So there was probably a ring of hardened crud in front of the seal, and the rubber part of the seal might have gone hard as well. You will need to clean that area of the crankshaft carefully before you reassemble it. Use of abrasives is problematic, since any grit that gets into the crankcase or is left on the shaft will cause rapid wear. Look for a tiny wear-groove in the crankshaft where the seal lip has been running against it. If the groove is significant, you will need to install the new seal in a slightly different position, so it runs on an unworn part of the shaft.
