Part 1: Short Block Shortcomings and Fixes

When introduced in 2001, there were plenty of naysayers worried that the 6.6L Duramax diesel would never hold up to the abuse and life it’d live in a 3/4 and 1-ton truck. As the years passed and performance technology evolved, it’s turned out to be one of the most durable and dependable engine platforms available to this day. Throughout its lifespan, the Duramax has gone through a few revisions to improve performance and drivability while reducing emissions. Some internal changes like different connecting rods, pistons and external changes like injector design, EGR coolers and VGT turbos have formed the latest Duramax (2001-2015 LML) into the best version yet producing nearly 400 hp from the factory. However, like any engine, there are always limitations and weak links. The Duramax engine can produce over 500 hp with basic bolt on parts like intake, exhaust, tuning, and a lift pump, but for those looking to take things a bit further, they’ll start to find the engines breaking points around 650 hp, give or take. At that point, a complete engine overhaul may be required and a lot of the stock parts will need to be replaced and upgraded with stronger product offerings from the aftermarket.

“The Duramax engine can produce over 500 hp with basic bolt on parts”

To go along with those performance parts, a good machine shop and precise assembly will be required, and since these diesel engines can be quite a bit different than the old school small block 350, finding a shop with previous Duramax experience can make or break the project before it even gets started. Custom Auto of Idaho Falls, ID, has been turning out some of the highest horsepower Duramax builds in the country for quite a few years now and was a simple choice when it came time to get the “insider look” at what goes into a performance-Duramax engine build. Owner James Brendle works closely with lead engine builder Danny Baird, who does 100-percent of the engine machining and assembly himself in-house at Custom Auto. Let’s tag along for pictures of a couple different engine builds they were in the process of putting together.

The start to every engine build starts with the engine block. This LB7 engine block has already been thoroughly cleaned inspected and machined with the in-house equipment at Custom Auto and is now ready for assembly of the short bock, meaning it’s time to install the camshaft, crankshaft, pistons and rods.
Each crankshaft, whether one out of a core engine or a brand new production unit goes through strict inspection and machining processes to ensure the main and rod journals are all within the proper spec to accept top of the line Clevite bearings and Carrillo connecting rods with the proper tolerances.
Danny Baird, of Baird Racing Engines works within the Custom Auto building and is basically the one-man-band on staff doing each particular engine himself from start to finish. This ensures nothing gets overlooked and each engine gets his utmost attention before it’s finished and shipped out the door. Here, Baird is installing the ARP main studs that will keep the crankshafts main caps in place. You’ll also notice the extra step Custom Auto takes with all their engines is the additional dowel pins added for the main caps, which helps prevent cap walk under higher power levels.
The crankshaft and main caps have now been placed into the engine block and are ready to be torqued down. Like every engine build, each specific cap and its bearings are all machined and measured to be used in a specific location.

Connecting Rods

The stock connecting rods within the Duramax engine are often the limiting factor when it comes to making horsepower. Each particular engine family has shown a different limitation with the LB7 being the weakest with common failure point being around 650 hp, give or take. There are a few that have been pushed further, but we’ve also heard of rods bending in basically stock applications as well. The latest LML version seems to have a little better rod as there are more and more pushing the 750 to 800-hp range without issue, but it’s still a playing-with-fire situation and upgrading is still suggested at that level.

Custom Auto has been using the CP Carrillo 100-percent machine forged connecting rods in their high horsepower builds for years. Proudly made right here in the USA, the Carrillo connecting rods have been proven to support well over 1,700 rear wheel horsepower.
Engineered, developed, forged and machined entirely out of their Irvine, CA, headquarters, the Duramax connecting rods from CP Carrillo are their top selling connecting rods with more than 3,800 units produced in just the last two years. The demand for these rods is higher than current production and keeping up during the busy months can be quite a challenge.
The forged connecting rod is turned into a complete work of art after a few hours of machine time on top of the line 5-axis CNC mills. Rods are machined, and bushings and the bushings (also made in-house) are installed. Then, sets of 8 are run through an in-depth inspection and balancing process to ensure each set will be weighted the same for optimum performance and durability within the engine.
Mahle is a pretty pseudonymous when it comes to pistons, as a major player in the gas market, Mahle has also been producing OEM pistons for many light duty diesels as well. The Mahle Motorsports division offers multiple versions for the Duramax application. Their stock replacement unit will be used in this particular build, but a Race cast piston along with some fully custom forged pistons are available depending on the engine’s purpose and power goals.
A freshly machined crankshaft awaits its turn being dropped down into the mains of an engine block. Carrillo rods have been assembled into the pistons and will soon find their home within the engine block as well. Of course, piston ring gaps, piston-to-wall clearance, and piston protrusion are all measured and checked to make sure each engine will perform at its absolute best.

CP Carrillo of Irvine, California, is one of the biggest players in the billet connecting rod game and produces 100-percent machine forged rods that are engineered and built right here in the USA. While being made of a much stronger material than OEM rods, the design itself helps increase its overall strength and allows them to support well over 1,700 hp as proven by Custom Auto in multiple different engines. While they can support major power, their balanced weight and durability also proves to work well in any Duramax engine, whether it be a heavy hauler or competition application.


