It’s hard to say where GM’s heavy-duty pickups would be today if the Duramax had never been born. Back in the late 90s, the auto giant was desperate to get its hands on more than the dismal 3-percent slice it held in the domestic diesel truck pie. But after the 5.7L debacle of the late 70s and early 80s, and in knowing how dated and underpowered the 6.2L and 6.5L platforms were, GM knew its next oil-burning venture had to be a homerun. Turning to Isuzu, the Japanese vehicle and engine manufacturer it had been a longtime stakeholder in, the DMAX Ltd. joint venture was formed. Isuzu would handle the engine’s base design while GM would handle integration into its vehicles. As proof that GM and Isuzu got it close to perfect right out of the gate, the 6.6L’s basic architecture has remained the same for 20 years now. It still sports a deep-skirt, cast-iron block with induction-hardened cylinders, four-valve aluminum heads with six head bolts per cylinder, and clean-burning, quiet operating, common-rail injection. For a look at what puts the “dura” (for durable) in the Duramax name, we’ll examine the OEM hard-parts that make it possible in the following pages. Then, building upon an already-stout foundation, we’ll explore the time-tested aftermarket components that make GM’s 6.6L V-8 nearly indestructible.

There’s nothing glamorous about a block cast from gray iron, but (in addition to pleasing the bean counters at GM) the same overall design and material has proven highly durable in the case of the Duramax. It typically takes more than 1,000 hp to find the structurally weak points in the cylinder walls, but you need a vastly upgraded rotating assembly before you get anywhere close to that number. Throughout the Duramax’s production run, key areas of the casting have been beefed up, but the same 4.055-inch bore and 3.897-inch stroke stayed. It’s also worth noting that the Duramax’s 90-degree layout makes for a compact overall package, which eased the integration process into GM’s trucks and vans.
Unique to the diesel engines offered in the ¾-ton and 1-ton truck segment, the Duramax features induction hardening. Performed to significantly reduce cylinder wall wear from piston ring travel, the top two inches of each cylinder bore are treated to the distortion-limiting process.
Forged from 4340 steel, the crankshaft is supplied by Sumitomo in raw form. Once they arrive at the DMAX Ltd. plant in Moraine, Ohio, each crankshaft is machined, balanced, and then receives nitride (hardening) heat treatment. The process of nitriding provides a wear-resistant bearing face, along with excellent lubrication characteristics.
Over the years, rod and piston assemblies have undergone various changes, but the Duramax engine’s rotating assembly remains the lightest in the diesel pickup segment. The lightest con-rod assemblies GM used can be found in the LML (’11-‘16), with each one tipping the scales at approximately 2,370 grams (this includes rod, piston, wrist pin, and pin clips). By comparison, the LBZ and LMM’s con-rod assemblies checked in at roughly 2,522 grams apiece.
The duration of Duramax production has seen four-valve, aluminum cylinder heads employed throughout. Cast in Japan, the heads are finished at DMAX Ltd., which includes CNC machining for head bolts, valves, and injector hold-downs. To be sure, slightly different castings exist, with the latest versions bolted to the L5P showing the biggest departure from previous RPO code engines. The ’17-newer heads benefit from the largest intake ports ever offered, along with a shorter distance existing between the port and the intake valve.
Just as all Duramax cylinder heads are cast-aluminum, each version is attached to the block by way of six head bolts per cylinder, with sharing. Having six 12mm diameter fasteners per cylinder (18 bolts per bank) is a big reason why the factory head gasket can last 200,000 miles or more, even when seeing boost levels that are significantly higher than stock.
The Duramax valvetrain is more than sufficient at stock and even highly modified power levels. In particular, the rocker shaft system is exceptionally stout. The two forged-steel rocker shaft assemblies incorporate forged-steel rocker arms, each one utilizing a guideless bridge for valve operation. This system is so robust it’s been proven capable of living at power levels in excess of 1,500 hp and more than 100-psi of boost.
