Sealing oil filler on M50

[BubbaHoTetsuo]

I keep getting a creeping vacuum leak from my oil filler cap. I have replaced the cap already, but it still loses a seal after just a few drives (oil film on the gasket and on the lip of the filler) as evidenced by a reappearing and worsening idle bounce that goes away as soon as I clean the gasket off again.

The problem is the actual sealing face on the valve cover somehow got pitted and gouged toward the inside so it has a good 15-20% not-totally-smooth sealing surface. I was able to block sand it down some, but whatever material I remove is that much less for the cap to clamp against. The inside and outside edges are still smooth, nothing is gouged clear across the face, but it's deep enough to where air is creeping past since it's not a Hulk-strength cap tension.

Should I try creating a new sealing surface by filling & sanding with some JB Weld or something similar? I'm not sure what will both seal well and stand up to repeated opening/closing of the cap.

[BubbaHoTetsuo]

what ended up solving this for me was going back to block-sanding the aluminum and doubling up the gasket inside of the cap. I have a Meyle replacement cap in the original square style and a round stamped silver parts store cap - I took the gasket out of the parts store cap and it happened to fit the Meyle cap.

With both gaskets & the cap cleaned with carb cleaner and dried, it finally keeps the engine sealed.

I sure wish BMW had gone with a threaded filler instead of twist-on.