21-22 the Best Tutorial on| Connecting Rod of Marine Diesel Engine

Connecting Rod of Marine Engine

A connecting rod is a crucial component in the functioning of a marine engine. It is responsible for transmitting the power generated by the pistons to the crankshaft, allowing the engine to run smoothly and efficiently. The connecting rod in a marine engine has to endure extreme pressure, temperature, and wear and tear due to the demanding conditions of a marine environment.

In this blog post, we will delve into the design, construction, and function of the connecting rod in marine engines, and why it is an essential part of the engine’s anatomy. We will also explore the various materials used to make connecting rods, their advantages and disadvantages, and how they contribute to the overall performance of a marine engine.

Construction of Connecting rod

In modern marine engines, high-strength materials such as titanium and aluminum alloys are also used to manufacture connecting rods. These materials offer the benefit of being lightweight, thereby reducing the overall weight of the engine, which in turn leads to improved fuel efficiency. Furthermore, they also have excellent corrosion resistance, making them ideal for use in marine environments where saltwater can cause damage to metal components.

The process of forging the connecting rod involves heating the metal to a high temperature and then using a press or hammer to shape it into its desired form. This process strengthens the metal and makes it more durable, which is critical in marine engines where high stress and vibration levels can put significant strain on the connecting rod.

In conclusion, the construction of the connecting rod in a marine engine is a complex process that involves precision engineering and the use of high-quality materials. It is essential to choose the right material, manufacturing process, and design to ensure that the connecting rod is able to withstand the extreme conditions of a marine environment and function optimally, helping to keep the engine running smoothly and efficiently.

What is Connecting Rod of Marine Engine ?

Connecting Rod of Marine Engine
Image credit:- https://grabcad.com/

The connecting rod of a marine engine is a key component that helps transfer the linear motion of the pistons to the rotary motion of the crankshaft. It acts as a link between the piston and the crankshaft and plays a crucial role in the operation of the engine.

The connecting rod is typically shaped like a long metal rod, with a small end that is attached to the piston and a large end that is attached to the crankshaft. As the piston moves up and down within the cylinder, it pushes the connecting rod which in turn rotates the crankshaft, producing the mechanical power that drives the marine vessel.

The connecting rod in a marine engine has to withstand extreme conditions, including high temperatures, pressure, and vibrations. It is therefore important that the connecting rod is constructed from high-quality, durable materials, such as steel alloys, titanium, or aluminum, and manufactured to precise specifications to ensure that it operates efficiently and effectively.

Types of Lubrication

Splash lubrication

Splash lubrication is a simple and cost-effective method of lubrication that is suitable for small marine engines. However, it is important to maintain the proper oil level in the sump to ensure that the engine components receive adequate lubrication. If the oil level is too low, there is a risk of metal-to-metal contact between the engine components, which can result in damage to the engine and reduced efficiency.

Overall, splash lubrication is an important aspect of maintaining the health of a marine engine, and regular monitoring and maintenance of the oil level and oil quality is essential for ensuring the longevity and performance of the engine.

Pressure lubrication

Pressure lubrication is a more advanced and sophisticated form of lubrication compared to splash lubrication. Instead of relying on the movement of the connecting rod and crankshaft to distribute oil, pressure lubrication uses a pump to pressurize the oil and force it into the engine components.

In a pressure lubrication system, the oil pump is driven by the engine and circulates oil from the oil sump through a series of passages and channels to the various components of the engine, including the connecting rod bearings. The high pressure of the oil helps to reduce friction and wear between the moving parts, leading to improved performance and reduced engine wear.

Pressure lubrication is typically used in larger marine engines that are subject to higher stress levels and greater demands on the lubrication system. It is also preferred in high-performance engines where reduced friction and improved efficiency are critical.

One of the key advantages of pressure lubrication is that it provides a more consistent and reliable source of lubrication, regardless of the speed or load of the engine. This helps to ensure that the engine components are always properly lubricated and protected from wear and damage.

In conclusion, pressure lubrication is a crucial aspect of the functioning of a marine engine and is essential for ensuring that the engine operates efficiently and effectively. Regular maintenance of the oil pump and the oil system is important to ensure that the engine is properly lubricated at all times.

