Question for Paul McBride, the General Aviation News engines expert: I’m test flying an experimental that has a Lycoming O-360-A4G engine.
During my second test flight, I noticed cruise oil pressure at 55 PSI. As it has the longer housing regulator, I added three washers and replaced the check ball. The result was oil pressure at cruise around 65 PSI and 40 PSI at idle. It’s within spec, but I’d like some more room over minimums.
My thinking is that I have a weak spring and I’m going to order the white spring to replace the current uncolored one.
Decent idle pressure would point to the bearings being in good shape.
While I’m at it, I’d like to replace the existing regulator with the newer adjustable one.
I would appreciate your thoughts on this.
Charlie Smith
Paul’s Answer: This is a great question and one that doesn’t have a simple answer, but let’s see what we can come up with Charlie.
First of all, if you look in the Lycoming Parts Manual for the O-360-A4G (PC-306-1), you’ll see that like most other O-360s it uses the same part number oil pressure relief valve spring.
The most common spring used from my experience is part number 61084 with a wire diameter of .054 inches and its free length is 2.18 inches.
If, as you say, your engine incorporates the long dome oil pressure housing, then you are allowed to use up to a maximum of nine #10 washers.
If you’ve added three washers and found some improvement, then you may want to try adding one or two more washers to see if the results change.
The thing to remember is that the idle oil pressure has a minimum of 25 PSI, and during normal operations, a minimum of 55 PSI is within the limits, so anything above that is good.
You also mentioned that you suspect you have a weak spring, but I’m not convinced since you were able to bring the oil pressure up by adding three washers.
I understand you’re considering going to the adjustable oil pressure relief valve, with which you cannot use any washers.
My suggestion is to delay that expense and stick with the long dome valve for now and just add a few more washers.
Seriously though Charlie, I don’t think I’d be concerned about your oil pressure if the numbers you provided were using a calibrated master gauge and not the aircraft instrument because I’m very skeptical of the true accuracy of that gauge.
I hope this may offer you something to think about before you spend any money on a situation that I don’t think you need to be concerned about.
