The term bulletproof gets kicked around quite a bit in the world of heavy, light, and medium duty diesel engines. It’s a word that invokes a feeling that a particular power plant is capable of enduring any amount of abuse, lasting indefinitely, and one that’s indestructible, even unkillable. Unfortunately, every mechanical thing on planet Earth has a failure point—which means that no internal combustion engine is bulletproof. However, some prove more robust than others, and while these over-engineered, near-masterpieces aren’t bulletproof, they often earn the bulletproof moniker thanks to their proven, real-world durability.

But what does the term “bulletproofed” mean, and when is it applied? In our experience, the bulletproofing of an engine occurs when most (if not all) of its factory shortcomings have been addressed. The best example we can give here would be the 6.0L Power Stroke, the Navistar-sourced engine for Ford trucks that’s well-known for its extensive factory weak links. It was plagued by EGR, oil, fuel, coolant, and turbo problems, but with a laundry list of aftermarket upgrades performed, the problematic 6.0L can become a very reliable platform. Using the troubled 6.0L and a few highly reliable OEM power plants as examples, we’ll explain the difference between “bulletproof” and “bulletproofed” below.

What Is A Bulletproof Diesel Engine?

A bulletproof diesel engine is one that is extremely durable diesel engine in factory form. It is here that engines like International’s DT466, the Cummins 6BT, or Detroit Diesel’s  Series 60 come to mind. These workhorses, used in the medium-duty, light-duty and heavy-duty truck segments, respectively, enjoy legendary-like status when it comes to durability. What do they all generally have in common? They were well-engineered, overbuilt, and turn out relatively low yet adequate horsepower and torque for their given application(s)—perfect ingredients for long-term dependability. Now let’s get specific about the nuts and bolts that can help a bulletproof diesel engine earn bulletproof status.

What Makes A Factory Engine Nearly Indestructible: Burly Parts

On top of a simplistic design, such as an inline-six configuration (which enjoys natural balance), big, brawny hard parts play a major role in a bulletproof diesel engine’s ability to survive the cylinder pressure it sees. As you may already know, big torque output means high cylinder pressure. This is why deep-skirt and thick, cast-iron blocks are commonplace, along with heavy-duty, forged-steel crankshafts and connecting rods, pistons with provisions for ample cooling and lubrication, six head bolts or more per cylinder, and large diameter main, rod, and head fasteners. Of course, wet sleeve engines enjoy further durability, not to mention cost-effective overhaul capability.

What Makes A Factory Engine Nearly Indestructible: Big Bearings

Complementing those big, burly internals are bearings with large surface areas, namely the engine’s main and connecting rod bearings. Not only do large bearing surfaces lend themselves to longevity, but when combined with low rpm operation and sound  maintenance their dependability is practically guaranteed for life. Most inline-six diesels also incorporate seven main bearings (vs. just five main bearings in V-8 configurations), which means that each connecting rod benefits from having its own rod bearing journal (and that is separated by a main bearing journal). It’s no wonder bearing failures are so rare on the legendary engines like the DT466, 6BT, and Series 60.

What Makes A Factory Engine Nearly Indestructible: Lack Of Electronics

Nothing complicates an engine faster than adding electronics. And while electronic controls have served to improve horsepower and torque while simultaneously reducing emissions in the modern era, ECM’s and other electronic modules eventually fail, and wiring issues can be difficult to diagnose and expensive to repair. This is why most, not all, bulletproof diesel engines are graced with very little in the way of electronics. Mechanical injection pumps and fuel injectors, gear-driven lift pumps, and fixed geometry turbochargers keep things extremely simple, with no electronic solenoids, control modules, or VGT actuators required for their operation. Most mechanical fuel injection components can last upward of 400,000 miles, with a fixed geometry turbocharger living longer than that—if not just as long as the engine itself.

