The Cummins QSB6.7 is one of the most common diesels in workboats, pilot boats, patrol craft and generator sets, and its on-road twin, the ISB6.7, shares the same bottom end. When one of these engines is opened for low compression, blow-by, oil consumption or a failed piston, the parts that go back in are a matched group: the piston with its rings, the piston pin and the connecting rod that carries it to the crankshaft. Two records covering this group are now in the Sea Keepers approved catalogue. This guide explains what each record contains, how the Cummins numbers relate, and what to confirm before ordering a set for six cylinders.
Cummins QSB6.7 / ISB6.7: standard piston kit 4309259
SK-00058 is the genuine Cummins engine piston kit 4309259 in standard size for the QSB6.7 and ISB6.7 6.7-litre engines, including the non-certified 6.7-litre ISB and QSB engines covered by Cummins Pro. The kit contains piston 5284820, ring set 4309257 and the retaining rings that hold the pin in place. The piston pin 4931041 and connecting rod 4989162 are listed alongside it in the Cummins piston and rod list, so they are ordered as separate lines. The record also carries a third number, 2815FT1018514, which could not be traced to Cummins; the 2815 prefix is the NATO supply class for diesel engine components, so it is most likely a stock or NSN-style code used on the purchase order rather than a Cummins part number.
Cummins ISF3.8, ISBe/ISDe and QSB6.7: connecting rod 4989162
SK-00056-AV is the genuine Cummins engine connecting rod 4989162, which links the piston to the crankshaft. In the ISF3.8 parts catalogue it is listed with the alternative number 5257364, together with connecting rod bushing 5257363 and connecting rod cap bolt 4891179. In the QSB6.7 piston and rod list it appears next to piston kit 4309259 (SK-00058). One supplier lists the same rod for the ISBe, ISDe, ISF3.8 and QSB with the numbers 4989163 and 4943979. No supersession was found in the sources reviewed, so the engine serial number and CPL should be used to confirm which number applies before a set is ordered.
Why pistons and rods are checked together
A piston is rarely the only part damaged. Scuffing, a broken ring land or a cracked crown usually means the rod small-end bushing and the pin have also run hot, and a rod that has been overloaded can be bent or twisted out of limits without visible marks. When a QSB6.7 or ISB6.7 is stripped, each rod should be checked for bend and twist, big-end bore size and roundness, and small-end bushing clearance on the pin. A rod that is out of limits is replaced rather than reused, and new cap bolts are fitted because they are torque-to-yield on many Cummins engines. The cylinder bores should be measured at the same time, because a standard-size piston kit is only correct if the bore is still within the standard limits.
Related Cummins rods and crankshafts in the catalogue
Sea Keepers carries approved records for neighbouring Cummins engines as well. SK-00025-AV is the connecting rod 3901383 / 3924350 for the 8.3-litre C-series, 6C, 6CT and 6CTA, and ISC and ISL engines, which suppliers note is for use with crankshaft 3917320 only. SK-00034-AV is the crankshaft assembly 3908032 for the 5.9-litre 6B, 6BT and 6BTA, made up of crankshaft 3929037 and gear 3929026. These are not interchangeable with the 6.7-litre parts, so the engine model must be confirmed before a quotation is accepted. For the larger QSX15, the approved crankshaft 4925762 / 4393462 (SK-00031), valve crosshead 3086362 (SK-00039-AV) and crankcase breather 3102665 (SK-00010-AV) are covered in their own guides.
Fitting notes
Pistons go into the bores with the ring gaps staggered as the manual specifies and the piston orientation mark facing the front of the engine. The pin is fitted with new retaining rings, and the rod is checked for free movement on the pin before the assembly is installed. Rod caps must go back on the rod they were machined with and in the same orientation, and the cap bolts are tightened in the stages and to the angle given in the Cummins service manual. After assembly, crankshaft end float and the rotating torque of the engine are checked before the oil pan is refitted.
What to send with an enquiry
To match the correct piston kit and connecting rod, send the engine model, the engine serial number and CPL, the rating and application (propulsion, generator or auxiliary), the bore measurements if the block has been inspected, the number of cylinders to be rebuilt, and whether pins, bushings, cap bolts and gasket sets are needed with the order. Sea Keepers will check the current Cummins number, confirm any alternatives and quote delivery to the vessel or workshop.
For the business buyer
Confirm the intended vessel or facility, operating environment, quantity and required approval before requesting a quotation. Capacity, coverage and service interval should be matched to the risk rather than selected by headline price.
For technical experts
Verify the delivered model against the maker reference, rating, dimensions, materials, operating limits and certification scope. Record the serial or batch number and retain the manufacturer datasheet, declaration, test report and maintenance instructions in the vessel file.
Product comparison
Compare alternatives on certified performance, compatibility, installation requirements, inspection interval, spares availability and total life-cycle cost. A lower unit price is not a like-for-like comparison when approval documents or service support differ.
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