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How to read an oil dipstick?

This morning I pulled the oil dipstick out and it read half way between full and empty. I cleaned it and put it back in and pulled it out again and the oil only came up to the bottom line. So I pushed it back in, did my walkaround, and tried again: still the bottom line*.

I can see why wiping the oil off is important if a vehicle has just been in motion – as the engine tilts the oil moves in the sump and might rise to a higher level than normal, staining the dipstick up to this higher level. But in an aircraft that has been immobile for a week or two surely the oil will have time to flow down the dipstick so that the first reading will give you a true level. In which case, why the discrepancy the next time you measure it?

*The other thing I’ve learned with a tailwheel aircraft with a VW engine is that you get a significantly different oil level reading if you jack up the tail so that the engine is level, so an oil level at the bottom line of the dipstick is actually mid-range.

Last Edited by kwlf at 29 Apr 00:17

…also twins with dihedral sometimes have dipsticks marked “left” and “right” because the the tilt angle makes so much difference.

EGKB Biggin Hill

The oil at rest ‘creeps’ up the dipstick, hence the need to wipe and check again. In tailwheel aircraft it is set for the three point attitude, although not sure if this is the case for VW engines as they are level for cars – is the dipstick re calibrated during the homebuilding process?

The Piper twins with counter rotating engines (Seneca, later Comanches and Navajo CR series) do not have interchangeable dipsticks, one is in fact longer than the other so you would damage the engine if you attempted it.

Oxford (EGTK)

RobertL18C wrote:

The Piper twins with counter rotating engines (Seneca, later Comanches and Navajo CR series) do not have interchangeable dipsticks

The Chieftain I am flying today has contra rotating engines and double sided dipsticks.

EGKB Biggin Hill

..may just apply to the IO-360, is the dipstick on the same side of the engine?

Oxford (EGTK)

It always has to be done twice. Mine always reads 0.5 quarts higher on the first attempt. Oil creeps up the dipstick over time.

This is what I have found, on my IO-540-C4:

  • the oil takes a good number of hours (overnight really) to drain down from the upper parts into the sump, so looking at the dipstick for the purpose of measuring oil consumption is pointless before then
  • the dipstick level is only slightly affected by the front oleo compression (maybe 2mm)
  • I have not seen any evidence that the oil creeps up the dipstick, and if it does it should be consistent (if the engine is always cold when you check it)
  • wiping the dipstick and re-checking yields an initial low reading because the dipstick is inside a tube and the oil takes a while to fully climb back up the tube after the dipstick re-insertion (say a minute or so)

For the purpose of measuring oil consumption, I have been noting down the level as e.g. “9 minus 6mm”. The results are very consistent. Also, between the 6 and 9 marks, 1 quart is 12mm so one could do it that way i.e. the aforementioned would be 8.5 quarts.

It always has to be done twice. Mine always reads 0.5 quarts higher on the first attempt. Oil creeps up the dipstick over time.

What exact engine type is this on?

Administrator
Shoreham EGKA, United Kingdom

RobertL18C wrote:

is the dipstick on the same side of the engine?

Yes.

EGKB Biggin Hill

Peter wrote:

What exact engine type is this on?

My guess is a Continental IO550
_22 rings a bell? Put SR in front.

LFPT, LFPN

On all the ones I fly. IO-550, TSIO-520, O-360, O-320
I did ocasionally check the oil without starting the engine, and when I did it again a week later the reading was higher.

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