What Makes A Factory Engine Nearly Indestructible: Lack Of Emissions Equipment

When you consider how crippling the failures associated with modern age emissions control equipment can be, it makes sense that older, pre-emissions engines last longer—hence their being more likely to earn bulletproof diesel engine status. After all, an engine that’s free of exhaust gas recirculation (EGR) isn’t subjected to soot, grime, and carbon buildup throughout the intake tract, soot contaminating the engine oil, EGR valve and cooler failures, and engine coolant working overtime to keep both the EGR system and the engine running cool. Diesel particulate filter (DPF) and selective catalytic reduction (SCR) systems only complicate matters further on ’07-newer engines, each technology bringing with it additional points of failure.

Example #1: International DT466

This entry refers to the mechanically injected version of the International DT466 built from 1971-1993. Its robust general construction included a deep-skirt, cast-iron block, a forged-steel crankshaft with sizeable main and rod bearings, a block stiffener for added main cap anchoring, plateau-honed wet sleeves, and forged steel connecting rods that weigh a whopping 6.4 pounds apiece. The cast-iron cylinder head was secured to the block via six bolts per cylinder—and rumor has it that the DT466 can be pushed to 1,200 hp before a blown head gasket becomes a concern. To be sure, the DT466E produced from 1994-2003 was also durable, but its electronically controlled, HEUI injection system didn’t quite enjoy the same longevity the earlier, mechanical injection system did.

The DT466’s Few Faults

Overbuilt in nearly every way, hard part failure is highly uncommon with the DT466. In fact, major failures of any kind are extremely rare. The biggest weak link with this bulletproof diesel engine is its propensity of developing leaky fuel return lines—more of a headache than a major mechanical problem. With age, coolant leaks become more frequent also, which can lead to the overheating issues you may have heard about on older, higher-mile versions. The combination of low coolant level, a partially blocked radiator, and working the engine hard in high ambient air temperatures is the most common method of overheating a DT466.

Example #2: Cummins 6BT

Designed for hard use in everything from construction equipment to marine vessels, it’s no wonder the 5.9L Cummins 6BT became legendary in Dodge Ram applications. Like the DT466 mentioned above, it too utilizes a deep-skirt, cast-iron block, a forged-steel crankshaft (with induction hardened journals and fillets), and forged-steel (I-beam) connecting rods. Despite being a parent bore block (no wet sleeves), a properly maintained 6BT can easily last 600,000 miles, and many have even made it to the million-mile mark. Topping things off, the fixed geometry Holset HX35W turbo that many 5.9L’s came equipped with is arguably tougher than the engine itself.

The 6BT’s Few Faults

Believe it or not, the nearly-bulletproof diesel engine Cummins 6BT has a potentially fatal flaw: the killer dowel pin (or KDP). If it rears its ugly head, you could be out an engine. However, if the little, 8mm dowel never works itself free from the block (or is secured in place via preventative measures), the 5.9L will rack up hundreds of thousands of trouble-free miles. An arguably more common issue, found on P7100-equipped (i.e. P-pumped) 6BT engines, exists with the P7100 overflow valve. When its internal spring becomes worn or breaks, inadequate fuel supply pressure makes it to the injection pump. Last but not least, some ’96-’98 model year 6BT’s were equipped with EGR. As you can imagine, these engines, built specifically for use in California, did not fare well with EGR in the mix.

Example #3: Detroit Diesel Series 60

Another engine believed by many to be bulletproof is the Detroit Diesel Series 60. It also thrived in the era that predated EGR, DPF’s, and SCR, and it too featured an overbuilt, million-mile design. The Series 60 employs a forged-steel crankshaft with large main and rod bearings for a long lifespan, its beefy, forged-steel connecting rods are shot-peened for added fatigue resistance, and it boasts John Deere-inspired “iron crosshead pistons.” The cylinder head, which was also designed with John Deere input, features very short intake and exhaust ports to minimize pumping losses. Other highlights include eight head bolts per cylinder, injector rocker arms equipped with ceramic rollers, and the fact that its electronics proved very reliable in an age when electronically controlled diesels were in their infancy.

The Series 60’s Few Faults

It’s a very short list but, like any engine, the Series 60 had its shortcomings from the factory. In particular, the pulse-recovery exhaust manifold they came equipped with was prone to cracking. Low boost and drive pressure, poor overall performance, and a mess of soot all over the passenger side of the engine are common side effects of this fairly common Series 60 issue. Other than that, low oil pressure—especially at idle—is known to take out main bearings. Luckily, this type of catastrophic issue is typically reserved for high-mile, or neglected engines.

