The oil pressure relief valve 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...
Bidon de 5L d'huile moteur 20W50 Minerale MILLERS Classic Performance
OIL PUMP DRIVE GEAR MIDGET 75-79
Pompe à huile entrainement étoile 1300
The cap that sits on the end of the oil pressure release spring which should be lightly ground in to get proper seating.Cooper S engines had the shorter stronger spring AEA536 and the rest had 6K853 but most fit the AEA536. The alternative to this cap is a ball bearing BLS916. The washer that goes between the cap and block when undoing this is 6K431.