder head overhaul is essential to a major overhaul as it must be checked and tested regularly to ensure safety and efficiency. Ensure this requires the cylinder head to be overhauled and reused on board or even overhauled when new cylinder heads are delivered from shore.

The process of testing these cylinder heads is complicated but necessary. All components, including valves, valve springs, valve guides, gaskets, and other parts, should be thoroughly inspected before reuse or overhaul. This process is critical for the proper and maximum efficiency of diesel generators.

Conecting-Rod bolts test

The diesel generator under the cap contains the connecting rod bearing and is bolted to the bottom end. These bolts are exposed to high alternating loads during operation. It is, therefore, essential to test these bolts regularly to ensure they remain in good condition and can withstand these stresses.

One of the most important tests for this is the connecting rod pin test. This test examines and monitors the condition of the connecting rod bolts for signs of wear and fatigue. This test allows potential problems to be identified before they cause severe damage to the engine or other parts of the diesel generator set.

 Connecting Rod Bend Test

Connecting rods are under extreme pressure. When overhauling an alternator, check the straightness of the connecting rod by inserting a brass rod into an oil hole slightly smaller in diameter than the oil hole of the connecting rod. If the connecting rod is bent somewhat (not visible to the naked eye), the brass rod will not fit into the hole.

Fuel Valve Test

Overhauling a diesel generator set is a big job that requires the right tools and techniques. Fuel injectors play an essential role in this process as they supply fuel to the engine. Therefore, it is necessary to ensure that these fuel injectors are in proper working condition before reuse after overhaul.

Injector testing is a critical step in the diesel generator overhaul process. This test ensures that the injector’s internal components -Nozzle, needle, and valve are functioning correctly and efficiently. It also helps identify potential issues that can affect performance, such as: Clogged or worn components. The timely application of this test can extend the life of the diesel generator set.

 Starting air valve testing

The air start valve is overhauled and reused, as are the fuel injectors. To check its correct functioning, all starting air valves are tested for leaks with starting air before installation in the cylinder head.

Relief valve test

Cylinder head relief valves are pressure tested to ensure proper operation. It is an important part that prevents head explosion and combustion chamber damage due to overpressure. Pressure testing is performed on a bench-mounted test stand consisting of high-pressure air, a pressure control valve, and a calibrated pressure gauge. Screw the relief valve onto the accumulator flange and increase air pressure until the valve rises. The settings are made accordingly.

The current test

This test is an important test performed after completing the major overhaul procedure and before testing the generator with fuel. When the Engine is ready, the turning gear is activated, and the Engine rotates with the indicator tap open. The current is monitored continuously. A fluctuating or increasing current value indicates a blockage or problem with the rotating shaft.

Alarm and Safety Trips test

Generator alarms and safety trips are electrical systems with wiring and contacts. All prime mover alarms and shutdowns are tested to verify their correct operation, including lube shutdowns, high coolant temperature shutdowns, and Overspeed shutdowns.

Diesel Generator Overhauling Procedure

Diesel Generator overhauling is a process that is often needed when a diesel Generator has clocked the specified running hours as per the OEM recommendations. This process aims to get the engine running as best as possible so that the DG set can continue to run with the best performance. This post will cover the best procedures to help you perform the best overhauling of Diesel generators.

Diesel Generator Major Maintenance
10 Important Tips for Diesel Generator Major Maintenance 16

The DG Sets (Diesel generating sets) are the only source of Power on board a ship. It is not possible to operate any machinery without the Diesel generating sets.

As per the power requirement, various types of Diesel Generators are used.

For the proper maintenance of Diesel generators, Engineers should understand the the following.

  1.  knowledge of design, construction
  2. Working of the machine.
  3. Overhauling Procedure and the maintenance works:

There are standard procedures for the Diesel Generator Overhauling Procedure and maintenance of these machines, irrespective of their design and manufacturing.

Diesel Generator Overhauling ProcedureMajor Overhauling/De-carbonisation:

The process of dismantling, cleaning, and removal of carbon deposits from the following parts.

  • Piston
  • Cylinder Liner
  • Cylinder head

Major maintenance/ Decarbonisation is carried out on the following occasions.

