Why Is There Oil Under The Rotor Cover?

Please, feel really stupid...why is there crank oil under my rotor cover? Is it suposed to be there? I am installing the 10 oz rotor on the YX400 F and oil dripped out like it was a sump! Is there a seal that has failed or is it suposed to be there? Bike runs great, no problems...

Either I don't understand the 4 stroke or something is terribly wrong... Help would be appreciated...

Dave

Nor-Cal

76, 77, YX80's

1999 Yz 400F

It is the sump. The left main bearing and balancer shaft bearing live there.

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