DANGER ZONE: 650 RWHP
With the same rods and pistons as what you’ll find in the LBZ, the LMM Duramax possesses the same weak link as the ’06-07 trucks do: the cast-aluminum pistons. The theories as to why these pistons fail remains the same as well, but it doesn’t hurt to add excess stress from dealing with extreme EGT and high mileage fatigue to the picture—all of which can combine to cause a cracked piston (even at stock horsepower and torque levels).
A lot of cracked piston scenarios we come across in LBZ and LMM trucks stem from the owner knowing the risks, yet trying to get the most out of his parts combination without crossing that red line. For instance, knowing that the danger zone was in the neighborhood of 650-700 rwhp and piecing together a setup good for 625 rwhp. The only problem is that that 650-hp number isn’t a line in the sand. As we’ve mentioned, some LBZ and LMM pistons survive 700 or more horsepower while others bite the dust at 600. There is simply no way to know when it will happen.
Ironically enough, a lot of budget Duramax builds (LB7-LMM engines) entail the use of LBZ/LMM rods, but employ Lb7/lly-based pistons. Cut and coated (de-lipped) LB7 pistons were a hot item in the days before Mahle Motorsports and other piston manufacturers offered an aftermarket piston option—and for affordability reasons, they’re still a relatively popular choice. This type of build is typically safe for the 650-700rwhp range, which is as far as a lot of enthusiasts want to go anyway (financially speaking).
The 16.5:1 Mahle Motorsport cast-aluminum replacement pistons require an LB7 style wrist pin. This is ideal, being that one of the major weak points in the factory LBZ/LMM piston is thought to be its lack of material in the wrist pin area due to the use of wrist pin bushings.
Quick-lighting compound turbo arrangements such as this one, where a stock Garrett VVT and Borgwarner S475 are employed, can kill stock bottom ends in short order. The faster spool up leads to vastly more cylinder pressure (torque) at low rpm, which is the primary culprit in connecting rod and piston failures. We’ve also seen a handful of LBZ and LMM owners switch from a single, stock-based VVT turbo to a set of compounds, only to waste a piston not long afterward.
Known for their ability to handle more stress than LBZ/LMM pistons, many enthusiasts opt for a cut and coated version of the LB7/LLY piston in budget builds. The pistons shown here came from Socal Diesel, feature de-lipped fuel bowls, valve reliefs, and drop compression down to 16.1:1.