The oil pressure relief valve is probably one of the least understood parts in a Mini engine. To help us understand to workings of this valve let's look at the diagram below of the Series A engine's oilways.
The valve sits between the oil pump and filter and it's role is to limit the oil pressure by dumping excessive oil back into the sump. The valve opens when the oil pressure is greater on one side than the pressure of the spring on the other side. So by reducing the spring pressure by inserting extra washers under the retaining nut the valve will open at a lower pressure.
In the case of your engine it may be that the spring was too strong causing high oil pressure. The ideal oil pressure for a Series 'A' engine is about 65 psi. Higher pressures than this will not provide any more protection but will cost horsepower to drive the pump and cause stress to the oil.
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