Check first that the crankcase breather is not blocked and there is not excessive crankcase pressure.
I have been in this situation before when replacing crankshaft oil seals on behalf of a manufacturer on new diesel engines. The leaking seal would be replaced with a new seal, making sure that the new seal was pressed in square with the housing, there were no leaks around the outside of the seal, using a sealant if needed, the seal land on the crankshaft was not cut or worn, the lips of the seal were lubricated-everything was correct, but they still leaked when reassembled and tested.
The unofficial solution I came up with was to carefully remove the tensioning spring which is a bit like a minature piece of curtain wire, find the join where one end is "screwed" into the other, unscrew it, you will find one end has a slight pointed end which screws into the plain end to form a loop. Shorten the plain end by about 10 mm by cutting with side cutters and rejoin the spring to form the loop, this increases the pressure on the oil seal lips on to the rotating shaft .Refit this spring to the seal and fit the seal in the normal way.
A short time after this the manufacturer changed the part number of the seal so something was definitely wrong.
Try practising this on one of the old seals you have taken out first.
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