Read the related blog post:-The Best 21-22 tutorial on |diesel engine piston

Types of a connecting Rod of Marine Engine in Four Stroke engine

Marine Head type Connecting rods shape

Connecting Rod of Marine Engine-Marine head
Connecting Rod of Marine Engine-Marine head

The Connecting rod illustrated in above image is the marine head type of connecting rod

The joint of the connecting rod is above the bearing.

Bearing shells are similar to main crankshaft bearings manufactured with a thin shell having AlSn running layer.

Bearing Blocks and Bearing caps are joined together by waisted bolts.

Advantages of Marine head Connecting Rod of Marine Engine

  1. Can pull the piston without dismantling the big-end bearing.
  2. Ensures Operational safety since there is no change in position and no new adaption. 
  3. Marine head bearing reduces the height dimension for the piston dismantling. 


Connecting rod and bearing body is of CrMo steel. The modern material is drop forged with high tensile steel.

Carbon Manganese Molybdenum alloy steel.

Parts of Connecting Rod of Marine Engine

1.   Big End

2. Small End

3. Bearing Insert

4. Bearing Shell

5. Bearing Bush

6. Beam

7. Shank

8. Nut and Bolt

Big End; – The side of connecting rod connected with the crankshaft pin is Big-end

Small End;- The side of connecting rod connected to the piston via wrist pin /Gudgeon pin is a small End.

Bearing Insert;- Bearing insert is inserted in the big-end attached to the cap. 

Shank; – Shank connects the big End and small End of the con- rod.

Bearing and bush: – Crankshaft will have bush and bearing at both ends—the bush inserts in the small End of the con- rod. 

The piston pin connects the small End and piston.

Big End connects the con- rod with a crankshaft pin. The big End is in two halves for easy mounting over the big End bearing shell. Nut and bolts join both halves.

Bush bearing is of bronze or gunmetal.  

The big End bearing is in two halves. The big End Bering shell is of thin-shell tri-metal in modern engines.

Marine Head Con rod Maintenance

Inspection of Faces (Visual)

  1. Inspect the con- rod faces for Damages, visible wearing marks, pitting, or even cracks because of the corresponding movement of faces.

Visible wear marks and cracks not felt by a fingernail. Pitting and impact marks perceptible with fingernails.

Handle the piston rod and marine head with utmost care. Mishandling will cause damages and will result in improper tightening without the ovalness of the big-end bore.

  • Record the observed damages for future reference.
  • Smoothen out the single raised spot in the faces caused by pitting impact marks by filing with a small file.

Inspection of Piston rod and Marine Head Bolts

  1. Inspect the hydraulic nuts for seizure in the threads and pitting on contact faces of the threads.
  2. Turn the hydraulic nuts on the bottom of the bolts
Inspection results         Action
If the bolts and nut have a seizure in threads and pitting on the surfacesRenew the bolts and screws
Hard/cant turn in to bottom position by handRenew the bolts and screws

Big-end Bore measurement

  1. Assemble the lower and upper bearing caps without the shells. The upper. While assembling the position of indent nos. Of bearing, caps and piston rod must match
  2. Assemble the upper and lower bearing caps with hydraulic bolts.
  3. Tighten the bolts with prescribed torque as per the torque datasheet.
  4. Install the con- rod
  5. Measure different diameters
Connecting Rod of Marine Engine-indent no
Connecting Rod of Marine Engine-indent no
measurement positions
measurement positions

Record the gauging readings in the format.

 Connecting Rod of Marine Engine- record of gauging.
Connecting Rod of Marine Engine- record of gauging.
  1. Calculate the maximum ovality by subtracting the maximum and minimum readings.
  2. Check the maximum ovality readings comparing the specified limits.
Inspection resultsAction
Maximum ovality has exceededRenew the complete marine head unit, piston rod, screws, and bearing shells
Maximum ovality is within the limitsRe-use the marine head.

Inspection of Bush for Piston rod

  1. Inspect the piston pin and piston rod bush surfaces
  2. Measure the piston pin and bush clearance.
  3. Check if the clearance is excessive. Change the bushes with new bushes if the measurement is more than the specified limit.

Big End Bearing shell Inspection.

