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Oil Pressure - correcting relief valve problems

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twas_brillig

Jedi Knight
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There's been some discussion in the past regards reasons for low engine oil pressure, and one is a poor seal between the oil pressure relief plunger and the block (due to a weak spring or the metal to metal seal), allowing oil to bleed past the metal cup and back into the sump before doing its job.
Here's a note from www.spridget.com page 8 of their catalogue:
Oil Pressure Relief – "Note: Cup should be lapped in block, also grind spring as to stand vertical when placed on a table."
The lapping had occurred to me before as something to do during a rebuild, but the 'grind spring' is a new one to me.
Doug
 
Makes sense to me.
 
Yep and some even use a 9/16 ball bearing to replace the cup, that take a bit of spring shortening and even more some of us, myself included make adjsutable pressure relief valves. Of course if you lap the cup in, this needs to be done prior to final engine block cleaning in a engine rebuild, you for sure would not wnat lapping compund in your oil travel, it would not be very good to your bearings.
 
Oil relief cups tend to wear on both the angular seating surfaces and on the outer circumference of the body. Also, trash scores are often seen on the outer circumference of the body. Trying to lap a badly worn cup is a futile effort. Better to use a new cup or one in better condition. Also check the fit of the relief cup in the bore. Make sure it slides smoothly. The bore can be honed with a small "Flex-hone", a homemade flap stick or maybe even a worn 2-stone brake wheel cylinder hone.

(Flap Stick = About 4" stick of 1/4" steel rod with a 3/4" long hacksaw split at the end to slide strips of 1" sanding/grinding cloth in. Used for general cleaning, polish of ID's of things. Used in air/electric grinder or electric drill.)

With the spring, the key is to find a good straight spring so that the spring doesn't bind on the relief passage wall as it functions. It should move back and forth in the bore easily. There is also a "performance" spring, usually referred to as the Cooper S spring that can be used as well.

I'll usually put a small AN washer in the cup or in the cap to increase spring tension on the relief cup and increase the relieving pressure.

Cornell University publish some information in "SportsCar" magazine (SCCA club mag) about 20 years ago describing a research project that identified oil losses in engines. IIRC, most oil pressure is lost in excessive rod side clearances, worn cam bearings and the valve train.

Make sure the relief passage behind the cup is open and not sealed up with oil crud. The relief cup can also hydraulically bind if this passage isn't open.

HTH,
Mike Miller
 
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