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The oil pressure relief valve in a MINI consists of an aluminum 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. Now throw away the aluminum plunger and replace it with a steel ball (BLS916). The harder ball will never become galled or pitted and therefore won't get stuck. This spring (6K858) and ball assembly should give you a steady 75 PSI @ high RPM. To prevent oil leaks, it is always a good idea to replace the copper washer (6K431) under the domed nut.