Piston & Connecting Rod Overhaul (with Angle Tightening) in Full Procedure on Marine Diesel Engine

Step-by-step piston and connecting rod overhaul on marine diesel engines. Full disassembly, inspection, reassembly, and angle tightening procedure explained.

Piston & Connecting Rod Overhaul of Marine Diesel Engine (4 stroke)

A piston and connecting rod overhaul is a fundamental yet major engineering procedure critical for restoring the power and efficiency of a 4-stroke marine diesel engine. This comprehensive process involves the meticulous removal, disassembly, and inspection of these core components for wear, scoring, and fatigue. Key steps include measuring piston ring clearances, inspecting piston crown and skirt for damage, checking connecting rod alignment, and examining the small and big end bearings. Successfully executing this overhaul is paramount for restoring optimal engine compression, ensuring efficient fuel combustion, and preventing catastrophic engine failure. For vessel operators, this essential maintenance directly translates to regained engine performance, significantly reduced fuel consumption, lower emission levels, and enhanced operational reliability, protecting your valuable asset and avoiding costly unplanned downtime.

For: Piston & Connecting Rod Overhaul Full Procedure on Marine Diesel Engine (4 stroke) with Angle Tightening

The full procedure for a piston and connecting rod overhaul on a 4-stroke marine diesel engine is a precision task that demands strict adherence to manufacturer specifications, with angle tightening being a critical step for integrity. After the meticulous removal, cleaning, and inspection of components, the precise reassembly of the connecting rod cap to the rod itself is paramount. This is achieved using stretch bolts that are first tightened to a specific pre-load torque and then precisely tightened further to a defined rotation angle, a method that ensures an exact and consistent clamping force far superior to torque-only methods. Neglecting this precise angle tightening procedure can lead to catastrophic bearing failure, rod bolt fatigue, and even engine seizure. Therefore, following the full technical protocol, including calibrated angle tightening, is non-negotiable for guaranteeing a secure and reliable overhaul, ensuring maximum engine performance, operational safety, and long-term reliability at sea.

The provided training video demonstrates a complete piston and connecting rod overhaul procedure, including disassembly, inspection, and reassembly with angle tightening of bolts. In this expanded guide, we combine the video’s step-by-step tutorial with additional insights from:

• Engine manufacturer manuals (MAN B&W, Wärtsilä, Sulzer).
• IMO and STCW requirements for marine engineers.
• SOLAS and classification society guidelines for safe maintenance.
• Real-world experiences, case studies, and troubleshooting lessons from the maritime industry.

By the end, you’ll understand not only how to overhaul a piston and connecting rod, but also why each step is critical, what can go wrong, and how to ensure full compliance with international standards.

1. Understanding the Piston and Connecting Rod Assembly

Functions

• Piston: Withstands combustion pressures (up to 200 bar in slow-speed engines), transmits force to connecting rod.

• Connecting rod: Converts reciprocating motion into rotary motion at the crankshaft.

• Piston pin (gudgeon pin): Connects piston and rod, allowing oscillation.

• Bolts & nuts: Secure the assembly, requiring precise tightening.

Operating Stresses

• Thermal stress: High temperatures on piston crown (500–600°C).

• Mechanical stress: Fatigue cycles with each revolution.

• Lubrication stress: Risk of scuffing/seizure if oil film breaks down.

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2. Preparations Before Overhaul

Tools and Equipment

• Liner protection ring and piston support tools.

• Resin hammer and soft slings for lifting.

• Torque wrench and angle gauge for bolt tightening.

• Hydraulic jacks for bolt loosening.

• Micrometers, inside calipers, and dial gauges.

• Penetrant testing kit.

Safety Measures

• Inform bridge/engine control room.

• Stop engine, allow sufficient cooling.

• Isolate starting air, fuel, and lube systems.

• Ensure lifting tools are certified.

• Wear PPE (gloves, goggles, safety boots).

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3. Step-by-Step Piston Removal

1. Remove cylinder head (if not already done).

2. Fit liner protection ring to prevent liner damage.

3. Remove cooling water and lube oil connections to piston.

4. Disconnect piston rod bolts using hydraulic jack system.

5. Lower piston rod carefully, support with lifting tool.

6. Use overhead crane to lift piston and rod assembly vertically.

7. Place assembly on clean overhaul stand.

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4. Inspection of Cylinder Liner

• Measure diameter at several heights (top, middle, bottom).

• Check for ovality and wear.

