Building A Better Duramax

Since 2012, D&J Precision Machine has been the place to go for high quality replacement Cummins engines. Between the company’s successful Heavy Hauler and Ready Run programs, its performance series 5.9L and 6.7L’s, or its legendary Enforcer series, D&J has a solution for any late-model, Cummins-powered Ram. Now, the Cambridge, Ohio engine specialist has expanded its portfolio to include the Duramax—and it begins with the LML platform. D&J developed two long block options for ’11-’16 GM owners: Essentials and Precision Plus. The Essentials long block offers a value-driven choice while the Precision Plus adds select upgrades for improved sealing, retention, and oil cooling—along with providing a more completely assembled engine option.

First and foremost, both long blocks address one of the LML’s most notorious weak links: crankshaft failure. Each option comes with a keyed (and reconditioned) crankshaft and camshaft, as well as remanufactured heads with re-cut valve seats and new valves. From there, the Precision Plus adds ARP head studs, a higher flowing oil cooler and a pinned oil pump, as well as the rocker assemblies, valve covers, intake manifold, harmonic balancer, and flex plate. Whether you need an engine for your daily driver, your weekend play toy, or the tow rig you depend on for a living, D&J’s LML long blocks are designed to get you back up and running quickly and reliably.

D&J Precision Machine’s Essential package gets you an LML long block that’s been CNC blueprinted, bored, honed, and deck surfaced for precise cylinder geometry and proper piston fit at its state-of-the-art, southeast Ohio facility. With it you get a crankshaft that’s been checked for straightness and size (and repaired as needed), new, OEM-spec Tier 1 pistons and rings for a proper fit, seal, and performance, fresh bearings and thrust washers throughout, and remanufactured cylinder heads. The reman heads boast re-cut valve seats, new valves, and the L5P Duramax’s top-hat valve stem seal design to prevent valve seal failure.

Building on what the Essentials long block comes with, D&J’s Precision Plus brings ARP head studs into the equation for added head-to-block clamping strength, a larger (L5P) oil cooler to keep oil temp from skyrocketing under heavy loads, and a pinned (and shimmed) oil pump for added reliability and oil pressure. And as the Precision Plus long block represents the premium LML long block option, it also includes the rocker boxes, valve covers, intake manifold, harmonic balancer, and flex plate—items that serve to speed up installation time by reducing component transfer.

Each D&J-built long block begins with teardown and inspection of a core LML engine. Here, the block and heads are hot-tanked and inspected to make 100-percent certain they’re reusable. From there, each unit is CNC blueprinted, bored, honed, and deck surfaced. Throughout both the machining and assembly process(es), critical engine dimensions and clearances are verified and all OEM torque specs are adhered to for utmost quality control. At the outset of each long block D&J builds, a fully refundable core charge of $3,000 is assessed.

One standout feature that comes standard with either the Essentials or Precision Plus long block option is the inclusion of a keyed crankshaft and camshaft. Both components are fitted with 5mm diameter alignment dowels from the factory, and these pins can become damaged or even shear completely off in high load applications—which allows crankshaft and camshaft timing to get out of sync and essentially wreck the engine. By machining a keyway in the crankshaft, the balancer and cam gear are permanently secured. And with a keyway also present in the camshaft the cam gear can’t rotate on the camshaft.

Both the Essentials and Precision Plus long blocks receive the same cylinder head treatment. It calls for the factory cast-aluminum heads being resurfaced and then fitted with re-cut valve seats and new valves for proper sealing and reliable valve operation. And not only do the new, high-quality valves provide for improved long-term durability and performance, but the heads are also treated to an upgraded, L5P-style valve stem seal design. This “top-hat” valve stem seal is known to help prevent valve seal failure. More on that below.

Because the valve seal controls the amount of oil that lubricates the valve stem, a worn (or failed) seal will inevitably allow excess oil to leak past the seal, down the valve stem, and make its way into the combustion chamber. With the L5P-style valve stem seal design implemented in D&J’s LML long blocks, the lower valve spring plate is incorporated into the seal assembly itself in order to keep it from riding up the valve stem. This is what prevents excess oil from leaking down the valve stem and reaching the cylinder. Fresh factory valve springs are also employed in all cylinder heads, with Precision Plus versions receiving rocker boxes and valve covers to cut down on installation time.

One of the biggest issues high-mile LML Duramax owners face is blown head gaskets. To make sure the most vital gaskets in your Duramax go the distance, D&J replaces the factory torque-to-yield head bolts with ARP head studs on each Precision Plus long block it builds. ARP2000 fasteners are heat-treated, have their threads rolled after heat treat, and won’t fatigue or stretch like the factory head bolts do. Combined with Tier 1 MLS head gaskets, ARP head studs all but rule out the possibility of ever lifting a head in your ’11-’16 HD—even a high mileage candidate that’s been treated to added horsepower.

Elevated engine oil temperature and even cracked pistons are common occurrences with factory LML engines. D&J addresses this in its Precision Plus long block with the addition of an L5P Duramax oil cooler. The L5P-intended, liquid-to-liquid heat exchanger benefits from having a larger cooling stack area for better heat rejection—and it just so happens to be a direct bolt-on for the LML. And because it’s an OEM upgrade, you get an oil cooler that was designed to last 500,000 miles or more. That’s tremendous peace of mind when you’re climbing 6-percent grades with 20,000 pounds behind you.

The OEM Duramax oil pump is known to be on the edge of what it can supply for pressure (and volume) from the factory. On top of that, the lock nut on the oil pump’s drive gear is notorious for backing off, allowing the gear to free spin, which can spell disaster for your engine when all oil pressure is lost. This is why every D&J Precision Plus long block comes equipped with a pinned and shimmed oil pump. The shims provide an increase in oil pressure, and the pin makes sure that pressure is always on tap.

Once complete, every D&J long block is tested using the company’s RMC ETS-100 equipment to verify oil pump pressure and volume, cylinder compression, and cooling system integrity. This comprehensive in-house testing allows D&J to identify any potential issue before the engine leaves its facility. It’s an added tier of quality control that helps ensure each completed long block is ready for a lengthy service life. Then it’s off to the races…or the jobsite. Whether it’s the budget-minded Essentials long block or the Precision Plus, D&J’s LML Duramax options are designed to get you back on the road, back to work, and keep your ’11-’16 Chevy or GMC reliable for the long-haul.

Written by Mike McGlothlin