So I’ve been noticing, since day one really, that my oil pressures have been on the low side. Lycoming has the green arc for normal operating pressures between 55 and 95psi. However, my plane was operating in the mid-40s to low 50’s at cruise power settings. Granted, it has been very hot outside, but still lower than I would like. Here in the first graph you can see my oil pressure and oil temps during a recent break-in flight.
Luckily, oil pressure is adjustable. Lycoming has three types of oil pressure relief valves. All work by tensioning a spring against a large ball bearing that controls the pressure. The most modern, is a fully adjustable valve that has a nut you can turn to adjust pressure. Then there are two non-adjustable valves. A short and a long tower valve. I have the long tower non-adjustable. Non-adjustable doesn’t mean you can’t adjust it, you just can’t do so by turning a setting. These valves are adjusted by way of either a different tension / length spring, and / or placing washers under the spring to shim it and increase tension.
Here is some guidance I gathered from a couple places on the web:
There are also springs of varying tensions and lengths which can be interchanged if the above adjustments do not yield the desired results. Some of the springs are color-coded to help differentiate them from one another. The most commonly used ones are the white LW-11713 springs (thick, heavy springs that are used to increase oil pressure at all settings), the 68668 (purple springs that are short and have much less tension than the others), and the 61084 non-color-coded spring that is standard equipment on most regulators.
Longest and shortest spring doesn’t equal toughest and weakest springs. as you can see the toughest spring is the second to shortest. and that spring is a really, really, high tension spring compared to any of the others.
Highest tension to lowest are:
- LW-18085 1.93 inches free length
- LW-11716 2.12″ (White)
- 61084 2.18″ (Most common spring) (No color)
- 68668 2.04″ (Purple)
- 77467 1.90″
Use the 61084 spring with an adjustable tower or a tall tower and a 68668 with a short tower. Use the 77467 in a short tower if you can’t get the op low enough even without adjusting washers and using the spacer with two gaskets. That was my starting points and worked out really good 99% of the time. You should never need to use the top two springs in any normally aspirated engines.
When I took mine apart, I discovered it had a purple spring. Which, as you can see from the above is one of the lowest tensioned springs, and normally for use in the short valve towers. Mine already had three washers present, and I added 3 more. The results you can see below. A little more pressure, but not great. My assessment was, that 9 washers (the maximum) was unlikely to get me the results I wanted, so best to get a new spring.
I got a new spring (two actually), the LW-11716 2.12″ (White), and the 61084 2.18″ (Most common spring) (No color). I decided to install the one with no color since it is the most common. I sat it next to the existing spring with 6 washers, and decided to make it the same length, so I added 2 shim washers. The resulting oil pressure is now within the green arc, but barely.
So I added two more washers, and now I’m pretty comfortably in the green arc. Temperatures in Maryland are in the 90’s at the moment, so oil temps are probably a little high, and thus oil pressure a little lower. I could probably add another couple washers to center the oil pressure a bit more squarely in the green arc, but I’m going to just fly it for a little and observe for awhile before doing so.
The procedures for this adjustment couldn’t be simpler. All you need are a couple AN900-20 crush washers, some washers, possibly a new spring if needed, and some safety wire.
Here is the relief valve on the engine on top of and behind cylinder #3. You couldn’t get any easier access!
Here is the valve removed. I just left the ball bearing in place for most swaps.
And finally, what the tower looks like when removed. For returning the spring and tower to the engine, I just sit the ball bearing inside, and then push it back into position with the spring as I seat the tower. Even with safety wiring, it is a 5 min job.