The pistons in the LB7 models have proven to be some of the best through the different engine models, as the LLY and LBZ have experienced more cracking at the 600 to 700-hp level. There are a lot of engines that get built with stock LB7 pistons that just get some basic machining around the inner bowl to improve durability (AKA “de-lipped pistons”), but when going for more than 650-700  hp, it’s smart to look into a complete aftermarket replacement.

 “Forged pistons have proven to the strongest option, but they’re not known for the their longevity and are better off in competition-only engines”

For this particular LB7 build, the goal is 800 hp, while keeping great street manners, even at the higher 4,500-ft. the truck lives at, so keeping close to the stock compression ratio was required. Aftermarket Forged pistons have proven to be the strongest option, but they’re not known for the their longevity and are better off in competition-only engines, as the ring lands around the piston rings can wear out prematurely in a daily driver. So the cast versions from Mahle Motorsports are a great choice for a street application under say…1, 000 hp. The Mahle stock replacement and cast Race models offer a little better design compared to OEM and should improve durability in higher horsepower trucks.

One of the weakest and common points of failure within the Duramax engine is the camshafts locating dowel that aligns the camshaft gear. These dowels are prone to bending and shearing off in 550-plus horsepower applications. This pin shearing off can lead to catastrophic failure of the engine, so the weak dowel will be replaced with a much stronger keyway.
The Duramax engine has a pretty solid and consistent oiling system for all the internals, but this particular core engines had some pretty substantial damage to the cam lobes. While the engine showed no signs of problems when it was running, the flaking and pitted lobes would’ve just continued to get worse and eventually fail altogether. This could be from lack of maintenance, poor oil quality, lack of oil pressure, or even a lifter failure.
The new performance engine will be assembled with an aftermarket Stage One Alternate Fire camshaft from Empire Diesel of Dubuque, IA. In 800-plus horsepower applications, crankshaft failure can be more common than one would want; this is assumed to be due to the firing order of the engine creating unwanted harmonics within the crank. To combat this, the Alternate Fire camshaft will change the firing order to bring the harmonics back into sink and hopefully help save crankshafts from failing in big power engines.


The stock camshaft profile works quite well in most applications, but in the search for higher horsepower and better durability, there are some aftermarket options available. It’s also extremely common for the factory cam’s dowel pin that keeps the cam gear in place to bend or shear off completely. This pin breakage can lead to catastrophic engine failure, so the Custom Auto machining process always includes replacing the weak dowel design with a much stronger keyway system. Cams that offer more lift and more duration can help get more air in and out of the cylinders allowing for more power and the opportunity to help spool larger turbochargers.

One of the newest and most unique camshafts on the market for the Duramax application is the Alternate Fire Camshaft design, like that offered by Empire Diesel of Dubuque, IA. Changing the firing order with the camshaft to relieve stress on the front portion of a crankshaft in higher performance applications is becoming an extremely popular upgrade when building the Duramax engines. The factory firing is 1-2-7-8-4-5-6-3, but this new camshaft design changes that to 1-5-6-3-4-2-7-8. Reversing an engine’s firing order isn’t new science, however, as GM gas guys have been doing it for years in with the LS engine 4-7 swap. The Duramax family (LB7, LLY, LBZ, LMM, LML) can suffer from the crankshaft breaking just behind the snout back to the #3 rod journal, on everything from mildly modified daily drivers to full blown competition engines with internally balanced aftermarket billet cranks. There is just something within the engines harmonics at higher RPMs and bigger power levels that cause a lot of stress on the crank due to the factory firing order. The Alternate Fire Camshaft can help reduce those harmonics and prolong engine life.

– This quick schematic shows the comparison of a standard fire order Duramax with the Alternate Fire order developed through the use of their camshaft profiles. Along with the camshaft, the engines injector wiring will have to be altered to create the ignition in each particular cylinder when the camshaft indicates that it’s ready.

Short Block

The combination of Carrillo connecting rods, cast Mahle pistons and Alternate Fire camshaft makes for an extremely sturdy rotating assembly that can support substantial power, more than the truck this engine is going in will ever make—that’s for sure. But there are some other aftermarket parts that go into making a bullet proof short block. Clevite engine bearings are used throughout, along with ARP main studs to keep the main caps clamped down tight. One additional step Custom Auto takes is machining for dowel pins for the main caps, which helps keep “main cap walk” from happening. Obviously, all the engine blocks freeze plugs are replaced and external bolt-on pieces like the front timing cover, engine oil cooler, oil pan, rear adapter plate, etc. are fully cleaned and inspected before being installed.

Every engine will need to have the camshaft degree’d to make sure the crankshaft gear and camshaft gear are installed in their proper locations. It’s vital to the engine’s performance that these two pieces be timed together perfectly to ensure intake air enters the cylinders, pistons hit top dead center, and the exhaust valves open at precise moments so the engine makes maximum horsepower throughout the RPM range.

With the short block assembled, the part of the build will turn to finishing out the long block assembly and torqueing down some worked-over cylinder heads, upgraded injectors and injector lines, CP3 pump, exhaust manifolds and up-pipes and all the other engine accessories that go along with it. With the help of James Brendle and Danny Baird of Custom Auto, and the right combination of parts, this engine build will take a daily driven LB7 from its already fun 550-hp stock setup to over 800-streetable tire shredding horsepower. DW

Custom Auto, LLC

CP Carrillo

Mahle Motorsports

Empire Diesel Performance

ARP Bolts


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