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6CT Vs. 6BT: Cummins’ Fully Mechanical, Ultra-Reliable Light And Medium-Duty Siblings

Written By, Mike McGlothlin

In a lot of ways, the 8.3L and 5.9L Cummins are very similar. Both all-iron, I-6, gear-driven, mechanically injected diesel power plants enjoy reputations for steadfast reliability in the transportation, agriculture, construction, RV, and marine sectors. However, in many other ways the C and B series inline-six mills produced from the mid 1980s to 1998 are quite different—and rightfully so. After all, the C-series engines were designed for bigger workloads than the 5.9L could handle, not to mention the fact that the 8.3L featured wet sleeve technology for enhanced durability and in-frame serviceability when (and if) it was needed.

However, the first of their distinct differences begins with the physical size of the two engines. Pickup truck enthusiasts, where the 5.9L (and now its predecessor, the 6.7L) Cummins rules the roost, are drawn to the 8.3L upon first learning of its existence. After all, knowing that the 5.9L Cummins boasts endless performance potential, what could be better than a larger displacement version of the same thing? Unfortunately for performance-seekers, the 6CT is longer, taller, and more than 500 pounds heavier—and that’s just the tip of the iceberg. Below, we’ll shed light on more notable distinctions between these two venerable engines, as well as their biggest pitfalls.

 

No KDP On The 8.3L (Advantage 6CT)

5.9L Cummins 6BT Killer Dowel PinPerhaps the biggest point of leverage the 6CT holds over the 6BT is that it’s void of the killer dowel pin (KDP) that is notorious for working itself loose and wrecking the geartrain of the 5.9L. On the assembly line, all Cummins B-series engines (the 4BT included) were fitted with a 5/16-inch diameter steel dowel pin in order to align and attach the front gear cover to the block. Years later, following thousands of heat cycles, countless miles, and constant vibration, the dowel pin sometimes works itself free. When this happens, the KDP can fall into the camshaft gear which, in a chain-reaction event of all-out destruction, causes the pistons to make contact with the valves. Of course, the dowel pin can also snake past the geartrain and simply fall into the oil pan, but that outcome is never certain.

Wet Sleeves On The 8.3L (Advantage 6CT)

8.3L Cummins Wet Sleeve Cylinder LinerA major design separation between the 8.3L and 5.9L is the 6CT’s use of wet sleeves in its cylinders. By effectively allowing engine coolant to circulate around the exterior of the combustion area of each cylinder, efficient heat transfer occurs and the cylinders themselves remain perfectly round. Dry sleeve and parent bore blocks (the 6BT utilizes a parent bore block) don’t enjoy this luxury, which adds significant longevity to an engine. But on top of that, with wet sleeves essentially making each cylinder a stand-alone unit, in-frame rebuilds are not only possible but highly feasible to perform in the field.

Weight And Size (Advantage 6BT)

Cummins 6BT Diesel Engine in Dodge RamWhile the 6BT is no feather weight, it packs substantially less heft than its big brother. Most dressed 5.9L Cummins power plants tip the scales at roughly 1,100 pounds. Depending on its intended application, a fully dressed 8.3L can weigh as much as 1,650 pounds. Not surprisingly, both engines measure similarly in width. But the larger bore and longer stroke of the 6CT predictably gives it more overall length and height than the 6BT. The following length, height, and weight specifications should be carefully pored over for any diesel conversion or engine swap project where the 5.9L and 8.3L are both being considered.

 

5.9L 6BT

Length: 40-inches

Width: 24.9-inches

Height: 37.9-inches

Weight: 1,100 lbs

 

8.3L 6CT

Length: 47-inches

Width: 28-inches

Height: 45-inches

Weight: up to 1,650 lbs

 

Ag Application Issues For The 8.3L

6CT Cummins Engine in Case Combine HarvesterThe biggest problems associated with the 8.3L Cummins stem from agricultural applications, and specifically the 6CT’s that were stuffed into combine harvesters. These engines, which were equipped with different pistons, a unique cylinder head, and that were often plagued by cooling issues, are known for overheating and blowing the head gasket. In particular, coolant leaving the water pump is forced through a 90-degree radiator hose (which significantly hampers flow) and these engines are designed to turn higher rpm than their road-going counterparts. The higher operating rpm in these applications has also been linked to connecting rod fastener failure, a failure that becomes more and more conceivable once the 7,000-hour mark is surpassed.

