LMM

Ultimate Diesel Builder's Guide - - Udbg Tech -

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.

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