Not only did GM bring common-rail injection to the domestic pickup market, but in doing so it also brought cleaner emissions and introduced the first “quiet” diesel in the segment. Working together with a CP3 capable of producing 23,200-psi worth of rail pressure, the solenoid valve injectors offered the finest in-cylinder atomization technology available at the time, as well as the ability to fire off a pilot event before the main injection (hence the engine’s lack of injection clatter). In the generations of Duramax that followed, the engine would become even quieter and cleaner thanks to advancing injection technology.
Regardless of the pump or injectors downstream of it, the Duramax has always employed common-rail injection, from all the way back in 2001 through to present day. At the heart of the LB7-LMM Duramax engines’ fuel systems, you’ll find a Bosch CP3 (shown). With roots in the medium application world, the CP3 was and still is well-known for its reliability. Installed at the front of the lifter valley, it’s gear driven via the cam gear, but is neither timed to the crankshaft or camshaft. However, from ’11-’16 the Bosch CP4.2 that was employed in conjunction with piezoelectric injectors is timed with both the camshaft and crankshaft. Currently, a Denso HP4 (along with Denso solenoid valve injectors) is being used on the L5P platform.
In addition to pioneering aluminum cylinder heads, induction-hardened cylinders, and common-rail injection in its respective segment, the LB7 Duramax represented the first diesel engine to hit the 300 hp mark. For utmost quality control, every Duramax to ever roll off the assembly line at DMAX Ltd. has been hot-tested before leaving. In comparison, we know for a fact that Cummins doesn’t hot-test every engine it assembles at CMEP, but rather selects 6.7L’s at random for fuel-fired dyno testing.
You can’t talk about GM’s iconic 6.6L without mentioning the legendary Allison transmission that’s been offered behind it from the very beginning. On top of being the first manufacturer to offer a diesel engine packing 300 hp, GM also brought the first five-speed automatic to the ¾-ton and 1-ton truck segment. The release of the commercial-grade Allison 1000 predated Ford’s 5R110 TorqShift by two years and Ram’s 68RFE six-speed by more than six.


Being completely electronically controlled, coupled with the abundance of fuel the common-rail system brings to the table, you can squeeze 500-rwhp or more out of every Duramax built to date on tuning alone. The best way to do it? EFI Live. For well over a decade, its software and communication devices have allowed tens of thousands of Duramax owners to throw an additional 200hp or more at their trucks with few negative side-effects.
This one isn’t necessarily required on L5P models (as they come equipped with an electric in-tank lift pump), but installing a chassis-mounted electric lift pump on ’01-’16 versions of the Duramax is a big deal. On ’01-’10 engines (LB7, LLY, LBZ, and LMM, respectively), an all-in-one low-pressure supply system such as the kits offered by FASS, Fuelab, and AirDog help ensure the CP3 sees a steady 8-to-10-psi of pressure on the inlet side, which makes an already reliable pump even more durable. On ’11-’16 LML engines, an aftermarket lift pump system is a mandatory addition. Being equipped with the problematic CP4.2, a pump that can self-destruct if it’s exposed to air pockets, there is no downside to feeding additional fuel pressure (air-free, no less) to the failure-prone high-pressure CP4.2.
No matter the setup, any Duramax that sees 40-psi of boost or more on a regular basis is eventually going to have a head gasket issue on stock head bolts. For most daily-driven trucks in the 500 to 700 hp range, ARP’s standard head studs (ARP2000 material) are a great upgrade. For more serious builds, Custom Age 625+ head studs are recommended and feature a 260,000-psi tensile strength. Even more serious yet, 14mm studs are available through SoCal Diesel, though it requires drilling and tapping of the block and heads, and also calls for reaming the heads and modifying the head gaskets in order to accommodate them.
Under elevated boost, the restrictive two-piece Y-bridge on most Duramax mills is notorious for blowing apart. Thankfully, companies like Wehrli Custom Fabrication, HSP Diesel, and Deviant Race Parts produce 3-inch diameter Y-bridges that not only increase air volume entering the engine, but (thanks to their design) don’t blow out under big boost. As a bonus, opening up the Y-bridge’s size yields a 20-to-30hp gain on a truck sporting the stock turbo, pump, and injectors.