What Is A “Bulletproof’ed” Engine?

A bulletproofed diesel engine is one that has had its factory shortcomings addressed, and oftentimes solved for good. Usually, the areas of remedy come by way of the aftermarket, but sometimes the OEM steps up and offers a sound fix-all. A recent example of the latter includes Fiat Chrysler Automobiles’ (FCA’s) handling of a high-pressure fuel pump problem on ’19 and ’20 model year, 6.7L Cummins-powered Ram heavy-duty trucks. With the Bosch CP4.2 pump having a tendency to fail catastrophically, the automaker not only changed back to the tried-and-true Bosch CP3 pump on ’21-newer models, but it recalled over 200,000 ’19 and ’20 Ram trucks to have a CP4 to CP3 pump swap performed at no charge to the customer.

The 6.0L Power Stroke

014 6.0L Power Stroke Diesel V8 Ford Super DutyIn the world of light-duty diesels, aftermarket upgrades performed to improve the reliability of the 6.0L Power Stroke V-8 really helped “bulletproof” terminology take off. Now, saying your 6.0L has been “bulletproofed” lets others know—or at least implies—that the engine’s biggest factory weak links (there were a handful) have been addressed. At a minimum, it conveys that the 6.0L’s EGR and oil cooler issues have been solved, and that its head bolts have been replaced with head studs. However, there are many more areas that require attention in order to truly make a 6.0L Power Stroke bulletproof. Here’s the complete list.

Oil Cooler Failure

Although the 6.0L Power Stroke is perhaps best known for blown head gaskets and EGR failures, its oil cooler is its biggest (and most commonly experienced) problem. Many don’t know that a cracked EGR cooler can be traced back to the factory oil cooler more than 90-percent of the time. This is because the microscopic passages within the liquid-to-liquid heat exchanger plug up due to a combination of suspended solids from the antifreeze and casting sand from the crankcase and heads. The blockage prevents adequate coolant flow from making it to the EGR cooler, causing it to superheat and eventually rupture.

Oil Cooler Fixes

The ultimate fix for the 6.0L Power Stroke oil cooler problem was developed by Neal Technologies, Inc., the corporation that owns Bulletproof Diesel.com, and it entails the installation of a remote-mount, air-to-oil cooler. The Bulletproof Diesel engine oil cooler essentially routes hot engine oil out of the lifter valley (the location of the factory oil cooler), through the air-to-oil heat exchanger, and back to the engine, which cools engine oil temperature and also eliminates the restriction of the engine coolant having to flow through the OEM oil cooler. Another common remedy is to start with a fresh, OEM oil cooler and add an aftermarket coolant filtration system at the same time. The replaceable coolant filter keeps contaminants from plugging up the passageways within the factory oil cooler’s heat exchanger.

EGR Solutions

When bulletproofing a 6.0L Power Stroke, the exhaust gas recirculation system should be the second highest priority (behind the oil cooler). Once the oil cooler has been addressed, the likelihood of the EGR cooler cracking drops considerably. However, exhaust flow through the EGR cooler will still be reduced over time due to soot and grime buildup. Your local emission laws and regulations will likely dictate that the EGR cooler (and its corresponding EGR valve) can’t be removed. If removal isn’t an option, there are aftermarket EGR coolers that outperform the factory version, such as the units offered by Bulletproof Diesel. The company’s EGR cooler holds twice as much coolant, is TIG-welded, and utilizes stainless steel tubing to better resist fatigue.

Ruling Out Blown Head Gaskets

It’s well known that the 6.0L Power Stroke is synonymous with blown head gaskets, but this problem is actually the fault of the factory torque-to-yield head bolts, which tend to stretch when the engine is worked excessively hard or sees additional cylinder pressure (courtesy of added horsepower and torque). Adding insult to injury, the engine’s design boasts just four head bolts per cylinder (with sharing) vs. six on the 7.3L that came before it and the 6.7L that later followed it. The ultimate fix? Replace the head bolts with ARP head studs—a process that, if performed correctly, entails resurfacing the heads, checking the block’s deck surface for flatness, and using factory spec, MLS head gaskets.

