Written By, Mike McGlothlin
B-series Cummins engines aren’t generally known for blowing head gaskets. In fact, even with high mileage the failure remains rare for either the 4BT or (and especially) its big brother, the 6BT. But that doesn’t mean it doesn’t happen. Age, added performance, abuse, or even a single overheat scenario can contribute to that vital head-to-block seal ceasing to exist. Of course, a combination of all of the above can lift a head, too. So whether you’re on the verge of dropping that take-out B-series into a project, are about to throw a few extra horsepower at your battle-worn 4BT, or you’re considering a complete performance engine build, it pays to brush up on the most common causes of a blown head gasket so you can avoid it.
Beyond age and structural integrity, maintenance neglect has a role to play in head gasket failure as well. And so can an owner’s failure to plan ahead in the form of using head studs as opposed to bolts (for improved clamping strength) and/or cylinder head O-rings (for better combustion sealing) in high boost, performance applications. Then there are the notorious cracked head issues that wield the potential to wreck any plans of performing a relatively-affordable head gasket repair. We’ll touch on all of that and more below, along with a word on how to get the best combustion seal possible.
Boiling Over: The Most Familiar Form Of Head Gasket Failure
Symptoms of a blown head gasket can surface in several ways, but seeing coolant exit the radiator overflow is a universal sign of the failure. In these cases, once a spot in the head gasket loses its perfect seal (be it due to the head lifting or the gasket’s fire-ring failing), compression is permitted to infiltrate the cooling system. The combustion gases pressurize the coolant, the coolant bubbles up and—once the pressure overcomes the rating of the radiator cap—overflows. Diagnosis of a blown head gasket can vary from easy to difficult depending on the severity of the failure and also the location of the failure on the gasket itself. For example, depending on the area of failure, compression can leak into the cooling system, or it can be leaked outside, into another cylinder, or even into the crankcase.
Age, Miles And General Use
Outside of stacking additional horsepower on top of a stock engine, age often plays a lead role in 4BT head gasket failure. High miles accumulated on the odometer or excessive operating time racked up on the hour meter have a lot to do with it, along with years (decades even) of heat cycles. This combination of contributors, which is par for the course in the life of an industrial-use diesel power plant, can bring an inevitable end to the useful life of the head gasket. This is precisely why we stress rebuilding a used 4BT prior to installing it. At the very least, a re-seal will help you gain some insight as to the engine’s overall health.
Cooling System Neglect
Surprise surprise, cooling system neglect is a leading cause of 4BT head gasket failure. Routine coolant level checks, inspection of radiator hoses, and ensuring the radiator sees adequate airflow may seem like trivial maintenance measures, but they can all contribute to a blown head gasket if ignored. Coolant loss leads to higher operating temperatures, which can take its toll over time, not to mention the fact that it’s often the root cause of a warped cylinder head. Regular checkups and replacement of your engine’s radiator hoses, thermostat, water pump, and antifreeze will limit the risk of exposing your 4BT to excessive heat—and the subsequent problems that go along with it.
Added Heat And Pressure
This is frequently the result of subjecting a second-hand 4BT Cummins to added stresses. Any time an aging engine is forced to cope with increased intake air, exhaust gas, coolant, and oil temps (which usually come from added fueling and boost pressure) this can be the result. Even something as simple as turning the maximum fuel screw on a rotary pumped version of the 4BT can push a high-mile engine to its breaking point. Blown head gaskets are frequent on engines equipped with the P7100 (a.k.a. P-pump) that’ve been treated to a significant advancement in injection timing. An earlier start of injection (SOI) creates increased cylinder pressure—pressure that the head gasket and head bolts have to contend with.
Why Blown Head Gaskets Are More Common In The 4BT Than The 6BT
Despite being designed with essentially all of the same parts (both internal and external), a blown head gasket is more likely to occur on a 4BT as opposed to a 6BT. But why? In the performance realm, our theory proposes it’s because most 4BT owners are starting with 105 hp as opposed to 215 hp (6BT). Because of this, there is a tendency to get overly aggressive with pump fueling and timing—the idea being to gain low-rpm drivability to better help get what is often a full-size vehicle up to speed more effectively.
With its displacement disadvantage, the 4BT can’t provide the effortless low-speed drivability that the 6BT can. To bring better drivability into the equation, some 4BT owners install a compound turbo arrangement. At this point, and because both boost and drive pressure are more than double the factory level, the OEM head bolts can no longer adequately keep the head clamped to the block. While the significant increase in cylinder pressure at low engine speed (i.e. torque) provides a great improvement in drivability, it often comes at the expense of a blown head gasket or (worse) internal engine damage.
For added holding strength in high boost, performance applications, the factory head bolts have to be upgraded. In comparison, head studs provide deeper thread engagement in the block, and the standard alloy steel studs offered by ARP (ARP2000, PN 247-4206) are a common go-to for 4BT builders. Their 220,000-psi tensile strength rating, as well as the shot-peening and heat-treating processes they undergo, makes them a solid choice for performance builds on a budget. For even more clamping force, ARP offers Custom Age 625+ studs, which boast a 260,000-psi tensile strength rating from the same diameter fastener. In milder horsepower engines, the factory head bolts will suffice and can even be reused if their measurements check out (i.e. they aren’t stretched).
O-Rings And Fire-Rings
In high performance engine builds, not only is it wise to run head studs but it’s also great added insurance to machine the head to accept O-rings or fire-rings. An O-ringed cylinder head calls for circular grooves being cut in the head for each cylinder, with those grooves accepting a pressed-in steel wire. While setting the head on the block, the wire lines up perfectly with the fire-ring in the factory head gasket, and the O-ring places added pressure on the fire-ring to better seal combustion. A fire-ringed cylinder head requires machining grooves into the head (or the head and block) to accept a thicker steel ring, and the fire-ring on the head gasket is eliminated. The result is (arguably) an even stronger combustion seal than what O-ringing provides.
Resurfacing The Head
Of course, any time you’re addressing a blown head gasket on a 4BT the head should be removed and checked out by a professional. One critical part of the inspection process exists in the practice of magnafluxing, which calls for a magnetic powder being used to illuminate cracks in the cylinder head. This step is always carried out before any machining of the head takes place. After magnaflux, and provided that the head is determined to be salvageable, the next vital piece of the head gasket repair puzzle commences: resurfacing. This is where achieving a roughness average (RA) within the Cummins-specified range of acceptability and proper flatness are crucial not only for good cold sealing, but a head gasket seal that lasts indefinitely.
What If It’s Worse Than A Blown Head Gasket?
In some cases, the symptoms of a blown head gasket are actually that of a cracked cylinder head. While it isn’t a worst-case scenario (that would be a dropped valve seat), it is more expensive to purchase a new head over simply reconditioning your original. However, finding a cracked OEM head is common on the 4BT Cummins. Again (and not unlike the causes of a blown head gasket), high miles and hours, decades of use, and abuse can all contribute to the failure. In some cases, the hairline cracks don’t warrant scrapping the head. In other cases, namely where the crack(s) protrude into the valve seat, replacing the head is the only remedy. Exposure to excessive EGT (exhaust gas temperature) is believed to be the biggest contributing factor to cracks appearing near or in the exhaust valve seats.