So you couldn't see where the oil was coming from? It could be something simple like a bit of hardened locktite got stuck between the gasket and a surface? Might be worthwhile to set the rocker lower cover on a surface plate with some wet and dry sandpaper to see if its flat. The most common leakage area is the vent path.. I'd also check to make sure that none of the rocker tower bolts are too long and that the bolt holes are clean.

 

 

 

My last thought is to ask if the bike had the older breather assembly with longer bolts and they installed the new breather that needed shorter bolts - or just used too long of a breather bolt. too long of a breather bolt can break through the blind hole in the head. Look into the small breather bolt holes in the head and make sure they didn't break through.

 

Replace the lower gasket and clean the front area real well. Dust with baby powder and take it for a short ride. The powder will help you locate the leak.

 

Regarding my post 14 - can also happen also if there was a bunch of oil in the hole for the breather bolt and it broke the blind hole in the head out.