The VGT Fix

Sticky or completely stuck turbo vanes are routine on 6.0L Power Strokes that are seldomly driven, spend inordinate amounts of time idling, or that see very little “spirited” driving. In these conditions, the unison ring, which moves the turbine vanes in a uniform fashion, succumbs to rust and corrosion. As a result, turbo responsiveness suffers. When the vanes stick in the closed position, the engine has ample low-rpm response but has virtually no midrange or top-end power. When the vanes seize in the open position, the engine has no bottom end response, some midrange power, and runs relatively normal at high rpm. The aftermarket solution for this common problem is to install a high-quality stainless steel unison ring, such as the corrosion resistant units offered by KC Turbos.

Stiction Solutions For Fuel Injectors

Fuel injectors are yet another point of concern on the 6.0L Power Stroke. Specifically, the phenomenon known as “stiction” is highly frequent. It occurs when varnish and sludge build up on a vital internal component known as the spool valve, the spool valve being responsible for allowing pressurized engine oil from the rail into the injector to fire it. This buildup hinders the spool valve’s ability to open and close effectively (and quickly), causing hard starts, rough idle, engine missing, poor performance, and excessive smoke. The solution is simple: 1) run a high-quality engine oil (typically a 5W-40 synthetic, not conventional 15W-40), and 2) add a stiction-fighting oil additive such as Stiction Eliminator from Hot Shot’s Secret or Rev X’s Stiction Fix at every oil change.

Completely Redesigned High-Pressure Oil Pump

Hydraulically actuated fuel injectors definitely have their drawbacks when the high-pressure oil pump (HPOP) that’s required for the system to work properly is notorious for dying suddenly. Two different HPOP’s were employed on the 6.0L Power Stroke, with both the early (’03-‘04) and late (’04.5-‘07) version being prone to failure in out-of-the-blue fashion. DieselSite.com developed a redesigned, direct replacement HPOP, complete with a revised housing and upgraded internals, that’s proven highly reliable. Unfortunately, it’s only available for ’03-’04 engines with the early style pump. To date, there is no real way to bulletproof a later style HPOP. All we can say is that keeping fresh, clean oil in the engine is always a good idea.

FICM Remedies

Looking at the fuel injection control module (FICM) on the 6.0L for a moment brings us to the most common failure point on the electronics side of the engine. The FICM is responsible for sending a 48-volt pulse to the injector coils in order to fire them. Unfortunately, the logic side (i.e. power side) of this module is known for dying an early death—and believe it or not, dying batteries are the leading cause of a dead FICM. Repairs include splitting the FICM open and replacing the logic board, shipping it out to have it repaired, and replacing the half shell (logic board side only) with a new one, but the closest thing to bulletproofing the FICM is by purchasing a quality, high voltage aftermarket unit with upgraded internals. The unit shown here is offered by Bulletproof Diesel and boasts a completely redesigned internal circuit board.

A Stronger, Longer-Lasting Water Pump

As if the 6.0L Power Stroke didn’t have enough cooling issues, its factory water pump features a plastic impeller that is prone to cracking at the hub, leading to coolant flow issues as well as overheating. Once again, Bulletproof Diesel came to the rescue, developing a replacement water pump with an aluminum impeller that’s stiffer, stronger, and that won’t crack or flex under heavy load. This also means less cavitation and erosion within the 6.0L’s front cover. Available in both a 90mm (’03-early ‘04) and 100mm (’04.25-‘07) version, the Bulletproof water pump sports a precision-machined, billet housing, a top-quality bearing assembly, and a high grade seal for maximum longevity.

