Or, another cause is when the seal creates a groove in your crankshaft surface which will cause a leak even with a new seal being installed over it. I found that after some measuring where the groove is and installing the seal either further in or out of the carrier depending on that measurement will make the seal miss the groove. I did this on my original 77KMI engine that had a groove in the crank every time I had the transmission out and it never leaked. My current engine doesn't leak however it is an EFI B280F. I have not looked into speedi-sleeves but that's another possibility.