Here is a nice example of modifying the oil pressure relief valve to be adjustable. It is a nice idea for testing and fiddling around with frequent pressure adjustments. But the same effect can be achieved by simply adding a shim at one end of the spring.

On 6/18/2020, Randall Everson wrote:
"My twin cam runs at 50 psi hot and 2500 rpm. If this valve opens at 50 psi and dumps back to the sump does that mean that the pump is delivering at 100psi?

I made an adjustable cap as shown with new valve and spring. Screwed the adjustment in to increase the spring force on the valve but it made no difference to the running pressure. Does that mean the valve is not seating or do I need to look at the bearings".

The short answer is, if compressing the spring more does not increase the relief pressure shown on your pressure gauge, there is something else seriously wrong. The MG engines have a positive displacement oil pump. If the relief valve never opens, or the oil return port in the block might be plugged, the pump can develop obnoxiously high oil pressure at higher engine speeds. This abnormally high pressure has been known to explode a sheet metal spin-on oil filter canister, or blow out the rubber seal ring at the oil filter, or even damage the oil pressure gauge with substantial over pressure.

There are half a dozen other tech articles higher up the list discussing how the engine develops back pressure due to oil flow rate, and how the pressure relief valve works to dump excess oil flow to avoid over-pressure. Maximum oil pressure can be almost anything you like depending on how strong you set the force on this spring. Pressure of 50-psi is enough to properly lubricate the engine for pretty much any normal operating conditions. Some racer types like to set the relief pressure up to 80 or 100 psi (or even higher). I suppose it is mostly a matter of superstition, thinking it should do something good when it probably doesn't help at all. Higher pressure will cause excess wear on the oil pump drive gear (and the driving gear on the camshaft. It will also make more heat in the oil, driving the need to install an oil cooler to prevent overheating the oil.

Here is a commercial version of an adjustable oil pressure relief valve. This one is Moss Motors UK part number MT1113. I suppose the little flat washer is the bearing plate between tip of the screw and the original PRV spring.