Connecting Rod Components for Cummins QSB 5.9L Engines

The Cummins QSB 5.9L engine is renowned for its durability, power delivery, and adaptability across marine and industrial applications. Whether operating in commercial fishing vessels, passenger ferries, workboats, power generators, or heavy industrial machinery, the performance of the QSB 5.9 depends heavily on the integrity of its connecting rod assembly.

Connecting rods form the crucial link between the crankshaft and the pistons, converting reciprocating motion into rotational power. In the QSB 5.9, these rods operate under high combustion pressures, extreme temperatures, and continuous load cycles. For this reason, proper maintenance and timely replacement of connecting rod bearings, bushings, and bolts are essential to preserve engine health and prevent costly downtime.

This catalog section covers all connecting rod–related components for the Cummins QSB 5.9L, including bearing sets for both fractured and non-fractured rods, piston pin bushings, and high-strength connecting rod cap bolts.

Bearing Sets for Cummins QSB 5.9L

Connecting rod bearings act as precision-fit sleeves between the connecting rod and crankshaft journal. Their purpose is to allow smooth rotation under load while protecting both surfaces from direct metal-to-metal contact.

In the QSB 5.9L, bearings are available in two primary styles depending on the rod type:

• Non-Fractured Rod Bearings – for earlier rod designs with a machined cap interface.

• Fractured Rod Bearings – for modern fracture-split rods, which offer superior alignment and fit.

Bearings are also offered in multiple size specifications:

• Standard Size – used when crankshaft journals remain within original factory tolerances.

• Oversize Bearings – used after crankshaft journals have been machined to restore roundness and surface finish, typically in oversize increments of 0.25 mm or 0.50 mm.

Standard Size Bearings – Non-Fractured Rods

For QSB 5.9 engines equipped with non-fractured connecting rods, the standard size bearing set includes an upper and lower half to service a single rod. These bearings restore factory fit and maintain optimal oil clearance.

Key Features:

• Manufactured from durable tri-metal materials for long service life.

• Maintains correct oil film thickness for load support.

• Compatible with earlier B series engines including 4B, 4BT, and 6B.

Oversize Bearings – Non-Fractured Rods

Available in 0.25 mm and 0.50 mm oversizes, these bearing sets are designed for crankshafts that have been reground. Oversized bearings restore proper oil clearance and protect against metal-to-metal contact in rebuilt engines.

Benefits:

• High load-carrying capacity for heavy-duty applications.

• Precise dimensional tolerances for consistent fit.

• Ideal for engines undergoing major overhaul.

Standard Size Bearings – Fractured Rods

Fracture-split rods, introduced in later QSB designs, offer a naturally interlocking cap-to-rod fit for maximum strength and alignment. The standard size bearing sets for fractured rods maintain OEM specifications and ensure proper lubrication.

Advantages:

• Superior alignment reduces bearing wear.

• Designed for high horsepower and torque loads.

• Compatible with ISB, QSB, 4BT, and 6BT engines using fracture-split rods.

Oversize Bearings – Fractured Rods

When crankshaft journals have been refinished, oversize bearings (available in 0.25 mm and 0.50 mm) provide the correct internal clearance for fracture-split connecting rods.

Highlights:

• OEM-grade construction for marine and industrial service life.

• Engineered for consistent oil flow under continuous duty.

• Reduced risk of premature bearing fatigue.

Connecting Rod Bushings

Connecting rod bushings are pressed into the small end of the connecting rod to provide a bearing surface for the piston pin. In the Cummins QSB 5.9L, these bushings play a critical role in maintaining piston alignment and reducing friction at the wrist pin connection.

Features & Benefits:

• Made from precision-machined bronze or similar high-wear alloys.

• Designed for exact piston pin fitment to prevent noise and wear.

• Essential for top-end rebuilds and piston replacement jobs.

Regular inspection of rod bushings is important, as excessive wear can lead to piston misalignment, poor combustion efficiency, and increased stress on the crankshaft.

Connecting Rod Cap Bolts

Connecting rod cap bolts secure the cap to the rod, holding the bearings firmly around the crankshaft journal. Due to the high tensile loads and fatigue stresses in the QSB 5.9, these bolts must be replaced whenever a connecting rod is serviced or bearings are replaced.

There are two primary types of connecting rod bolts for the QSB 5.9L, depending on rod design:

Early-Style Bolts – Non-Fractured Rods

• Designed for traditional machined-cap connecting rods.

• Manufactured from high-strength alloy steel.

• Provide secure, consistent clamping force under continuous duty.

Fracture-Split Rod Bolts

• Engineered for modern fracture-split connecting rods.

• Precision-threaded and torque-to-yield for exact preload.

• Critical for maintaining cap-to-rod alignment in high-load operation.

Maintenance Tips for QSB 5.9 Connecting Rod Components

1. Always Replace Bearings During Overhauls
   Even if wear appears minimal, bearing surfaces degrade over time and can develop microscopic fatigue cracks.

2. Check for Proper Clearance
   Use plastigauge or micrometer readings to confirm oil clearance meets Cummins specifications.

3. Replace Cap Bolts After Each Disassembly
   Reusing connecting rod bolts can lead to bolt stretch, loss of clamping force, and catastrophic failure.

4. Inspect Bushings for Ovality
   Any distortion in bushing shape can accelerate piston pin wear and lead to knocking noises.

5. Lubricate Components Before Installation
   Apply assembly lube to bearings and bushings to ensure immediate oil film protection at startup.

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• 40,000+ Satisfied Customers worldwide trust Diesel Pro Power for genuine and high-quality aftermarket Cummins components.

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• Expert Technical Support for correct part selection and installation guidance.

• Extensive Inventory covering all Cummins QSB 5.9 connecting rod configurations.