Diesel engines lack the spark of gas engines, both literally and figuratively. They don’t need a spark plug since they ignite the fuel using only compression, but also lack the ‘je ne sais quoi’ that inspires us to drive our cars. There are indeed diesel enthusiasts around, yet for most car lovers, gas is the way to go.

However, diesel engines are so much more applicable. The world’s smallest diesel engine — the Nano Bee — has a capacity of 0.006cc. The world’s smallest gas engine? 0.031cc. On the other end, the largest spark-ignited engine produces 25,000 hp, while the largest diesel is good for 107,389 hp. Both are made by Wärtsilä, a Finnish company that builds massive engines that power large ships.

Yet the main reason why diesel engines are so OP is their outstanding longevity. Diesels are workhorses, built from sturdier parts, which makes them inherently more reliable. They run at lower rpm, too, which further increases their lifespan. Now, sure, some diesel engines were a reliability nightmare, like the infamous Oldsmobile 5.7-liter V8. However, on this list you’ll only find tough and durable diesel engines that just won’t quit. Propelling everything from passenger cars to SUVs, trucks, and boats, these workhorses will accompany you for hundreds of thousands of worry-free miles.

Mercedes-Benz OM617 3.0L Inline-5 (1974 to 1990)

A few years ago, we found a 1979 Mercedes-Benz W123 diesel wagon with 782,000 miles on sale at Bring a Trailer — in mint condition. If you know just a bit about these cars, you’ll know that this isn’t too surprising: W123s are built like tanks, and that’s especially true for the OM617 diesel workhorse under the hood.

Yes, the naturally aspirated 3.0-liter inline-5 isn’t a speed demon. In the W115, the mill produced just 80 hp, though a revision to the cylinder head pushed that to 88 hp in the W123. Not that it mattered much — 62 mph still arrived in 17.8 coma-inducing seconds, and the top speed was 96 mph. Fortunately, Mercedes-Benz introduced a turbocharged version in 1977. The boosted version made 111 hp initially, though later versions were good for 125 hp and 184 lb-ft of torque. Still tame, but now good enough for the Mercedes W116 S-Class.

As underpowered as it may be, though, the OM617 simply refuses to die. Covering over 500,000 miles before a rebuild is very common, and many consider the OM617 a million-kilometer (620,000-mile engine). Heck, a Thessaloniki taxi driver covered an astonishing 4.6 million kilometers (2.85 million miles) with his W115 240D! Sure, he might’ve done a rebuild or two, but the fact that the engine still ticked is remarkable. It’s also surprisingly refined; cluttery, but smooth. I drove many OM617s over the years, and the smoothness always impressed me. That, and the mileage indicator. I’ve never driven one with fewer than 200,000 miles, yet the engine always ticked with the same mechanical precision.

Volkswagen 1.9L TDI inline-4 (1991 to 2009)

VW’s 1.9-liter TDI (Turbocharged Direct Injection) engine has a near-mythical status in Europe. You can still see Golf III and IV models equipped with this four-cylinder unit refusing to retire. With regular maintenance, they can go well beyond 300,000 miles, with some drivers using them for over 500,000 miles before a rebuild is necessary. That’s mightily impressive for a small turbodiesel not destined for heavy-duty applications. Sure, the 1.9 TDI isn’t without issues, but none of them will wreak havoc on the internals. Easily repairable oil leaks, faulty injectors, and a clogged EGR valve are common in most diesel engines.

Not all 1.9 TDIs are equal, though. In general, earlier versions are more reliable, particularly the ALH and AGR. However, we’d argue that units equipped with the more advanced ‘Pumpe Düse’ direct injection system offer the best balance between performance, fuel economy, and durability. These engines had much higher injection pressures (2,050 bar/29,700 psi), which resulted in best-in-class power and efficiency figures during the 2000s. Notably, the 2001-2003 VW Golf IV 1.9 TDI produced 150 hp and 236 lb-ft of torque, went from zero to 62 mph in 8.6 seconds, had a top speed of 134 mph, and its fuel economy was 44-mpg combined. Impressive stuff.

But the 1.9 TDI is also one of the best production engines ever made because of its tuning potential. Ah, yes, you didn’t see this coming. Simple ECU remaps bring the 150-hp version to a hot-hatch-rivaling 204 hp and truck-like 322 lb-ft of torque! For reference, that’s more than the Golf IV GTI 1.8T engine’s 178 hp and 173 lb-ft of torque.

