Step 1: Eliminate all of the air in the line from the engine to the oil pressure gauge. This is not easy and requires you to open the line at the gauge. You can do this without removing the oil pressure gauge. to do this, just lie on the cockpit floor and reach up and disconnect the line at the back of the gauge. Use a 9/16" and a 1/2" open end wrench. You can then remove the #2 nipple fitting from the back of the oil pressure gauge and remove the gauge from the panel. Once the gauge is out you can file down the fitting at two of its angles so that you can fit the gauge back into the panel with the nipple in place. Fill the gauge with oil and insert a nipple with the pin point orifice before reinstalling the gauge. Also, disconnect the line at the engine and back fill the line to the engine with oil. I had someone watching the old oil as it came through the line and he did see an air bubble during the procedure.
Step 2: If you still have any needle flutter then you can install a snubber which you can buy from www.visionmicrosystems.com ask for Dave. The down side to this snubber is that the oil has to get warm before the gauge functions.
The reasoning below shows that my approach was entirely wrong. The problem was probably with the Frati inverted oil system letting air into the oil system.
This letter came from a SF260 owner.
With your restrictor fitting in place, all the pressure fluctuations would have been confined to the inside of the engine. The hoses on my aircraft are all the same type as the ones that were on the aircraft when it came from Marchetti. All the oil plumbing used 1500 PSI hoses. This seems to be excessive overkill, but I guess it does no harm except that the hoses are really stiff.
My take on the whole problem is that when an air bubble goes through the pressure relief valve, the pressure drops and the valve slams shut. With the valve shut, the pressure overshoots because it takes a short time for the ball to move back open. I suppose I could make a PHD Thesis out of all this, but there arenít supposed to be any air leaks in the oil pickup lines. One note: when we changed the oil and filter, the pressure showed the oscillation problem until all the air was out of the system, which took about a minute.
My mechanic friend told me to get over it and when I suggested putting my fancy, high speed, pressure gauge on his Stearman, he told me to stay from his airplane. He assured me that the oil pressure went to zero for a bit, whenever he rolled upside down, and it was OK. Bob
I did some more testing today. I took the pressure relief valve apart and it was fine. I hooked my digital storage oscilloscope to the fancy electronic pressure transducer I bought, and found that when the oil got hot, it really was jumping around from 40 psi, to over 100 psi. I talked to Lycoming and they had a lot of things to keep me busy but did stress the importance of making sure there were no air leaks in the oil pick up. I decided to disconnect the inverted oil system, and make the engine into a standard non inverted airplane. The oil fluctuations seem to have disappeared. I am now going through the external oil plumbing to see what I can find. Thank you for the name and phone number of your friend. I will call him and see what he had to do. Bob
AND NOW THE FINAL ANSWER
From Mr. Garrison:
I approached the oil pressure problem with a high speed (500 microsecond response time) electronic gauge, a chart recorder, and a storage oscilloscope. The chart recorder was too slow, but the scope told no lies.
The oil pressure really was bashing back and forth from 20 psi to over 100 psi, at about 300 times a second. I disconnected the inverted oil system, and put the engine back the way it would be without the external plumbing. Everything was fine. This eliminated engine problems which are always expensive. In removing the sump fitting from the bottom of the engine, I noticed that it wasn't tight when we took things apart. Note, no oil ever leaked out even after sitting for a week. After putting the inverted system back on the engine, with the sump fitting tight, the problem was gone. My take on it all is that a little air was leaking in when the engine was running, and as the bubbles went by the pressure relief valve, the ball started bouncing. This caused the spikes in the oil pressure. After all that, I installed a 0.015" restrictor in the pressure gauge line, and all has been happy ever since. Note: The Marchetti parts book calls for a restrictor fitting in the line. At least, I found the problem, and didn't just damp out the response time of the gauge.
Here is a picture of the Christian Inverted oil system. It came on my SF260W but was not on my SF260D that developed the flutter in the oil pressure needle.
The pictures require intense study. The top picture is with the engine upright, in level flight. The second picture has the engine inverted. There are several chances for air to get into the system but this was never a problem for me. In both my experimental SF260 and the Giles 202 I which had the Christian inverted oil system I never had any oil pressure needle flutter.
My final thoughts.
It seems that the Frati designed inverted oil system will fail somehow and let air bubbles into the oil system. These bubbles wrack havock with the oil pressure relief valve causing the needle fluctuations. I think the correct solution is to repair or take out the Frati oil inversion system.