our billet rear main seal from bcr products uses orings to seal. the original ones were cast aluminum. sometimes the casting flash interferes with the seal sitting properly causing a leak. sometimes those side wedge seals slip out or don't seat all the way and cause a leak. we eliminate the side seal for a dual o ring seal on the ends. ours is also billet aluminum so there is no casting flash to interfere with sitting properly and our oil pan bolt holes go into a solid section and don't poke through. I have seen a few of the cast aluminum ones pop a hole in the casting if someone used too long of a bolt and then you get a drip from the bolt hole unless you rtv it in place.