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PostPosted: Tue Oct 20, 2009 12:39 pm 
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Joined: Sun Sep 23, 2007 6:12 pm
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Many Thanks To drmini in aust for this how-to on replacing a scored oil pressure relief valve with a more reliable option.

Fitting a ball in place of the valve will:
1. give better pressure at idle when hot,
2. avoid getting jammed with swarf (like the valve does) if any gets picked up.
NOTE, the Cooper S did NOT have a ball fitted as standard. It is a competition modification.


Here's how you do it.

1. Undo the hex cap, remove the spring and old relief valve plunger.
(If plunger won't come out, put a 10mm OD dynabolt inside it and tighten its nut gently, then pull out.)

2. Get a 9/16" steel ball from a bearing shop, or out of an old Mini CV.

3. Cut 3 - 4mm off one end of the spring. Grind it flat.

4. Cut a piece of 5/16" or 8.0mm unthreaded steel rod (off a long bolt is OK) 40mm long. Put it inside the spring. Make sure it can slide.

5. Put the ball in the block, fit the spring (and rod) with the cut end of spring outwards.

6. Fit the hex cap.

7. Check oil pressure, it should be (with a good motor) around 60+ psi hot, at 3000 rpm up.
You can adjust this a bit if too low, by putting some 3/16" (5mm) flat washers inside the hex cap. If pressure is too high, grind a little more off the cut end of the spring.


The steel rod is to keep the spring straight- if it bends, it can slip around the ball, which then won't seat properly.
At 40mm long there is still room for the ball to lift 6mm off the seat, which is heaps more than needed.

Note:
If the old relief valve plunger is badly scored or out of round, it should be replaced.
If you decide to fit a new plunger instead of doing this ball mod, ensure the new one moves freely in the block without sticking.


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