Modern crank oil seal size..

[Chris KC]

First thing I would suggest is check what's actually in there. You don't say what engine you have but the later ones at least have a thrower which is sometimes left out or fitted the wrong way round. Personally I've always found this a tolerable arrangement (no worse leakage than everywhere else).

A modern seal won't last forever, but to survive at all needs to run on a smooth, even surface. Those among us who use them adapt the flywheel boss with a purpose-made stainless steel sleeve (SKF part if I remember correctly? - others will chip in with details I'm sure).