  • Completion of running hours as per the planned maintenance program of OEM (Original equipment manufacturer).
  • Accident/Breakdown.
  • Before continuous survey in which process of change of running parts like and piston rings etc.

There are standard procedures for the maintenance of these machines, irrespective of their design and manufacturing.

Planning;- Planning of maintenance is to done keeping the following points in view.

  • MakeShift arrangements for the emergency in the event of Power failure
  • Special Tools and tackles to be beforehand in proper operating condition.
  • Measuring Instruments and tools: All measuring tools gauging and measuring devices should be in suitable working conditions with a reasonable calibration period.
  • Spare Parts: check and ensure the availability of spares needed to accomplish the maintenance.
  • To ensure the working of Power Packs if available on board.
  • Engineers/technicians to be involved in the care should know the procedure as per the Operating manual.
  • The maintenance team should have Knowledge of symbols and signs used to indicate the safety of machinery.

Real Life Incidences

  • Onboard, a ship inventory of the spares was not taken by the engineer in charge. While the maintenance on the way, during the inspection, it came to the light that the Con-rod bolts were elongated and needed replacement. Since the bolts were not available in the ship’s stock and the overhauling period was extended. Due to this delay company has to lose the revenue for not taking the container with perishable goods because of lack of power.
  • Before the hydraulic tool’s maintenance pump for loosening the Cylinder head, Main Bearing and Con-rod nuts were not building pressure. During the maintenance engineer –in charge found out that the nuts were not getting loosened up, and hence dismantling work couldn’t be started. Ultimately it caused the delay in maintenance.
  • Very often has been observed that due to the proper upkeep and maintenance of Measuring instruments/ calibration of the same in the stipulated period, many instruments fail to operate or lead to inaccurate measurements.

Preparation for Overhauling

Prior to maintenance following preparation is essential.

  • Quarantine
  • Tagging Out
  • Inspection of special tools supplied by Manufacturer.
  • Knowledge of functioning and procedure for operating the devices.
  • Referring to the correct tightening torque and hydraulic pressure values.
  • Complete understanding of parts assembly.
  • Preparation of safety regulations for safe working.

Quarantine: – Before the maintenance start, the machine is to be quarantined for care to avoid any operating fluids spilling and injury/accidents.

Both the Prime mover and alternator are to be quarantined.

Following steps are to be taken to isolate the Electrical and mechanical system.

  • Open the ACB ( Air Circuit Breaker) to cut off the link between the Alternator and the bus bar.
  • The power supply of Pre-Lube oil Pump.
  • Oil supply to Engine
  • Cooling water supply
  • Lube oil
  • Fuel oil supply
  • Air supply

A checklist of all the isolated areas is to be prepared, and the same is to be referred to before reopening the systems.

During the isolating process/opening procedure, a slow action with close observation is to be adhered to to ensure other equipment is not affected during this process.

Add on safety: While the maintenance work is in progress, additional tagging out is to be carried out to prevent any opening up of Valves or Electrical breakers resulting in an accident of serious nature.

The following special tools: supplied by the engine manufacturer are to check for their correct working, calibration, and leakage on the hydraulic device.

  • Injector and starting valve opening tool
  • Spanners
  • Cylinder head opening tool
  • Cylinder liner Removal Tools
  • Cylinder Liner Holding Tool
  • Cylinder head lifting tool
  • Piston Lifting tool
  • Bearing removal tool
  • Cylinder Head and con rod nuts loosening and tightening hydraulic tool
  • Valve grinding tool
  • Valve seat grinding machine/tool
  • Injector testing tool
  • Rope and ceilings with proper size D shackles.
  • Overhead cranes.
  • Engine Barring arrangements
  • Cylinder Liner Brushing tool
  • Piston Ring removal tool
  • Piston guide tool for inserting the piston.
  • All the pressure and temperature calibration instruments.
  • Electrical measuring instruments.

Points to be followed for assembly

  • Engineer in charge should know assembly of parts and doubt Instruction manual/Consult with Senior Engineer.
  • Store all the dismantled components in a separate box with the marking on significant parts. For each unit, a separate container to be used.
  • Mark all the bearings and con-rod bolts and wrap the threads with the cloth to avoid the threads’ damage.

Dismantling of parts

Overhauling of Generator is governed by the following factors.