Inspect the bearing shells and replace the shells as per the following conditions

  1. Replace the bearings shells If the observation is not satisfactory as per the Equipment manufacturer’s recommendations.
  2. Change the bearing shells as per the bearing changing schedule specified in manufacturer recommendations.
  3. Change the bearings shells In case of accident/ failure of bearings.

Four stroke medium speed Engine Connecting rod

 Connecting Rod of Marine Engine- Four stroke
Connecting Rod of Marine Engine- Four stroke


H shape section to curtail mass and enhance strength


The connecting rod is of die-forged technology.

 The big End is with inclined joint for the easy withdrawal of piston and connecting rod assembly through cylinder liner.

 The Connecting rod and bearing cap joint faces have serrations for exact location and arrest relative movement of parts.

Connecting has rifle bored whole to supply the oil from big End to small End.

The big-end bearing is tri-metal kind and is with a coating of running layer.

The design of the bearing shell is a precision type and to install it without scrapping.

The small end bearing is also tri-metal type and is shrinking fit into the connecting rod. The small end bearing has a circumferential groove for evenly distributing oil to bush and pin bosses.

Advantages of H shape design;- 

  • Increase in load carrying capacity by increasing the bearing Surface area.
  • Reduces Shear stress
  • Serrations Increased gripping size due to the serrations and also stressed withstanding capacity of connecting rod increases.

Inspection of the big-end bore

  • Clean all the machined surfaces on the connecting rod.
  • De-grease all the serrated joint faces and threaded holes with volatile solvent and blow with dry air.
  • Inspect the serrated joint faces. The serration on the connecting rod may show damages due to the relative movement between the mating surfaces, may show cracks and pitting marks in the high-loaded zone.
  • Record the observed damages in the con-rod inspection sheet.
  • CarefullysSmoothen the sharply raised points with the help of the file.

Be careful not to damage the serrations on the connecting rod and bearing cap while dismantling the con-rod. File to rectify/smooth the dent mark.

  • Reject the connecting rod in case of damage of serration.
  • Carefully clean the threads of the antifatigue screws.
  • Inspect the threads for any seizure. Test the threaded holes, and the test must confirm the turning of the screw by hand up to the last End of the thread. Change the screws if the test doesn’t prove re-usability.  

Big-end bore measurement

Big-end bore measurement
Big-end bore measurement
  • Assemble the con-rod with a bearing cap without the bearing shells. The indent no. on the con-rod and bearing cap should match.
  • Tighten the con-rod Nut at prescribed torque/hydraulic pressure.
  • Measure 30 mm from the serration point.
  • Take the bore measurement with the help of an inside micrometer.
  • Measure five different diameters as per the measurement points in the figure.
  • Record all the measurements in the sheet for the con- rod measurement.
  • Calculate the maximum ovality by the difference between the largest and smallest.
  • Reject the complete con-rod if the difference in ovlty is exceeding the Manufacturer’s prescribed limits.
  • Use a new complete con-rod with screws and new shell bearings.
  • Fill all the observations in the squares of the measurement sheet.
Record of mesurement
Record of mesurement

Bush for Connecting Rod of Marine Engine

  • Inspect the surface of the piston pin and the small end bush.
  • Measure the clearance between the bush and piston pin.
  • Reject the bush in case the reading measurement exceeds the prescribed limits.

Two-Stroke ConRod

 Connecting Rod of Marine Engine-two stroke connecting rod
21-22 the Best Tutorial on| Connecting Rod of Marine Diesel Engine 57

Popularly a Connecting rod is known as Conrod. A connecting rod looks like the one shown in the above picture. A connecting rod consists of an upper forked section that fits on the crosshead bearings, while the lower part of the Conrod fits on the crankpin bearing.
The connecting rod is subjected to heavy axial loading.

The loading reaches a maximum at the TDC (Top dead center) due to the addition of pressure created by the combustion gases and the inertia forces. A connecting rod fails due to abnormal working conditions like momentary loading in peak pressures and piston seizures causing severe stress.
Connecting transmits the motion. Oil is supplied through the connecting rod for piston cooling and the lubrication of Crosshead bearings.

The material of the con-rod is forged steel. The shape in each end accommodates the suitable bearings.

