I have the lever-action type dipstick on my IO360-A1B6. I figured I couldn’t lose too much oil out the tube if the stick popped out, because it could only come out an inch or two with the door closed. I never really considered the crankcase pressure just sending vapor out, and creating an oil loss that way (if the stick popped out of the tube and just hung on the lip). There is a section further down the stick that looks like it has an o-ring groove (but no ring). Should I check for the right o-ring on the annual this coming month?
This item was originally posted on MML.
The Lycoming parts manual does show an o-ring on the lower lever-type dipstick bushing, at least for the IO360-A1B6. However, take care if
you find the ring missing, and decide to add it. Any oil at all that is in the tube will get raked out each time you remove the dipstick. There is supposed to be an o-ring on the top bushing, where it mates into the neck of the tube. If the top ring is in good shape, and you are seeing signs
that oil or vapor is getting past this top ring, give the lower ring a try.
In my case, it was missing when I got the plane, so I added it. I wound up deciding to take it back off, after raking out oil half a dozen times. Any oil that drips into the upper part of the tube, during oil checks or additions, seldom has time to fully drain back before you reinsert the stick. It gets trapped above the lower ring where it accumulates, then gets raked out on the next stick removal.
I never have any indications of oil getting past the top bushing o-ring. There should never be enough crankcase pressure to actually pop the stick out, unless the top ring is missing or flattened, and something has drastically increased blow-by gases inside the engine (or the case is being pressurized by a blown nose seal).