We have 2 2007 GMC C4500s both have the LMM 6.6. One broke a crank, the other we are about to tear down and see what broke. Both had the Harmonic Balancer come apart and reek havoc. Both plus a 2004 with in a 2 week span, the 2004 managed to go without damage internally... Dealer says there are no recalls or service bulletins pertaining to this matter. We just got a long block for the first truck that broke the crank, installed it 2 weeks ago and can not get it to start on it's own, it will fire with either. Called the local Truck shop that deals with Duramax and he pointed me to a pain in the arse priming process, after a week we started looking into other things like internal timing not being right for the computer. I pulled the paper work, and called the dealer for which engine belongs to this truck. Dealer says its the mid year 2007.5 model having the newer LMM engine setup. They place we got the engine send us the Long Block labeled LB7. I have talked to the sales/mechanic that sold us the long block and he told me the LMM, LB7, LBZ, and 1 other all share the same block. Only difference are the computer detuning, possibly turbo, emmisions, and sometimes the heads and intakes. Anyone with Knowledge on Duramax's please lend me some info as to whether this is correct or not. If it is correct, point me in some type of direction to look. The mechanic working the job has 7 different codes, starting with Cam sensor, then crank sensor, intake temp sensor, airflow, and 3 others that he says did not matter, prolly just set off from any one of these. we transferred all parts from original engine which was not having any issues...Also replaced the cam and crank sensors because of the original internals busting loose.