Types of bearings in Two-stroke Engine

  1. Old design;- Old design uses White metal thick wall bearings. In these bearings, adjustment of clearance is by adding or removing shims between the bearing halves.
  2. Modern Engine;- thin wall tri-metal precision bearings. Tin-based, with a thin layer of white metal with a minimum of 88% tin ( Sn). 5-7% antimony ( Sb), 3-5 % copper ( Cu) is the material composition of these bearings. A tin aluminum alloy is consisting 20 to 40 % tin, remaining aluminum bounds with steel shell backing.

This type of bearings design doesn’t need clearance adjustment. Reject and change the bearing when the clearance exceeds the specified limits.

A rifle whole through the center of the rod allows passage for the lubricating oil flow between the bearings- in a downward position.

Conrod is with the smaller length for the limitation of the overall height of the Engine. Short con-rod provides greater angularity more enormous side thrust. 

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Marine Diesel Engines

Marine Diesel Engines 1

This book has been written with the view to fulfilling the need of marine engineers to be in touch with up-to-date information on present day engines, which have replaced the older series. In this age of technological advancement, it is of vital importance that today’s marine engineer keeps abreast of these developments and equip themselves with equip themselves with thorough knowledge of the engines that they work on a regular basis.

A distinctive feature of this book is that the text matter is presented in a ‘easy-to-understand’ point form, for the benefit of marine engineering students. Besides providing an in-depth understanding of the basic principles of marine diesel engines, this book also gives an insight into working of modern engines.

This book will be useful to candidates appearing for the Certificates of Competency examinations.

The book covers the following topics exhaustively.

  • Dry Docking
  • Watch Keeping
  • Engine running problems
  • Camshaft-less electronically controlled intelligent engines
  • Indicator card analysis
  • Engine performance and testing
  • Latest developments
  • Engine overhauls
  • Engine emission
  • Starting and reversing
  • Maneuvering
  • Bridge control
  • VIT and Super-VIT
  • Faults, defects and problems of all engine components

Lamb’s Questions and Answers on the Marine Diesel Engine

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The book is comprehensive and includes almost all topics needed to understand marine diesel engines. The book explains the role of heat in engineering science, the principles behind internal combustion engines, the role of fuels and lubricants, the cooling systems, the lubricating systems and the heat exchangers.

The book also explains the various parts of the marine engines and their functions. Air-storage tanks and air compressors are also dealt with in the book. Information on balancing and vibration, various instrumentation and control methods, along with the safety measures are covered in the book.

Lamb’s Questions and Answers on the Marine Diesel Engine was published by Butterworth-Heinemann in 1990. The eighth revised edition is available in hardcover.

Pounder’s Marine Diesel Engines and Gas Turbines 

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Pounder’s Marine Diesel Engines and Gas Turbines, Tenth Edition, gives engineering cadets, marine engineers, ship operators and managers insights into currently available engines and auxiliary equipment and trends for the future. This new edition introduces new engine models that will be most commonly installed in ships over the next decade, as well as the latest legislation and pollutant emissions procedures. Since publication of the last edition in 2009, a number of emission control areas (ECAs) have been established by the International Maritime Organization (IMO) in which exhaust emissions are subject to even more stringent controls. In addition, there are now rules that affect new ships and their emission of CO2 measured as a product of cargo carried.

  • Provides the latest emission control technologies, such as SCR and water scrubbers
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A Pocket Book of Marine Engineering: Questions and Answers

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A Pocket Book of Marine Engineering: Questions and Answers is a book that covers the topic of air filter for engine in great detail. This book is a great resource for anyone who wants to learn more about this topic. The author, John Myatt, is a world-renowned expert on marine engineering. In this book, he covers everything from the basics of how these engines work to more advanced topics like choosing the right air filter for your engine. If you’re looking for a comprehensive guide to air filter for engine, this is the book for you.

This handy pocket-sized guide is filled with hundreds of questions and answers on marine engineering, covering all the basics from engines and propulsion to steering and safety. It’s perfect for anyone who wants to learn more about this fascinating subject, whether they’re studying for a qualification or just interested in how boats work. With clear explanations and plenty of diagrams, this book is an essential reference for anyone with an interest in maritime engineering.

Blog Conclusion

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