Written By, Mike McGlothlin
It’s an engine that’s widely renowned for being tough as nails, but the 6BT Cummins does have its limitations. Granted, many of this iconic diesel’s weak links are discovered after turning up the power, something that—thanks to its mechanical injection system, can be done with simple hand tools (and on the cheap)—many enthusiasts can’t refrain from doing. However, at least one failure point is directly linked to the 5.9L Cummins’ factory assembly process. If you want a truly fortified, all-but-bulletproof 5.9 Cummins, there are five key areas to address on this venerable inline-six, and we’re here to tell you what they are.
Weak Link #1: KDP
You’ve heard of the 6BT Cummins’ killer dowel pin (KDP) before, but you can’t talk about
the 5.9L’s few vulnerabilities without starting here. On the original assembly line, an 8mm diameter steel dowel was pressed into the block in order to align the front gear cover to the crankcase. Over time, and due to vibration, age, and expansion and contraction, it can work itself free from the block, fall into the geartrain and unleash all sorts of mayhem. It’s no guarantee that the KDP will become dislodged from the block, but it’s something that can cause more harm than any other weak link on our list if it occurs. This is why it is paramount that you address it sooner rather than later.
Permanent Solution: Tab The KDP
Being proactive is the only way to ensure the KDP doesn’t wreck your 6BT, and it’s one of
only two weak links on our list that can occur without any additional horsepower in the mix. To take KDP failure out of the equation, staking or tabbing the pin in place is the best and most common solution. Gaining access to the front of the engine and removing the gear case is required, as is a new seal for the gear case and crank. It’s a laborious task and often takes a skilled mechanic four to six hours to complete—and is exactly why we at Big Bear Engine Company have machined the gear housings differently to completely solve the problem on all of our new remanufactured component builds. No need to worry anymore!
Weak Link #2: Rod Bolts
Because the 5.9L Cummins is built so robustly, its hard parts can withstand immense increases in cylinder pressure, heat, and even rpm. However, there is a limit to what the factory connecting rod bolts can handle, and they become a significant weak link once power levels crest 800 hp (or roughly 1,500 lb-ft of torque). At this point, the 7/16-inch diameter OEM rod bolts can stretch, which can culminate in a catastrophic rod and/or crankshaft failure, or even result in a windowed crankcase. For optimum insurance, the factory rod bolts should be addressed in any project where horsepower is going to exceed 750 hp.
Permanent Solution: Upgraded Rod Bolts
For high rpm and high horsepower 6BT Cummins builds, upgrading the factory rod bolts
is all but mandatory. A proven solution has long been the direct replacement fasteners produced by ARP. Made of the company’s L19 material, these rod bolts are 23-percent stronger than the OEM versions despite being the same 7/16-inch diameter. And because they can be installed without pulling the engine, they’re a highly common bolt-in option for budget-minded performance builds. For engines with everything from mild to wild parts combinations, they provide peace of mind when pursuing serious horsepower.
Weak Link #3: Head Bolts
In a performance setting, where horsepower and torque levels are double the factory ratings or more, head bolt failure can become a worry. Opinions vary on just how far you can push the 6BT Cummins’ factory head bolts, and some swear they can be re-torqued beyond OEM specs to hold more boost, but once you’re pushing the limits anything can happen. When the factory head bolts fatigue and stretch, their clamping force is drastically reduced. This can break the perfect seal that once existed between the cylinder head and the block, ultimately leading to a blown head gasket.
Permanent Solution: Head Studs
To ensure 5.9 Cummins head gasket issues never surface, aftermarket head studs (and namely the versions produced by ARP) are money well spent. These ARP head studs are made from higher quality material than the OEM bolts and allow deeper thread engagement into the block, which equates to improved clamping force. A properly torqued set of studs can keep a head gasket alive with triple-digit boost pressure in the mix (and no O-ring or fire-wing machine work performed on the head, or block). Again, we’re speaking in terms of increased horsepower, torque, and cylinder pressure applications here. Your 215 hp 6BT will be perfectly reliable, and likely never have a head gasket issue, if you stick with OEM head bolts.
Weak Link #4: Excessive Boost
Diesel enthusiasts know about the vast untapped potential that exists within the
mechanically injected 6BT Cummins, be it the Bosch VE or P7100 equipped version. Once wrenches are turned and fueling is increased, the boost pressure the turbocharger produces increases as well. On top of fueling mods, it’s also commonplace for 12-valve tinkerers to disable the OEM wastegate. This can allow boost levels to skyrocket, not to mentione drive pressure and turbine shaft speed. The Holset H1C, WH1C, and HX35W turbos employed on 6BT Cummins engines are tough, but they aren’t immune to overspeeding or contributing to a head gasket issue.
Permanent Solution: Limit Boost or Upgrade The Turbo
To avoid overspeeding the turbo and piling added strain onto the 6BT’s workload, it’s wise
to limit peak boost pressure. For the HX35W, 40 to 45-psi should be the maximum boost threshold. For the earlier H1C and WH1C units, a 35-psi peak should be adhered to. A highly fueled 5.9L Cummins with a factory turbo that’s on the verge of overspeeding is very hard on head bolts and head gaskets. Outside of those boost parameters for the OEM turbochargers, a larger turbo (or two) may be in the cards—and often is in performance applications.
Weak Link #5: Increased Coolant Temps
As with many inline-six engine designs, the rearmost cylinders on the 6BT Cummins are starved for proper airflow and (more importantly) coolant flow. Over time, and especially in engines turning out elevated horsepower levels, this lack of sufficient cooling takes its toll on the number 6 cylinder. Throughout the years, throngs of mechanics have traced scored up cylinder walls and piston ring damage back to excessive heat from poor coolant flow. Adding insult to injury, the added heat results in excess pressure, which pressurizes the cooling system and can even blow the freeze plugs out of the block.
Permanent Solution: Coolant Bypass System
Adding a secondary thermostat, which works off of temperature and not pressure, Fleece
Performance Engineering’s take on a coolant bypass system for the 6BT solves the toasty rear cylinder issue like no other. By functioning according to temperature, it solves the issue before pressure can become a problem. That means no possibility of losing a freeze plug (in the block or head) and experiencing rapid coolant loss. The Fleece system also ensures consistent temperature exists across all six cylinders. It’s ideal for everything from tow-rigs to sled pullers, and daily drivers to dedicated drag racers.