  • No. of Running hours clocked.
  • Company’s policy.

Order of wear down of individual parts is as under

  1. Cylinder Head & Piston Rings
  2. Cylinder Liner and Connecting Rods
  3. Bearings.

Due to the forthcoming Voyage or defect/incident, all the Dismantle all the significant parts including Main and con-rod bearings.

In addition to the above Engine parts, undertake the overhauling of the following components as per the maintenance schedule laid down in the Instruction manufacturer’s manual.

Carry out the following jobs.

Cylinder Heads

  • All cylinder heads are de-carbonized.
  • Inspect cylinder heads.
  • Inspect Valve spindles.
  • Carry out the Inspection of Inlet, Exhaust valves & Inlet valve guides. Record the measurements. Replace the Corroded & worn-out valve guides with the new one.
  • Inspect valve seats and reject the excessively dented, pitted, and uneven working surfaces. Accepted valve seats are to be re-grounded with the machine.
  • Dismantle all the valve rotators for inspection. Observe for deep markings on the inclined surfaces of ball pockets.
  • Inspect all the Injector Sleeves and record the observations.
  • Check and adjust the operating pressure of safety valve.
  • Carry out the inspection of Indicator valves.
  • Inspect cylinder head cooling water spaces. 
  • Carry out the lapping of cylinder head seating face.
  • Carry out the pressure testing of all the cylinder heads. 
  • Carry out all Cylinder head valve seat grinding work fo. Old/new seats. 
  • All Water guide jackets are to be removed, clean, inspect and fit back with new O rings.
  • Tighten all the nuts for cylinder heads with prescribed torque as per the OEM instructional manual.

Cylinder Liner

  • Pull out all cylinder liners for inspection.
  • Inspection/ bore measurement and honing to be carried out at workshop as per OEM procedures for Inspection and Honing of Cylinder liner.
  • Enter the records of size into the forms.  
  • Carry out the lapping of the cylinder liner’s upper face.
  • Replace all the flame rings with an excessive worn-out working surface.

Piston and Piston Ring

  • Withdraw all pistons for inspection and overhauling.
  • De-carbonization of all the pistons (The pistons are to be cleaned both from inside and outside )
  •  Clean all piston and oil scraper ring grooves, inspect and measure for wear with groove measuring tool supplied by OEM. Record the measurements.
  • Replace all piston rings with the new one.
  •  Confirm the piston ring position is offsetting the ring joint 1800 to each other, and the spring joint of the scraper ring also fitted 1800 offset to ring joint while mounting the Pistons.
  • Check the Piston plug tightness on specified torque.

Central Tools 6434 Sleeve Height and Counter Bore Gauge

Counter Bore Gauge c4
10 Important Tips for Diesel Generator Major Maintenance 17

Quickly measures the cylinder sleeve height using pointed contact; Counter-bore depth using flat end contact; Top dead center; Piston-to-deck height; Valve protrusion; Easy readings around entire bore. This is a compact and very versatile gauge.

Connecting Rod

  • Inspect all Connecting rod big-end bearing shells and replace if found with the deep grooves and excessive wear down more than the accepted norms for the shell thickness and free spread.
  • Check the Connecting rod big end bore and record the measurements.
  • Check the Small end bearings.
  • Check the clearance between the wrist pin and bush.
  • All the piston pins are to be subjected to an ultrasonic test to check hairline cracks.

Connecting Rods Industrial Tools 

Connecting Rods Industrial Tools
10 Important Tips for Diesel Generator Major Maintenance 18

Thumb rule for oil clearance calculations

Thumb rule for oil clearance calculations

0.01905mm to 0.0254 mm. (Three quarter to 0.0254 of a millimeter) of clearance per mm of a shaft diameter is the reasonable starting point.

For example 2 mm shaft diameter would require 0.0381mm to 0.0508mm (0.01905×2=0381 and 0.0254×2=0.0508mm)

For the super performance, engines add 0.0127 to the maximum calculated value. The recommendations for 2 mm shaft will be 0.0508 mm + 0.0127=0.0635mm.

( For the purpose of maintaining hydrodynamic lubrication between the shaft and bearing the maximum ovality should not be greater than the maximum oil clearance. If exceeded then there will be bearing failure due to boundary lubrication.)

