Cylinder stud o-rings

[Blower1]

Are those front center stud o-rings mandatory or replaceable with standard o-rings without blocking the cooling oil return to the crankcase?

I ordered o-rings from local Suzuki shop and get 17mm o-rings instead of 15mm, which is the right size.

[Gixer1460]

Aren't those two studs open to the elements and are sealed in the head - hence the rust? Anyway, path of least resistance - oil will go down cam chain tunnel!