Cummins 6BT 5.9L Inline-6 (1989 to 2007)

In 1989, Dodge started offering a 5.9-liter 12-valve turbodiesel engine in the first-gen Ram truck. Little did they know at the time that the Cummins-sourced 6BT unit would become the greatest truck engine of all time.

You can argue with that statement, sure, but the facts prove our point. For instance, the diesel-equipped Ram has been a sales miracle since its launch. But of course, buyers still didn’t know about 6BT’s reliability at that time. They bought it because it was powerful and efficient. As the first truck engine equipped with direct injection, turbocharger, and an air-to-air intercooler, the 5.9-liter produced 160 hp at 2,500 rpm, and a meaty 400 lb-ft of torque at just 1,700 rpm. But that was just the beginning. Cummins refined the formula over the years and incorporated advanced tech like Common Rail direct injection. As a result, the latest 6BT produced 325 hp and trailer-pulling 610 lb-ft of torque.

Then there is the fact that the 6BT is as stout as it gets in an internal combustion engine. It’s a relatively simple engine with a sturdy cast-iron block and head, factory-installed oil cooler, and oil pump cavity for durability. The 6BT’s internals are very sturdy, too; Cummins opted for a forged-steel crankshaft and rods, induction-hardened fillets and journals, and cast-aluminum pistons. Unsurprisingly, most 6BTs last hundreds of thousands of miles before rebuilding. Still, 1994-1998 6BT engines are the most sought-after, mainly thanks to the super-durable Bosch P7100 P-pump, which lasts over 500,000 miles. Moreover, the P-pump can supply a lot of fuel, allowing 1,000-plus-hp tunes of this amazing diesel engine.

Navistar-Ford 7.3L Power Stroke V8 (1994 to 2003)

What Cummins did for Dodge with the 6BT, Navistar did for Ford a few years later with a potent 7.3-liter V8 turbodiesel. Destined for Ford’s heavy-duty trucks and International trucks, the Power Stroke V8 was appreciably more potent than its inline-6 rival, initially producing 215 hp and 425 lb-ft of torque, and 225 hp/450 lb-ft two years after its launch — hardly surprising given the much larger capacity. Still, Navistar used all the tricks up its sleeve to make this V8 more competitive, including a HEUI hydraulic injection system, large injectors, an air-to-air intercooler, and a Garrett TP38 turbocharger.

Yup, the Power Stroke V8 was advanced for its time. However, it was also simple and built from overengineered parts. The block and head were built from cast gray iron and tightened with six bolts per cylinder. The pistons were made from cast aluminum, the crankshaft and steel rods were forged steel, while the valve lifters were hydraulic. Caterpillar’s HEUI injection system was also more reliable than Common Rail, despite offering similar injection pressures. Even the injectors, which often fail on other diesel engines, were very durable. To top all that, most components were relatively unstressed — the Power Stroke V8 never went beyond 275 hp and 525 lb-ft of torque.

Sure, Ford 7.3 PowerStroke V8 owners still face common issues, like a faulty camshaft position sensor and leaking exhaust manifold up-pipes. What they don’t face is a clogged EGR valve, as this engine had none. Overall, the 7.3 PowerStroke V8 will last for between 400,000 and 500,000 miles before a rebuild is necessary.

Toyota 12H-T 4.0L Inline-6 (1985 to 1990)

Toyota has a long lineage of diesel engines that just won’t quit: 1HZ, 1HD-FTE, 1VD-FTV, 13BT, 2B, and 3B, the list goes on and on. So, we played ‘duck, duck, goose’ and chose the 12H-T at random. It’s a joke, of course: this 4.0-liter turbocharged inline-6 deserves a place on any list of the most reliable engines.

With a cast-iron block & head, forged crankshaft, and mechanical fuel pump, the 12H-T was designed as a workhorse engine to power the facelifted HJ61 Land Cruiser, launched in 1985. However, it replaced the seriously underpowered 2H naturally aspirated diesel from the pre-facelift model, which produced just 102 hp. The 12H-T is no powerhouse, but with 134 hp at 3,500 rpm and 232 lb-ft of torque at a relatively low 1,800 rpm, it had a much easier job moving the Land Cruiser 60 around.

Sure, Toyota’s 1985 Land Cruiser 60 for 2025’s SEMA may run circles around it, but the Tundra-sourced 3.44-liter twin-turbo V6 won’t be nearly as reliable. The 12H-T, on the other hand, can survive north of 400,000 miles with regular maintenance. The unfortunate part is that it was only five years in production, so parts are scarce. So, if you want to build a project Land Cruiser, we’d recommend going for the 1HZ. This naturally aspirated 4.2-liter inline-6 is slightly lower on power (129 hp and 210 lb-ft of torque), but Toyota still sells it in Africa, so parts are readily available.

