I know this subject has been covered a lot but i want to add this information.i have replaced the  rear main seal in my 312 three times and it still leaked.one time i installed the bottom seal in the retainer backwards and it really leaked.the other two times i really took my time and followed Teds tips and it still leaked.when i took it apart this time i noticed that the two halves  of the neoprene seal were a different width and there is a .025 gap between the retainer and the thinner seal.i still had the one that leaked before and it was the same.  apparently the oil pressure pushed the seal back and let oil seep between, the seal and retainer.I called Armin Brown at Best Gasket and he said that they had been (recently) aware of that problem.he did send me new seals,both rope and also neoprene and the neoprene are both the same width and are a tight fit in the block and retainer.now i have to decide which to use,rope or neoprene.   Carl    Ohio