|See the attached JPEG clipping from the JH Parts Manual.
On the back end of the oil pump housing, next to the bore where the distributor installs, you'll see a second bore with a pressed-in steel plug. Under that plug there is a sleeve/ cylinder with ports cut in it's sides, a piston in the sleeve, and a coil spring pressing the piston down to the bottom/ far end of the bore. The bore is a through hole, and breaks into the oil pump's 'working' area, on the high pressure side of the rotor and annulus (ring).
As the pump builds oil pressure, the pressure pushes the spring loaded up the bore. Eventually the piston moves far enough up the bore to uncover the ports in the sleeve/ cylinder wall, and oil vents out of the port. The ports dump oil into an angled drilled passage that goes back to the inlet/ low pressure side of the oil pump.
The port position (how far up the cylinder bore it's placed) and the strength of the coil spring determine the pressure at which the relief valve vents. That's designed and built in, and is not intended to be adjustable. However, some racers in search of a little more pressure have been known to shim the coil spring with a washer or two. Or, old springs can fatigue and weaken over the years, allowing the piston to move too easily and vent oil at too low of a pressure.
I do not know the correct spring rate, and to the best of my knowledge, replacement parts are no longer available. If any new s do exist do exist, they will be from JH specialists, since Lotus never did sell them. Instead, Lotus preferred that you buy a replacement oil pump that is correctly calibrated.
The piston can stick in the bore for a number of reasons, including galling, sludge gumming up the works, and corrosion. If the piston sticks at the bottom of the bore, no oil pressure will ever be vented, and peak pressure can become quite high... in excess of 60 psi. If the piston sticks at the top of the bore, with the ports uncovered, then oil pressure will vent and be low to non-existent. The piston can stick anywhere along the bore, so the failure symptoms can be either very high or very low oil pressure. Or it can be irratic.
Oil pressure that builds very slowly on cold start-up is probably caused by something other than a faulty pressure relief valve. Never say never, but don't focus exclusively on the relief valve and over-look other possibilities.
Other possibilities include a plugged breather hole in the auxiliary housing, just inside where the oil pump bolts on, a plugged oil pick-up screen, or an air leak anywhere on the suction side of the oil pump or the passages that feed oil lto it.
Attachment: 9XX Lubrication - Oil Pump, Pressure Relief Valve - Exploded Parts - All 9XX.jpg (Downloaded 20 times)