|Classic mini oil pressure valve & spring. The oil pressure relief valve consists of an aluminium plunger with a heavy spring behind it. Access is through the 15/16″ domed cap nut beside the dizzy. Once this nut is removed the spring will be visible. The plunger is beneath the spring and will probably be stuck, requiring encouragement to come out. Briefly cranking the engine over will usually free the plunger; in severe cases a thread tap can be screwed in to grip the plunger from the inside to draw it out.
Part number for reference: 6K853 (spring) , 12H865 (valve cup)