Given the fact that, depending on how much low-rpm torque is on the table, LB7 and LLY rods are on borrowed time between 550-600 hp and LBZ-LML rods are in the same boat around 650 hp, it stands to reason that stronger rods are incredibly popular in the Duramax aftermarket. Among the most popular rods are the forged-steel H-beam units from Carrillo and both variants offered by Wagler Competition Products: the company’s competition series I-beam forged rods and its budget-friendly, as-forged units. According to Carrillo, its Duramax rod is one of the company’s best sellers. Its H-beam rod (pictured here next to an OEM LBZ I-beam unit) checks in at roughly 1,100 grams apiece, which makes it lighter in addition to being at least three times stronger than stock.
This one is actually an aftermarket process rather than a part, but it’s highly recommended any time the crankshaft is removed. With the 5mm dowel pin known to fail in high horsepower applications (and all the destruction that occurs when the crank and cam get out of time), it gets removed, followed by a long keyway being machined into the crankshaft’s nose, and a steel bar welded into place. From this point forward, the balancer, cam gear, and reluctor wheel are all permanently secured. It should be noted that keying the camshaft is common practice as well.
As horsepower rises, main cap walk increases, so it stands to reason why so many Duramax owners opt for ARP main studs, whether their build ranges from mild to wild. Installing main studs will call for an align-hone of all five main bearing bores to ensure roundness, but that’s just one of the many machining processes that ensue during the course of a full-on Duramax build. For competition level engines, billet main caps or a main bearing girdle should also be considered.
In engine’s that see high rpm, the excessive pressure created inside the factory water pump can actually cause the impeller to spin on the shaft or drive it off the shaft altogether. For peace of mind in a daily driver, or a virtual guarantee that the water pump in your competition engine will never fail, a welded or keyed water pump is the answer. Merchant Automotive offers a drop-in water pump with both the impeller and drive gear TIG-welded to the shaft. As a bonus, Merchant includes a cast-iron impeller, not a plastic impeller like the one employed on mid-2006 and later engines.
Already outflowing the cylinder heads offered by the competition from the factory (approximately 180 cfm on the intake side), the aluminum heads on the Duramax can be opened up to flow an additional 35 cfm over stock with relative ease. In Wagler Competition Product’s ported aftermarket-cast versions equipped with significantly larger intake and exhaust valves, beehive valve springs, and a precision 5-angle valve job, maximum flow can be increased to as much as 320 cfm on the intake side and 270 cfm on the exhaust side. Just as they are with alternative firing order cams, Wagler and SoCal Diesel are very knowledgeable in this arena.
Although the nightmare that is crankshaft failure on high-powered Duramax’s hasn’t been eliminated, alternative firing order camshafts have helped cut down on them tremendously. By changing the factory firing order from 1-2-7-8-4-5-6-3 to 1-5-6-3-4-2-7-8, less stress is concentrated in the first rod throw area at the intersection of the second main journal, where the crankshaft typically breaks. SoCal Diesel and Wagler Competition Products both offer more than one alternative firing order camshaft.
Taking a Duramax beyond 550-rwhp calls for more fuel, and the aftermarket is more than capable of building high-flow injectors with factory-like reliability. The biggest names in common-rail fuel system components for the Duramax are S&S Diesel Motorsport and Exergy Performance, with each of them offering injectors fitted with EDM’d nozzles capable of flowing anywhere from 30-percent to 500-percent more fuel than stock (and even larger than that if you’ve got the air). This comprehensive injector size range supports everyone, from the enthusiast looking for 600 hp to the folks looking to go beyond 2,000 hp. The fact that they’re built and tested using genuine Bosch parts and equipment means you’re getting the highest quality performance injector on the market.
In support of the aforementioned injector upgrades available in the Duramax aftermarket, larger displacement (i.e. “stroker”) pumps have emerged, and in many instances even ruled out the need to run dual pumps. For example, S&S’s 14mm CP3 is capable of supporting a set of its 200-percent over injectors, which with the right turbocharger(s) in the mix can yield nearly 1,400 hp at the crank. The company’s 12mm CP3 can easily keep up with the demands of its 100-percent over injectors and allow for roughly 1,200 hp. 10mm stroker pumps are known to support 60-percent over injectors and somewhere between 750 to 800-rwhp.


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