• Clean carbon deposits from top.

• Record values in PMS for trend monitoring.

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5. Piston Disassembly

1. Remove piston rings carefully, check end clearances.

2. Remove piston cooling chamber cover.

3. Withdraw piston pin using extraction tool.

4. Separate piston crown and skirt from connecting rod.

Inspection of Piston Components

• Piston crown: Penetrant test for cracks.

• Ring grooves: Check wear limits with gauges.

• Skirt: Inspect for scuffing and lubrication marks.

• Cooling space: Flush with cleaning oil, check for blockages.

Inspection of Piston Rings

• Check butt gap clearance.

• Check side clearance in grooves.

• Replace if broken, worn, or stuck.

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6. Connecting Rod and Bearings

• Inspect crankpin and connecting rod bearings.

• Look for scuffing, metal fatigue, or overheating signs.

• Never reuse bolts beyond manufacturer’s specified hours (usually 20,000).

• Check serrated mating surfaces for cleanliness and tight fit.

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7. Bolt Fatigue and Replacement Guidelines

• High-strength bolts securing piston rods and big ends face cyclic loading.

• Manufacturer rules: replace after ~20,000 running hours or at each major overhaul.

• Never apply oil on bolt seating surfaces (can cause incorrect preload).

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8. Angle Tightening Procedure

One of the most critical steps in reassembly.

Why Angle Tightening?

• Torque alone cannot guarantee exact preload (friction variation).

• Angle tightening ensures bolts stretch elastically by a known amount.

Procedure

1. Tighten bolts initially to a set torque (e.g., 300 Nm).

2. Mark bolt head and reference point.

3. Tighten further by specified angle (e.g., 90° + 90°).

4. Verify markings align with required angle.

5. Stamp bolt heads to indicate completion.

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9. Piston Reassembly

1. Fit connecting rod to piston with new circlips.

2. Insert piston pin, ensuring correct orientation.

3. Fit piston rings — align ring gaps at different positions (120° apart).

4. Refit cooling chamber cover.

5. Lubricate all moving surfaces.

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10. Installation into Cylinder

1. Insert piston into liner using support tool and protection ring.

2. Guide connecting rod to crankpin journal.

3. Align fuel injection side marks between piston and rod.

4. Tighten connecting rod bolts using torque + angle method.

5. Check free movement and lubrication.

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11. Final Checks and Testing

• Adjust valve clearances if cylinder head also overhauled.

• Rotate engine on turning gear to confirm smooth operation.

• Conduct crankcase inspection for leaks.

• Record overhaul details in PMS logbook.

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12. Troubleshooting Reference Table

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13. Regulatory & Training Context

• SOLAS II-1: Machinery must remain reliable for safe navigation.

• STCW (Reg. III/1 & III/2): Engineers must demonstrate piston & rod overhaul competence.

• Classification Societies: Require inspection of pistons and rods during major surveys (5-year drydock).

• Maker Manuals (MAN, Wärtsilä): Provide exact clearances, torque values, and replacement intervals.

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14. Case Studies

Case 1 – Piston Ring Failure at Sea

A bulk carrier experienced high exhaust temperature alarms. Overhaul revealed broken piston ring due to insufficient clearance. Regular inspection would have prevented it.

Case 2 – Connecting Rod Bolt Fatigue

On a container ship, connecting rod bolts failed because they were reused beyond 25,000 hours. This caused crankshaft damage and four weeks of off-hire.

Case 3 – Misaligned Reassembly

During a training exercise, piston rings were installed with aligned gaps, leading to blow-by and scavenge fire during operation. Lesson: always stagger ring gaps.

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15. Best Practices for Engineers

• Always renew seals, circlips, and O-rings during overhaul.

• Replace bolts per manufacturer’s schedule.

• Record all clearances and measurements for PMS tracking.

• Never strike piston crowns with steel hammers — use resin hammers only.

• Keep overhaul area clean, free of dust and oil.

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Conclusion

The piston and connecting rod overhaul is one of the most demanding yet essential maintenance jobs on a marine diesel engine. As shown in the video and expanded here, it involves:

• Safe removal with proper tools and liner protection.

• Careful inspection of piston crown, rings, grooves, and bearings.

• Precise reassembly with angle tightening of bolts.

• Compliance with IMO, STCW, SOLAS, and class requirements.

By following these steps diligently, marine engineers ensure the long-term safety, efficiency, and reliability of the ship’s propulsion plant — safeguarding both lives and global trade at sea.