Best oil level for hot weather operations

[N2865C]

In the Operators Manual for Continental O-300 it says if oil temperature is high you should keep 8 qts in the sump for better cooling. Then in the C170B Owner's Manual it says that for hot weather operations (seaplane) you should keep the oil level below 6 qts on the dipstick (which I think would actually be 7 qts. in a seaplane) Both make sense to me... 1- more oil to absorb the heat, 2- more cooler surface area on the oil pan to cool the oil. I am curious if anyone has noticed a difference. I am about to leave on a long hot trip. I prefer to keep my oil level around 61/2 qts to keep down the oil out the breather, but I would be willing to sacrifice a little oil on the belly for lower oil temps.
jc

[Bruce Fenstermacher]

I've not notice a change in oil temperature regardless if I have 8 quarts or 5.5 quarts which is as low as I go. More oil should be cooler.

[GAHorn]

JC, I don't see the advantage of running the sump lower on oil as a method of cooling oil. If the sump has air blowing against it, then the only oil which will be cooled will be the oil in contact with the sump, and that would indicate keeping the sump full. The air above the oil level in the sump will not be assisting in oil cooling...in fact it's mostly composed of hot blow-by gases and the less amount of room for that (in other words a full sump) the better.
I'd bet that most of us, when checking the dipsticks of our cars just before a long highway trip, will add a quart of oil if it reads a quart low. In fact, I'll bet most of us would even consider splitting a new quart bottle, just to top it off to the full mark for such a trip, because we all want our cars to last a long time and not miss a single drop of that precious lubrication on a long, hot, summer trip. Yet, our cars have liquid cooling and run at more even temperatures and virtually never suffer risk of overheating the oil. And even if we did run low and damage an engine, all we have to do is pull over to the side without a lot of risk to our safety. But we're discussing leaving on an airborne trip in our precious airplanes with less than what we give our cars?

At the risk of starting something else, (putting on my Nomex now) I also believe it to be an old wives tale and a poor practice about departing with the oil level low to avoid blowing oil out the breather.
First of all, the breather outlet is at the TOP of the crankcase, well above the sump and well above the crankshaft and all other moving parts in the engine. The oil that leaves the breather is, at the most, misted by all the thrashing that goes on in there from "sling" off the crankshaft. That oil then can be blown out the beather if excessive gas-pressures, improper breather elbow placement, or both exist...but neither of those conditions have anything at all to do with the amount of oil in the crankcase sump....because even a full sump (8 qts) has the oil level well below the crankshaft throws...and an 8 qt/full oil level drops even further when the engine is running due to some of that oil being "in transit" throughout the engine's lubrication systems, i.e., the rocker boxes, pushrods, gear/accy case, etc. (And it matters not that if/when the oil level may be intentionally kept low...oil still slings off the crankshaft and blow by gases still flow out that breather tube, so if that is the problem then keeping the oil low will not arrest it.) I operate my engine with 8 quarts, and when it gets below 6.5-7 qts on the dipstick I add a quart. My oil consumption hovers around one in 10 hours, and I don't see any evidence of "excessive" beather blow-by. (A couple of two-bit sized spots will fall out of it onto the ramp about 15 minutes after shut-down sometimes, but I consider that normal.)
The seaplane SHOULD have a different dipstick if the operator wants it to read accurately. But it's not required, nor is it even common.* (Most seaplanes (170's) were/are not equipped with the 172/tricycle-gear dipstick because the floats were considered an alternate gear on a standard 170. Lots of operators switched the airplane back and forth, and besides, many 170's actually were float-equipped by an aftermarket STC rather than by Cessna. Consequently many float-equipped 170's operated with the standard conventional-gear dipstick installed however, and overfilling a sump will lead to frothing and overheating of the oil, hence the Owner's Manual comment.)

*In later years Cessna's very successful model 172 far exceeded the production output of the 170. Consequently many more O-300 engines were produced for a tricycle stance than a conventional stance. The engines have been swapped around on airframes almost without regard to the type landing gear utilized. Therefore it is very common to find 170's on conventional gear with the INCORRECT tricycle gear dipstick in them. This results in the engines being run with LOWER than intended oil levels. This situation also results in those seaplanes using conventional-geared dipsticks being subject to OVER servicing with oil, which leads to frothing and overheating...therefore it's necessary to run a seaplane which has a conventional-gear dipstick with an oil level that READS about 6....even though it has 8 quarts of oil in the sump making that reading on that dipstick.

TRIVIA: Keep in mind that the mfr considers these engines airworthy with oil consumption rates almost a whole quart per hour! (at max continuous) And their engineering/maintenance data state that. They also state the MINIMUM oil level for safe operation to be 4 quarts. So, in order to avoid potential engine damage, only if a sump is filled with 8 qts of oil can it fly for 4 hours (the average endurance of the airplane at that power setting) at the maximum allowable consumption and still land with sufficient oil to avoid damage to the engine. (Some aircraft designs have such endurance that oil sumps must be enlarged for certain engines to accomodate certification criteria. This usually results in a "dash" number/designation change for the basic engine model.)

[cessna170bdriver]

George,

Is there a drawing or any other kind of reference against which we can check our dipsticks to determine whether we have the right one?

Miles

[GAHorn]

This information is posted in "The MX Library".

The service bulletin regarding such "oil guage rods" is: http://www.tcmlink.com/pdf2/SIL00-7A.pdf


In short, the correct dipstick has a part number etched.stamped onto it, and for a 170 the correct dipstick is PN 628410A1.
The dipstick for a 172 and/or a seaplane would be: PN 628413A1

[170C]

Then based upon the data I have just read, my dipstick in my O-300 B engine, which is in a C-172 (came from the factory as a trike) but is now a conventional geared airframe, is likely an incorrect dipstick. I usually fill my sump with 8 qts of oil, knowing that 1 qt will be in the oil filter at all times. Would this mean that I am actually only having 6 qts in the sump and if so should I be putting in 9 qts at oil change?

[GAHorn]

If...IF...you have a 172 dipstick in your taildragger...and if you put in 8 qts...then your sump will have 8 qts in it. (But it'll read HIGH...like you have more than full.)
If you have a 170 dipstick in a tricycle gear 172, and you use the dipstick to determine oil level, then you'll also be OVER servicing your engine. (You may froth up the oil and overheat, also.)

The problem is having the INCORRECT dipstick and using that dipstick's indications to service your sump. The 172 dipstick in a 170, if used to determine oil level, will result in UNDER servicing your taildragger. This can lead to engine troubles because the operator will inadvertently be running his engine LOW on oil. The 170 dipstick in a tricycle 172 will also result in an incorrect OVER service of the sump, if the dipstick indices are relied upon. (That' why the seaplanes, which would be sitting more in the tricycle attitude, would need to operate at an INDICATED lower dipstick level.)

BUT....8 qts....poured into an empty sump is still a correctly filled sump regardless of what it reads on the dipstick.....and it's a simple matter during an oil change to confirm that when you add 8 qts....your dipstick reads 8 qts before you start the engine and let the oil pump fill the filter. (Although the usual spin on filter does not hold an entire quart...it actually holds about a pint is all.) Then you'll know if you have the correct dipstick....and if you don't....then you'll know what an incorrect dipstick reads when it has 8 qts in the sump.

Of course, the best recourse is to get the correct Part number dipstick, AND confirm it's readout at oil change time.