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O-540 Oil Pressure Adjustment

Slidin Gator

Well-known member
It's the summer repair and maintenance season for me. I thought I would document this operation here for others to reference. I will also note that status of the oil pressure relief is a good item to check when buying a new motor, you don't want a motor with a pile of shim under the spring and just barely making pressure. I hope the following helps new guys looking to purchase.

According to my Lycoming Operators manual, my O-540 oil pressure at operating temp and 2,100 RPM should be between 55 to 95 PSI. Confirm the following for your model.


My motor has been running around the low end at 55 PSI, so I decided to adjust it and document it here for others to reference. My goal was about 75 PSI at cruise, the middle of the operating range. Pressure is controlled by an oil pressure relief valve, located in front of the #5 cylinder.

Here is a description of the relief valve options from the over haul manual, I have the "Non-Adjustable" style:

Here is a diagram and parts list from the parts manual, note the range of spring options listed. You can get different springs to get different performance of the relief, so my findings on shims may not match your results.


Here are pics showing removal of the valve assembly:




The overhaul manual mentions a spacer to decrease pressure, which would be located between the copper gasket and the engine case. My relief did not have any spacer. Item #3 on the parts list is is a Lycoming #10 Washer, which are pretty thick, at least 1/16". Here is a screen shot of the washer from Aircraft Spruce:

With the spacer in place, you can add up to 3 of these washers, otherwise, delete the spacer to increase oil pressure. Without the spacer, you can add up to 9 of those thick washers, so at least a 1/2" thick stack of washers, to increase oil pressure. I used standard SST #10 washers, which are 0.030" thick. Initially I put in 10 washers and tried it. Cold pressure went off the gauge. So I dropped down to 5x of these washers for a total of 0.15" of shim. Oil pressure is now 75 PSI at 2,100 rpm. So 0.15" of adjustment added 20 PSI of oil pressure to my motor.

Finally, here is a picture of the shim washer stack in the correct location (between the cap and spring.


Stay safe when entering the cage. :thumbleft:
Nicely done. They say if you can’t explain something simply, you don’t understand it well enough. You got this one licked!