Camshaft and Camshaft Bearings

  • Inspect all gear wheels bolt and connection as per OEM instructions.
  • Replace all Camshaft bush if found worn-out
  • Check the condition of a camshaft.
  • Check the lubrication of the camshaft. Observe the flow of lube oil from all the bearings and spray pipe nozzles. The oil jet should be hitting at the gearwheel right at the meshing point.
  • Carry out the tightness of locking devices and fasteners.
  • Inspect all pushrods for straightness.
  • Adjust inlet valve and exhaust clearances as per the OEM recommendations.

Inlet Valve, Exhaust Valve, and fuel injection pumps operating mechanism

  • Inspect the Roller Guide for
  • Event of any marks.
  •  Deformation on the spherical stud.

  • Inspect the Surface of rollers for any mark or deformation.
  • Check for free play between the rollers and the bush and the pin.
  • Check the Valve Gear, springs, and Pushrods.
  • Inspect all the valve bridge springs, Rocker arm brackets, and pushrods.
  • Check the Roller Guide for fuel injection pumps.
  • Inspect all the roller guides for any marks.
  • Check the Surface of rollers for any defects and deformation.
  • Check for free play between rollers, bush, and pin.
  • Check the lubrication of all the operating gears.


  • Overhaul the Governor.                                                                      
  • Change the Governor oil.

Crankshaft and Main Bearings

  • Inspect all the crankpins: Polish all main &CR pins with glossy paper.
  • Inspect & replace thrust bearings if wear is more than the prescribed limit.
  • Re-tighten counterweights at the specified torque.
  • Dismantle all main directions & Polish all the Crank-pin.
  • Check the runout.
  • Carry out the Tightening of main and guide bearing caps at the prescribed torque.
  • Check and replace all main bearing if found with abnormal wear down.

Turbocharger & Charge Air Cooler checks during Diesel Generator Overhauling Procedure

  • Remove the TC cartridge.
  • Change nozzle ring.
  • Dismantle the cartridge and inspect the bush and labyrinth ring, and change if required.
  • Re-assemble the cartridge
  • Check the clearances as per the procedure.
  • Carry out the cleaning of Charge Air Cooler (CAC).
  • Conduct the pressure test of Charge Air Cooler.

Engine Frame and Bed Plate

  • Re-tighten of Holding down bolts.
  • Re-tighten the bolts between the engine and base frame.
  • Perform the safety cover’s functional test.
  • Check the AVM paid adjusting screw clearance and adjusts if required.

Rechargeable LED Flashlight

Rechargeable LED Flashlight
10 Important Tips for Diesel Generator Major Maintenance 19

Fuel oil System

  • Calibrate the Fuel injection valves.
  • Check all high-pressure fuel pipes & replace “O” rings.
  • Check FIP roller, and measure nominal height. 
  • Overhaul all the Fuel Injection Pumps. Replace barrel, plunger & ‘O’ ring kit.

Lube Oil System

  • Remove the engine-driven Lube oil Pump for inspection. Dismantle the complete pump and change the bushes. 
  • Apply the correct torque for tightening the self locking nut.
  • Remove old oil from the engine and charge new oil.
  • Overhaul Pre-lube oil pump.
  • Replace the Lube oil filter cartridge.
  • Clean and replace the kit of Glacier (bypass filter).
  • Clean the Lube oil PHE ( Plate Heat Exchanger) by circulation with the chemical.

 Cooling Water System

  • Dismount LT (Low Temperature) and HT (High Temperature) water pumps from the engine.
  • Carry out the complete overhauling of pumps.
  • Change the rotating sealing and both ball bearings of the HT Water Pump.
  • Change the rotating sealing and both ball bearings of the LT water Pump.
  • Tighten self-locking nuts of both the pumps at the specified torque.
  • Carry out the overhauling of the HT water thermostatic valve.
  • Clean the LT/HT PHE.

Mechanical Over-speed

  • Overhaul the Mechanical over speed device.


These points are very important for the Diesel Generator Overhauling Procedure.

