View single post by Esprit2
 Posted: 06-15-2017 12:12 am
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 321
Please excuse the redundancy, but I just posted this photo in another thread on this same general subject. All the parts together make the pressure relief valve. These parts are typical of the Lotus and Mk II JH oil pumps, while the Mk I JH has a different piston config. Look past the details, and both versions work basically the same way.

At the far left is the 'Insert'. It's just a hollow sleeve that serves as a spacer for the black 'Sleeve'. The longer the Insert, the higher the Sleeve and it's ports, and the higher the vent pressure.

Second from the left is the 'Piston'. It's a hollow cup, open to the left in the photo. It's closed end on the right also has a tapered, or domed end. The closed end must face the pump's rotor & annulus. The open end faces right in the photo, and the coil spring slips into it.

Third from the left is the black 'Sleeve'... ie, the cylinder. It has ports, or open holes through it's wall. As the piston moves right through the cylinder, it eventually uncovers the ports, venting some oil and relieving the pressure.

Fourth from the left is the coil 'Spring'. It slips into the cupped piston on one side, passes through the sleeve, and slips into the cupped cap on the other side. It's length and spring rate are fixed and not adjustable, but some people have been known to shim it with a washer or coin in order to increase oil pressure. Springs can fatigue with age and become weak. When that happens, the relief valve will vent early, and the oil pressure will be low.

Finally, on the far right, is the 'Cap' and it's rubber O-ring. It's a press fit into the pump housing. The Cap contacts the end of the Sleeve, pinching it securely between itself and the Insert at the far end of the bore. The length of the Insert therefore positions the Sleeve, and there for the ports; and that determines when the piston uncovers the ports to vent oil.

If the piston is installed backwards the vent pressure will be high. The coil spring will contact the piston's closed end, instead of slipping into the piston. That means the spring will be compressed more, resulting in higher pressure. Also, the piston's tapered/ domed end will be replaced by it's square end. That means it's edge will reach the ports later, also resulting in higher vent oil pressure.

Good luck,
Tim Engel

Attachment: 9XX Lubrication - Oil Pressure Regulator 06.jpg (Downloaded 101 times)

Last edited on 06-15-2017 07:59 pm by Esprit2