LB7 Duramax Rebuild

Replacing bent connecting rods and broken pistons

Published in the April 2009 Issue April 2009 Build, Duramax Ryan Harris

We figured there was a problem, but had no idea just how bad it really was. The truck, a 2004 Chevy with a fueled and hot LB7 Duramax engine, was a little harder to start than usual. When it did start, it blew blueish-white smoke at an idle. It had a slight miss on acceleration, but once the engine warmed up it seemed to run fine. Colby Hulse, the truck's driver, noticed an unusual noise emitting from beneath the valve cover on one of the heads.

The first thought was that one of the injectors was stuck open. At this point, you really want it to be something simple like a stuck injector, but there's a nagging feeling in your gut that it's not going to be that easy. Under the advice of the company who supplied the injectors, two-stroke oil was poured into the fuel tank. The theory there was that the lubricity in the two-stroke oil would free up any parts in the injector that were stuck. It didn't do much by way of helping the situation. The next step was to replace all eight injectors.

That didn't fix the problem, either. The coolant system was building pressure, which usually means that an injector cup has lifted. So off came the valve covers and the stock injector cups, hold downs and bolts were replaced with SoCal Diesel cups, billet hold downs and studs (stock hold down bolts can be torqued to 35 ft-lbs while the SoCal studs can be torqued to 75 ft-lbs). That still didn't fix the problem. We were chasing down small issues with the engine, but nothing that was causing the greater problem.

The next step was basically an acceptance that something bad had happened. Colby and his brother Cam performed a compression check on the engine's cylinders. It was low. That usually means one of two things: the pistons either had holes or damaged sides to them (which, if that were the case, the engine wouldn't run decently like it did), or the rods had shrunk.

With what they were facing, it was easiest just to pull the engine out of the truck. With it on the stand, the cylinder heads were removed. The moment the first head came off the block, they could confirm that the rods were toast. At top dead center, the pistons sat about an eighth of an inch lower in the cylinder than they should.

When a rod shrinks, what really is happening is that the stress caused by the excess horsepower and torque is fatiguing the connecting rod to the point where the metal fails and the rod bends near the crank bearing. This fatiguing process happens over an extended period of time and the more you drive it and the harder you push the engine, the worse the final result once it fails. If the rods shrink a little, you lose compression and the engine runs funny (like this LB7 did). It's the first sign that you need to stop driving and get it fixed. If the rods shrink beyond that, they shorten enough that the bottom of the piston skirt contacts the crank journals on No. 1 and No. 8 cylinders. Now you have two busted pistons and metal oatmeal in the oil pan. That's bad, but what's worse is if the rods shrink beyond that. Then the crank journals contact the bottom of the piston's wrist pin bosses and pretty much send the whole assembly out the side of the engine block.

The pistons on this LB7 had connected with the crank journals and lost the piston skirts. It was very close to taking out the wrist pin. The pistons also showed signs of further fatigue, with hairline cracks on the skirts of all eight pistons.

According to TTS Power Systems' Steve Cole, a Duramax engine's OEM rods can begin to fatigue once you push the power levels past about 500-550 rwhp. Once the line is crossed, you have anywhere from 5 to 5,000 miles before the engine is in the same position this LB7 is (or until you see half a rod and a smashed piston tumble across the street beside your truck following a very loud noise).

The best course of action is to replace OEM parts with stronger aftermarket components. We turned to TTS Power Systems for a set of Duramax Billet Connecting Rods. TTS builds its rods from 4340 chromoly, using Cad/Cam technology and TTS's extensive research and development to increase strength at a connecting rod's stress points. The result is a rod that will stand up to anything you can throw at it in terms of modern torque and horsepower output levels. The TTS billet connecting rods eliminate the potential threat of shrinking rods again (and are best installed prior to the engine reaching catastrophic failure-just a tip).

We picked up a set of forged pistons from Mahle to replace the stressed OEM pistons. The Mahle pistons are forged, which gives the benefit of having the piston skirt formed together with the crown and pin bosses. This gives the piston more overall strength. There's still an open debate about forged vs. cast pistons and their application for daily drivers, but the intent with this LB7 engine is to leave the truck at its current power levels (just under 700 rwhp) and have a reliable truck. The forged pistons will more than accommodate that.

If you wanted to go all-out with an engine build, TTS Power Systems offers everything you need to complete the ultimate engine. From race or street ported heads with full high-performance valve trains to street, race and extreme camshafts, TTS has any part you'd need to improve the performance and durability of a fast diesel engine.

Sources:

TTS Power Systems
www.ttspowersystems.com

Mahle Piston Systems
www.mahle.com

Custom Auto
208-522-7166

Adrenaline Performance
208-357-5603

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