Cummins QSB6.7 marine engine spare parts

AMS Parts · Cummins marine engine guide

The Cummins QSB6.7 is one of the most widely used mid-range marine diesel engines operating in UK and European waters today. Its combination of compact dimensions, high-pressure common rail fuel injection and a broad power band has made it the engine of choice across commercial and high-performance leisure applications — from pilot boats and wind farm crew transfer vessels to fast passenger ferries and performance motor yachts.

There is a detail that matters to UK operators in particular: the QSB6.7 is built at the Cummins Darlington Engine Plant in County Durham, alongside production at Rocky Mount in North Carolina. It is, in a meaningful sense, a British-built marine engine — and one that AMS Parts supplies spares for from the UK.

This guide covers the engine's architecture, its marine applications, the spare parts most commonly required, and how to specify them correctly.

What is the Cummins QSB6.7?

The QSB6.7 is an in-line six-cylinder, four-stroke diesel engine displacing 6.7 litres (408 in³), with a bore and stroke of 107 x 124 mm. It is turbocharged and aftercooled, and built on Cummins' long-running B-Series platform, re-engineered for marine duty.

In marine propulsion and auxiliary configurations, Cummins publishes ratings of 184–405 kW (247–542 hp), with the wider QSB6.7/QSB7 family covering 172–410 kW (230–550 hp). The engine carries RCD 2 certification for recreational applications, with commercial ratings available to relevant marine standards.

Two design details are worth noting because they affect parts sourcing. First, the fuel system is Bosch HPCR with hardened components — specified to run alternative fuels including kerosene and JP8/JP5, which is why the engine appears in government and defence vessel applications. Second, the engine uses aluminium pistons, metric O-ring seals and edge-moulded gaskets, a sealing design intended to eliminate fluid leaks. Those seals and gaskets are specific to this design; generic substitutes are not equivalent.

What vessels use the Cummins QSB6.7?

The QSB6.7 is rated for both commercial and recreational marine duty. In UK and European waters, it is commonly found in:

• Pilot boats and fast patrol craft, where responsiveness and fuel efficiency over variable load cycles matter
• Wind farm crew transfer vessels (CTVs) — a growing UK and North Sea sector where reliability at sustained high load is critical
• High-speed passenger ferries and water taxis operating under noise and emissions scrutiny
• Commercial workboats running long duty cycles
• Performance motor yachts and luxury leisure craft
• Auxiliary power installations aboard blue-water sailing yachts
• Repower projects — the QSB6.7 is a common upgrade path from the older QSB5.9, giving higher power density with modern emissions compliance

Is the Cummins QSB6.7 suitable for commercial marine certification?

Yes. Commercial ratings are available with classification society approvals, and the engine's rating and CPL number determine which compliance level a given unit was built to. Because emissions specification varies between ratings, the CPL is essential when sourcing fuel system or air system components — two engines both described as 'QSB6.7' may be built to different standards.

What is the difference between the QSB5.9 and QSB6.7?

The two share the B-Series block family but are not the same engine. The 6.7 has a longer stroke and a different fuel system architecture. Parts are not generally interchangeable, even where components look dimensionally similar. This catches people out on repower projects where an old parts list is carried over to the new engine.

What is the difference between the QSB6.7 and QSB7?

They are closely related members of the same family, with the QSB7 typically configured for auxiliary and generator duty. Cummins publishes them together in most marine documentation, but the ratings and some ancillary components differ. Specify which you have when requesting a quote.

Which spare parts does the QSB6.7 most commonly need?

Fuel system: Bosch HPCR injectors and high-pressure pump

The Bosch high-pressure common rail system is the QSB6.7's most significant departure from earlier mechanical B-Series engines, and the component group that generates the most sourcing enquiries. Common rail injectors are precision components matched to the engine's calibration; fitting incorrectly specified injectors produces emissions non-compliance and performance degradation, not just a rough idle. Injector supply must be matched to ESN and CPL.

Turbocharger

The QSB6.7's turbocharger works under the variable load conditions typical of marine propulsion — aggressive throttle cycles, short periods at full boost, then idle at the dock. This produces different wear patterns from a constant-load generator application of the same engine. Several turbo configurations exist across the power range, so matching to the specific build variant is essential.

Piston-liner kits and overhaul components

At major overhaul, a matched piston and liner set plus a full overhaul kit — pistons, liners, rings, main and conrod bearings, thrust washers — should be quoted against the CPL to secure the correct compression ratio and ring specification. The QSB6.7 uses aluminium pistons, and the correct specification matters for both durability and emissions performance.

Gaskets, seals and O-rings

The QSB6.7's sealing design uses metric O-rings and edge-moulded gaskets specifically to prevent fluid leaks. Marine thermal cycling — rapid warm-up leaving port, sustained high temperature under load, cooldown at berth — stresses these differently from land-based applications. Full gasket sets, head gaskets and oil seals are standard at major overhaul intervals.

Sensors and Quantum System electronics

The QSB6.7's electronically managed architecture means sensors are integral to engine protection. Temperature, oil pressure, speed and coolant level sensor failures trigger derate or shutdown — which in a marine context means a vessel losing propulsion under way. Commercial operators are well advised to hold verified spares for the critical protection channels.

Raw water pump and cooling circuit components

The raw water (seawater) pump is the highest-frequency replacement item on almost any marine diesel. Salt, debris and continuous operation accelerate impeller wear well beyond what land-based pump applications see. Impellers should be treated as scheduled replacement items regardless of apparent condition at inspection.

Cylinder heads

The QSB6.7 uses a 24-valve cylinder head that contributes much of the engine's power density. Head overhaul or replacement on high-hours marine engines requires pressure testing and correct valve seat and guide specification for the rating concerned.

How do you identify the correct QSB6.7 part?

The QSB6.7 has been produced across multiple emissions tiers and power ratings, and the model description alone does not identify which components are correct. The engine data plate carries the Engine Serial Number (ESN) and CPL. Both are needed for accurate specification, particularly for injectors, the turbocharger and electronic components.

Photographing the data plate before requesting a quote removes a round of back-and-forth and speeds up pricing considerably.

Why source QSB6.7 parts from a UK supplier?

Many of the online parts catalogues that rank well for QSB6.7 searches are US-based. For UK and European commercial marine operators, that introduces import handling, customs clearance, longer transit and — most significantly — no UK-based technical contact who can verify compatibility before the part ships.

AMS Parts supplies genuine and OEM Cummins marine spares from the UK, with serial number verification built into the quotation process rather than left to the buyer. We do not supply aftermarket components.

What to send with your enquiry

Supplying the right information up front removes a round of questions and gets you a firm price faster. For any quote on this engine, please include:

Why source from AMS Parts

AMS Parts is a UK-based supplier of genuine and OEM spare parts for marine, power generation and industrial diesel engines. We verify every quotation against the engine serial number rather than leaving compatibility to the buyer, and we supply genuine and OEM parts only — we do not offer aftermarket components. Our full range of Cummins engine spare parts covers propulsion, auxiliary and generator applications, with worldwide shipping to vessels and service facilities.

Related guides

• Cummins QSM11 marine engine spare parts
• Genuine vs OEM Cummins parts
• How to find the right Cummins part using your ESN