  • Record Fuel pump index at Governor Index 0.
  • Check the compressed air system of the engine.
  • Check the fuel oil system and nozzle cooling system.
  • Carry out the functional tests of Safety shutdown switches and analog switches.
  • Calibrate all safety switches
  • If required, replace the Alternator bearing.
  • On completion of maintenance work, Start the D.G. Set and follow all moving parts’ sequence as per the standard procedure of Diesel Generator Overhauling Procedure.
  • Carry out the load trials of the D.G. Set as per the OEM protocol.
  • Record the set’s performance in stage load sequence from 25% to maximum available load for the machine prescribed in Manual.
  • Monitor the parameters and record them in Logbook.

Vibration Meter

Vibration Meter
10 Important Tips for Diesel Generator Major Maintenance 20

Data for pressure and Tolerances

Diesel Generator Overhauling Procedure-Data for pressure and tolerances
10 Important Tips for Diesel Generator Major Maintenance 21

All the procedures explained will help in for proper accomplishment of Diesel Generator Overhauling Procedure

Men’s Long Sleeve Zip-Front Coverall

Mens Long Sleeve Zip Front Coverall
10 Important Tips for Diesel Generator Major Maintenance 22

Timberland Men’s Pit Boss 6 Inch Steel Safety Shoes

Steel Safety Shoes
10 Important Tips for Diesel Generator Major Maintenance 23

Our top picks on diesel generator overhauling Books

Troubleshooting and Repairing Diesel Engines

Troubleshooting and Repairing Diesel Engines
10 Important Tips for Diesel Generator Major Maintenance 24

This hands-on guide shows, step by step, how to save time and money by learning to identify and fix virtually any diesel engine problem yourself. Thoroughly updated to cover the latest advances in diesel technology, Troubleshooting and Repairing Diesel Engines, Fifth Edition contains hundreds of drawings, schematics, and photographs to ensure success and avoid frustration. This fully revised classic resource equips you with all of the techniques you need to keep diesel engines running in top condition.

Diesel Generator Handbook

Troubleshooting and Repairing Diesel Engines 1
10 Important Tips for Diesel Generator Major Maintenance 25

Diesel Generator Handbook meets the need for an authoritative reference work covering the range of mechanical and electrical topics embodied in the practical design and application of diesel generating plant. It will be particularly welcomed in many parts of the developing world where the diesel generator is basic to the electricity supply system.

The discussion covers, in fifteen chapters, the prime mover, power ratings, synchronous generators, load assessment, control principles and systems, switchgear and controlgear, standby power, fuels and lubricating oilss, installation and commissioning, noise reduction, and plant operation and maintenance.

The book thus caters for all who are concerned with the selection, specification, testing, commissioning, operation and maintenance of diesel-based generator systems: not only the practising plant or services engineers, but also non-specialist engineers and users. The young technician or trainee engineer who is embarking on a career in the supply industry will find this handbook an invaluable investment.

L L J Mahon, FIEE, FBIM, after an engineering apprenticeship with BTH in Rugby, gained over 30 years’ experience in the design, development, manufacture, installation and commissioning of both stationary and highly specialised mobile diesel generator plant for a range of applications.

Introduction to Marine Engineering 

Introduction to Marine Engineering
10 Important Tips for Diesel Generator Major Maintenance 26

Introduction to Marine Engineering discusses machineries and related equipment in ships.
The book first gives an introduction to the kinds of ships and their machineries. The manuscript also discusses diesel engines. Gas exchange process; power measurement; compositions of two-stroke and four-stroke cycle diesel engines; starting air system; turning gear; and common marine diesel engines are described. The text also highlights steam turbines and boilers. Turbine construction, gearing, boiler arrangements, boiler operation, and coal-fired boilers are discussed. The book also looks at feed systems, pumps and pumping systems, fuel and lubricating oils and their treatment, air conditioning, ventilation, and refrigeration.

Blog Conclusion

We hope you enjoyed our blog post about the Diesel Generator Overhauling. In this post, we have covered Diesel Generators for Marine Engineers and help themlearn the right procedure. We hope that this article provided valuable information and is a helpful resource for anyone who is interested in learning more about Diesel Generator Overhauling. If you have any questions, please contact us at https://marinediesel.co.in/contact-us/. Thank you for reading, we would love to hear from you!

Hi, I’m Sham, Founder of marinediesel.co.in. A web site that provides authentic information regarding Marine Diesel Engines, and learn marine Engineering free with us

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