International-Navistar DT466 6.8L to 7.6L Inline-6 (1971 to 2016)

Few engines are as widespread as the International-Navistar DT466. This direct-injection diesel engine probably powered the school bus when you were a kid, as well as the tractor that ultimately brought food to your table. Heck, the DT466 did that for over four decades. The reason: it was so good and overengineered from the beginning.

See, International made a few decisions when designing this mill that would turn it into an unstoppable beast. The DT466’s deep-skirt block was made from sturdy grey iron, while the cylinders were dressed in plateau-honed wet sleeves. The wet sleeves protected the cylinders and improved cooling, but were also made from cast iron with a high chrome content for quicker break-in. The forged steel crankshaft was induction hardened for maximum toughness, while the cast-iron head housed hardened valve seats and ductile-iron rocker arms.

Over the years, Navistar continued to improve on the formula initially developed by International and gave us the diesel overlord: the MaxxForce DT466E. The newer 7.6-liter unit produced 300 hp and 860 lb-ft of torque, improving on the older units quite significantly. Crucially, though, it had a main cap girdle that made it basically indestructible, even when tuned. Pro Farm IH dragster tractors are often powered by the MaxxForce DT466E and produce beyond 1,200 hp. This high-horsepower diesel dragster, according to its makers, pushed the DT466 to 3,000 hp, for heaven’s sake! Yup, that might be an exaggeration, but the dragster still ran an impressive 7.62-second 1/4 mile.

Caterpillar C15 15.2L Inline-6 (1998 to 2007)

Caterpillar’s C15 did similar things for heavy-duty applications as the DT466 did for the medium-duty category. A true powerhouse, this 15.2-liter turbocharged inline-6 offered unmatched reliability, power, efficiency, and versatility, and was indispensable for the long-haul trucking industry. Thanks to its toughness and longevity, though, the C15 was also used in power generation, industrial & construction equipment, agricultural vehicles, boats, and more. In other words, you can run most of your fleet using this engine, which further simplifies the C15’s low-maintenance requirements.

The C15 is a million-mile engine with regular maintenance. That’s in no way shocking if you look at the materials Caterpillar used to construct this engine: Forged steel connecting rods and crankshaft; monotherm pistons; gray cast iron block with a deep skirt design, and high-volume oil and water pumps that ensure each of these parts is lubricated and cooled properly.

The most exciting part is that Caterpillar didn’t sacrifice performance for longevity. Depending on the generation and application, the C15 produced between 375 and 625 hp, and up to 2,050 lb-ft of torque! Because it’s so popular, the C15 also enjoys excellent aftermarket support, with numerous tuning upgrades that can push it beyond those figures. Even a simple ECU update will extract 750 hp and 2,460 lb-ft of torque, bringing the C15 in line with more modern long-haul diesel engines. With some upgraded components, like a performance head from HHP, you can push the C15 to 1,000+ hp.

Gardner 6LXB 10.45L Inline-6 (1966 to 1979)

It’s okay if you’ve never heard of Gardner & Sons, because the brand ceased to exist during the 1990s, after failing to embrace turbochargers as the hot new thing in diesel engines. However, Gardner once ruled the British industry with innovative diesel engines that surpassed their rivals in almost every way. Perhaps the best example of that is the 6LXB, a 10.45-liter naturally aspirated diesel engine that powered everything from buses and trucks to ships, power generators, construction equipment, and mining machinery.

Depending on the application, the 6LXB was available in 127 hp, 150 hp, and 180 hp tunes. Yes, not too impressive from today’s perspective, but it was the delivery that impressed the most. The 6LXB produced maximum torque from just 975–1100 rpm and peak power at 1,850 rpm, just a few hundred revolutions lower than the competition. All the while, the Gardner 6LXB proved impressively frugal, thanks to the 40% thermal efficiency.

All these excellent figures were packaged in a sturdy cast iron block and cylinder head, which gave the 6LXB unmatched durability. The internals were overengineered, too. Notably, the crankshaft main bearings were horizontally and vertically braced for outstanding rigidity, while the crankcase was made from a tough cast alloy. On top of that, Gardner never embraced mass production. Each unit was crafted by one skilled worker, so it’s not surprising that 6LXBs often last hundreds of thousands of hours and are still widely used today. Thus, next time you argue with someone that ‘they don’t make them like they used to,’ make sure you mention the unkillable Gardner 6LXB.