 

The Better High-RPM Candidate (Advantage 6BT)

12-Valve 5.9L Cummins Performance Diesel EngineIn contrast to the 8.3L, the 5.9L Cummins is much better suited to high rpm operation. To be sure, this is a benefit of being a smaller engine with less internal rotating mass. In factory form most 6CT mills offer a maximum engine speed between 1,800 to 2,100 rpm, while the 6BT’s peak rpm window sits between 2,500 and 2,700 rpm. In the diesel aftermarket, where the proper valvetrain and governor spring provisions have been made to sustain higher rpm, it’s not uncommon for the 5.9L Cummins’ factory rotating assembly to survive (and even regularly see) 5,000 rpm.

 

Parts Availability (Advantage 6BT)

6BT Cummins Diesel Engine Rebuild KitThanks in part to Cummins’ mass production run of the 5.9L for use in Dodge Ram 2500 and 3500 pickup trucks, a significantly higher volume of 6BT’s were produced. In the grand scheme of things, this means the 6BT gets the upper hand in part and component availability over the 6CT. In terms of servicing, replacing, and even rebuilding, parts availability is often a major factor in deciding whether to run a 5.9L or 8.3L engine in a piece of equipment, a truck, motor home, bus, or even a marine vessel. Be it from the local salvage yard, an eBay special, or a brand-new OEM component, 5.9L parts are generally easier to obtain—along with being more affordable.

 

Holset Turbochargers (Tie)

Holset HX40 Turbo on 6CT Cummins EngineJust as the 6CT and 6BT employ the kind of robust engine construction that lends itself to longevity, the turbocharger feeding each engine is highly durable also. A Holset HX35 model is common on versions of the 5.9L Cummins equipped with the Bosch P7100 injection pump, while the Holset HX40 (the HX35’s big brother) can be found bolted to the 8.3L Cummins. Both turbos are simple, fixed geometry units that utilize journal bearings. The HX35 makes use of a T3 turbine inlet (what attaches to the exhaust manifold) while the HX40 can be found in both T3 and T4 form (and with various A/R sizes). Each turbocharger has the ability to handle boost levels and shaft speeds that well exceed their OEM specs, with the smaller HX35 getting the slight edge in durability when forced to endure at elevated boost.

 

P-pumps (Tie)

Cummins 6CT Mechanical Diesel Injection PumpLike their reliable, fixed geometry, journal bearing turbochargers, both the 6BT and 6CT benefit from highly durable, mechanical injection pumps—as well as time-tested, pop-off style mechanical fuel injectors. One big difference between the inline injection pumps (i.e. P-pumps) that were employed on these two engines is that the version used on the 8.3L Cummins received its engine oil supply from a different location. Because oil supply feeds the P-pump from the gear housing and through the front cover, the 8.3L pump requires no external oil feed line (the 5.9L P-pump does). Other pump distinctions between the 5.9L and 8.3L include different cams, plunger diameters, and delivery valve holders.

 

No Electronics (Tie)

Mechanical 8.3L Cummins Big Bear EngineBeing that they were designed, produced, and implemented during the same era, the 6CT and 6BT engines lacked electronics (and more specifically, electronic controls). This not only offers both power plants simple functionality, but it’s also the driving force behind their timeless desirability. When adherence to modern day diesel emission standards isn’t a requirement, both the 6CT and 6BT remain go-to options for simplicity, sufficient performance, adequate fuel efficiency, and million-mile durability. It wasn’t until 1998 that the CAPS system was implemented on the 8.3L ISC and a similar fuel system, built around the use of the electronically controlled Bosch VP44, was added to the 5.9L ISB.

 

Which Cummins Is Right For You?

Comparison between Cummins 6BT and 6CT Diesel EnginesThough we’ve seen the 8.3L Cummins swapped into a smaller class of vehicle than it was originally intended for, the 5.9L Cummins is a much more commonly selected swap candidate. Much of the 5.9L’s popularity stems from its high parts availability, immense aftermarket support, and (last but not least) its size and heft. Based on its size alone, a 6CT is best-suited for medium-duty trucks and motorhomes while a 6BT is generally better served for lighter-duty applications like single axle dump trucks and ¾-ton or 1-ton pickups. Replacing a 6CT with a 6BT in a vehicle that will be expected to perform the work the 8.3L was previously carrying out would be a mistake.

 

Key Takeaways

For vocational applications where cost is the highest priority, go with the 5.9L Cummins (again, high parts availability and more were produced). When high-mile or high operating hours is of utmost concern, choose the 8.3L Cummins (its wet sleeve design and overall lower rpm operation better lends this engine to longevity).

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