That looks like its just a black piece of debris singh. like a curled paint chip?
I quite agree with you on that.
@Singh Ji
Anyway you could use a thin wire to feel the gap if its there. Mine does not look like that for sure.
Why dont you use some brush to wash that cavity and you will get a better picture.
It is an odd shape for a crack Singh - could well be a bit of debris. Can you squirt something down there and see if it'll clean up? A crack ought to look a bit more jaggy I'd have thought .
A quick blast with some compressesd air will probably tell you what it is. Looks like debris of some sort to me too.
Thanks GHG, SSR, WillW and r80rt
It is not a crack as you have correctly identified. I wiped the debris off the nut and surrounding area. The nut was intact.
After a little effort, I was able to unscrew it, cleaned it, and then put a copper washer (there is no washer originally) and screwed it back. This is an attempt to stop the seepage of oil that may be coming from incomplete contact of this flange nut with the surface. But there is no real smoking gun that indicates the real source of this oil. I do not see any oil drips off the rocker cover.
Thanks Rusty: I also thought a part was missing, as you have posted.