Have read quite a few threads here about LS rear main leaks, though none appear to cover this! While having the LS7 clutch replaced on my 6L Crewie late last year we found the rear main seal was weeping so had a new one installed at the same time. After a few months the rear main went from a weep to dropping a considerable amount of oil. Spoke to my repairer, who then stripped it down & mentioned it was leaking from the seal & it must be a faulty one! So it was replaced again. A few days later the same started happening again. In the meantime I started researching this leak referring to a few forums & GM shop manuals. I read an article about cylinder block & rear cover plate porosity issue that can occur with these ally blocks. GM in the states did a recall there where people were experiencing this leak. With this info I bought some more parts, thinking this could be my problem, also read up on the LS using a pvc barbell oil bypass behind the rear plate also. I bought a billet ally one to replace this as the rear plate was coming off anyhow. Whilst doing my research found the GM oem rear plates come with a seal already installed in the plate!! I referred to the engine manual in fitting the rear plate which described the installation using An alignment tool to correctly centre this plate around crankshaft rear boss. It appears GM spares are selling parts contradictory to how they should be installed!! With this knowledge I bought that plate & ordered one extra rear main seal. When I received the plate removed the seal & threw it away. Had an alignment tool machined up as this pic attached. With all spares in hand had my repairer strip it down for another go, removed rear plate & no signs of engine block porosity or rear plate problems either. What we found was the oem barbell appeared to be bleeding a considerable amount of oil from the block into the rear plate area. NB the rear section is not intended to be under full pump pressure hence over pressurizing the rear seal causing a premature leak. Traces of this bleeding off can be seen from the barbell to the right extending to the rear main seal. These barbells are generally firm in place & need to be picked out of the block, mine basically fell out. The new billet one fit firmly & have a blind thread in the rear to extract it. Fitted the new rear cover & gasket applied RTV to both bottom corners where meets sump edges. Fitted alignment tool & centred plate, torque all bolts & removed tool. Fitted seal & reassembled & reinstalled box. Been all good for the past 3-4 days & will be monitoring this, hopefully it could help someone else chasing this same problem/leak. In conclusion it appears GM spares are setting most repairers DIY guys up to fail by selling a complete Rear plate with fitted seal. When in reality fitting this would be hit or miss!!