[JFP in PA]

Actually, the spring is under compression when the shaft and seal (#4&5) are installed into the pump housing. With the spring pre compressed to its installed height, if it got hot, if anything it would tend to try and grow slightly in length, increasing the pressure it exerts against the relief valve (#26), raising the pressure at which the valve relieved. An argument could be made that heating the spring could lower its tension slightly at any installed height, potentially offsetting the slight gain from its increase in length, which would probably bring the relief pressure back to somewhere near its ambient temperature starting point.

This design had been around for a long time; if you pulled the oil pump out of a 1955 Chevy 265 V8, you would find it uses pretty much the exact same spring and valve set up. For years (before they realized that oil volume is more critical than pressure to component life), engine builders altered the pump’s pressure characteristics’ by shimming the spring (adding a thin stainless steel washer under one end of the spring) to increase oil pressures, often well beyond 100 PSIG, which often led to other problems. Today, the focus is on delivery volume rather than high pressures, becasue high pressure actually sap horsepower due to the load on the pump. With the M96 design, raising the pressure by altering the spring pressure brings another issue into play as the higher pressures created by the gear section increase the load on the oil pump drive shaft (#3), which is already a weak spot in these engines. LN Engineering developed a heat treated chromoly pump hex drive that sells for about $20 after seeing several engines blow when the OEM hex drive snapped in two. If you are contemplating altering your pump’s pressure profile, you might want to look into replacing the shaft as well.