Cracked Oil Branch Tubes

On the 6.0L Power Stroke, steel branch tubes route high-pressure oil from the HPOP to the oil standpipes, which connect to the oil rails that feed oil to the fuel injectors. Due to vibration, these hard lines sometimes develop hairline cracks, which inevitably leads to long-crank, hot restart scenarios and the injection pressure regulator (IPR) having to ramp up duty cycle percentage in order to compensate for the loss of injection pressure from the leak. Unfortunately, there is no permanent fix for this. Fortunately, it’s nowhere near as common as the everyday STC fitting failure (where late model engines are converted back to the earlier, threaded style fitting for a forever fix), and OEM replacements are the go-to here.

The Fix For Leaking Oil Standpipes

High-pressure oil leaks are rampant on the 6.0L Power Stroke, and this is a common culprit: leaky oil standpipes, the pieces present on ’04-’07 engines (not ‘03s) that tie the oil branch tubes in with the oil rails in the cylinder heads. Specifically, the O-ring at the top and/or the O-ring at the bottom of each standpipe is notorious for failing, causing leaks within the high-pressure oil system. The ultimate solution for this comes from the aftermarket. DieselSite.com’s reengineered standpipes use a shaft that accommodates a back-up O-ring for every parent O-ring (for insurance), and the company also spec’d its O-rings with the proper durometer (hardness). Additionally, the company doubled the O-ring count on the inside of the standpipe’s shaft and altered the tolerances to properly match the O-rings.

The Dummy Plug Fix: Better O-rings

Now on to the oil rails, where further high-pressure oil leaks often occur due to O-ring failure(s). Here, we’ll zero in on the dummy plug O-rings which, due to being aged, cut, or torn, can begin a small leak in the system. Again, hard starting when the engine is hot and getting stranded until the engine oil cools back down is par for the course. Just as the company did with its reengineered standpipes, DieselSite.com developed higher quality O-rings than OEM (the OEM-sourced replacements having gradually decreased in quality over the years) with improved hardness and that better stand up to the heat and pressure these O-rings are constantly exposed to.

The Cure-All For A Cracked Coolant Reservoir

Although not as devastating as blown head gaskets, a cracked degas bottle is a common headache many 6.0L Power Stroke owners face at least once. Typically, the factory degas bottle will leak from a split that develops in the seam, but they can also crack on the flat areas on the bottom of the reservoir. Cracks often develop due to age, as the plastic becomes more brittle. A quick fix is to throw an OEM replacement in. However, the permanent, near-bulletproof solution is to source and install an aftermarket, TIG-welded, aluminum degas bottle from a company like Mishimoto, Blessed Performance, or SPELAB.

Bulletproofing Isn’t Exclusive To The 6.0L Power Stroke

Make no mistake, the term bulletproofed can be applied to any engine that has had its factory shortcomings addressed. In fact, many of the same aftermarket names mentioned in this article offer problem-solving parts in the medium-duty and heavy-duty segments as well. A few examples include DieselSite.com’s high-pressure oil pump offerings for the Cat 3126 and International DT466E, Bulletproof Diesel’s nearly indestructible EGR coolers for Cummins ISX15, ISX12, ISL/ISC, and even ISL-G engines, and ARP’s head stud kit to solve the stretched head bolt problem on Cat C15’s. Even replacing a factory VGT with a tried and true, fixed geometry turbo or replacing the B-series Cummins’ press-in cylinder head freeze plugs with threaded versions (pictured) can be regarded as a bulletproofing measure.

How You Can Bulletproof Your Own Engine, From Top To Bottom

If you find yourself starting from the ground-up, your blueprint for going a million miles should look like this: Properly clean, inspect, and magnaflux the block and head(s), enlist a machine shop that specializes in your specific engine, source all new parts from reputable suppliers, balance the rotating assembly, insist on a precision valve job and, if possible, opt for oversize and hardened valve seats. During engine assembly, perform all work in a clean room, use the proper assembly lubricant (and/or the one supplied or specified by the fastener manufacturer), torque all fasteners (be they bolts or studs) to the correct torque spec—and make sure your torque wrench is calibrated beforehand. Also measure and then re-measure all clearances, and properly gap all of your piston rings. Attaining true bulletproof status with an engine may technically be unachievable, but following these crucial steps will get you as close to it as possible.