by Jim Hughes
Our aircraft engines have a crankcase oil capacity as specified in the 1949 CAA (Civil Aeronautics Administration) Technical Manual No. 107. On page 198, the oil supply tank requirements are listed.
This was certainly intended for the early radial engines that were set up loose on clearances, so they burned and leaked a lot of oil. This specification insured that there was still oil in the tank when the fuel was exhausted. Here is the page from my copy of the Handbook;
Since most of our flat engines were originally designed under this Power Plant Handbook, (and you thought that you have a ‘new’ engine), the oil supply was designed to the same requirements.
With the advancement in materials and machining, our engines use very little oil and leak almost none. Yet the design requirement remains for these old designs. New designs appear free to use a new standard.
Since the text above may be hard to read, I’ve listed the two requirements:
1 gallon, (4 quarts) of oil for every 25 gallons of fuel
1 gallon of oil for every 75 horsepower
So, which ever calculation yields the larger number, that’s what is required.
A few examples:
Cessna 172 with an O-300 and 39 gallon tanks.
- 39/25 gallons x 4 = 6.25
- 145/ 75 hp x 4 = 7.73
So, rounding up, 8 quarts is specified
Cessna 172 XP, IO-360, 210 hp, 52 gallon tanks
- 52/25 x 4 = 8.3
- 210/75 hp x 4 = 11.2
So, 12 quarts is specified. (I guess that they liked even numbers)
Cessna 182, O-470, 230 HP, and 64 gallon tanks
- 64/ 25 gal x 4 = 11.2
- 230/ 75 hp x 4 = 12.3
So, 12 quarts is specified
Most POH and engine operating manuals specify the minimum oil quantity for level flight, which is usually 3-5 quarts. But we have to allow for any oil use during a long flight, so we can safely use six to seven quarts at the beginning of a flight.
Interestingly, most engines don’t tolerate a full crankcase. The crankshaft will whip up the oil and either blow it out the breather tube, or it will flood the cylinders, overwhelming the oil rings, and the engine will burn it,
So, now you know why your crankcase hold so much oii!