I have not seen any clear data on this.
On a new engine you get a lot of blow-by past the rings, so there is a lot of pressure oscillation in the crankcase, so a lot of oil is lost as air+oil vapour, out of the breather. Same on an engine with shagged rings.
Shagged piston rings will also cause oil to be burnt, but you should then see the smoke 
Leaks may lose some oil but unless it is p1ssing out somewhere, out of an oil gallery, I find it hard to see how this could be operationally significant since the leaks will be from the crankcase cavity, and there is already the breather pipe which should make the biggest leak of all…
Then you have some engines consuming 1 quart in 3hrs and others consuming 1 quart in 25hrs. I don’t believe this can be explained by better or worse ring seating.
We had this thread on the specific topic of high oil consumption due to a bunged-up bottom ring(s), following a series of high altitude / low power flights.
Peter wrote:
Then you have some engines consuming 1 quart in 3hrs and others consuming 1 quart in 25hrs. I don’t believe this can be explained by better or worse ring seating.
I think a lot of it can, when I had the Cessna 140, it would burn 1 quart every 4 hours until we fixed it, when oil consumption fell to a more normal 1 quart every 12 hours.
Peter wrote:
Then you have some engines consuming 1 quart in 3hrs and others consuming 1 quart in 25hrs. I don’t believe this can be explained by better or worse ring seating.
I believe in most cases this is due to people filling up the oil level to above the “comfort” level of an engine because the POH says “between x and y quarts” and they believe y is better than x.
My engine is a perfect example. I always run it with minimum oil level and only top up every 20-25h (i.e. usually not at all between oil changes). If I fill it to maximum POH level, it will blow out those 3 extra quarts within 2 hours and make an incredible mess.
I think this explains 90% of all oil loss.
We did have a debate here on the minimum oil level, and it is astonishingly low.
It can be found in the TCDS sometimes.
In reality it is not wise to do a long flight with 3qts say (in an IO540 whose TCDS-max is 12 qts).
However, has anyone tested this i.e. flew say an IO540 with 3 quarts in it for a few hours? You can’t do this test on a dyno because you won’t have the airflow past the breather pipe outlet which carries away a lot of vapour.
I get through 1200 quid of oil a year and of that I put about 5 quarts in a year.
By far the worst are the “CPL” holders who fill the thing Up To 6 plus quarts fly 3 hours and think the aircraft burnt 2 quarts so fill it up again.
No matter how times I tell them it takes a few hundred maybe even a thousend hours before they believe me. And before that they think I’m some person who puts cost cutting before safety.
Once our C152 drops below 4 quarts it burns hardly anything.
You seem to have missed all the other places oil comes out – crankcase seals, crankshaft seal, all the ancillary seals, the ancillaries themselves, if they have oil flowing through them, filler neck seal (top and bottom) and so on.
I am always amazed that any oil stays in the engine
You seem to have missed all the other places oil comes out – crankcase seals, crankshaft seal, all the ancillary seals, the ancillaries themselves, if they have oil flowing through them, filler neck seal (top and bottom) and so on.
Actually there are a lot fewer seals (I mean “rubber” seals) in a Lyco engine than most people might expect. There is the big seal at the front of the crankshaft, and it is amazing that (a) that can be installed over the top of the crankshaft flange (it has to be heated up in hot water apparently) and (b) how long it lasts. Then there is an o-ring at the oil drain, and another one at the dipstick. And a seal at the vacuum pump drive. But most of these seals don’t have “solid oil” behind them; that is present only on the sump and bottom strainer seals, and the oil filter seal (which is under pressure but is changed at every service anyway). They have to seal only the air/oil mixture mentioned below. Actually the strainer seal is a deformable copper o-ring anyway.
Then there is a number of hoses but they should not leak at all, ever, unless the mating surfaces are damaged or the hose itself leaks. Nobody should be using anything other than teflon hoses for this which have no actual life limit (may have a regulatory life limit).
But there is a huge hole called the breather tube
and it amazes me how little oil is lost through there, when you have a crankcase full of air/oil vapour and a hugely oscillating pressure due to the pistons moving about – and that’s before considering any exhaust product blow-by which pressurises the crankcase. If this breather gets clogged up, a lot of oil starts coming out in various places e.g. the crankshaft seal.
One can do a crankcase pressure test as per this ( local copy ) and I did that here a while ago. This is a test a few days ago on my new engine
and that turned out to be OK (max rpm was 1500 in the above test).
IME the crankcase can leak fairly easily because of the way the joint is made. It is only a silk cord with some sealant on it and in places (e.g. the top of the engine) the mating surface is only about 4mm wide. The cord is supposed to be wrapped around the holes of the many bolts which hold the crankcases together at the edges, but this often fails too and you get oil reaching the bolt and this oil then always escapes along the thread and via the nut. Parallel threads do not seal… I have had a bit of this, hence this thread about what sealant can be used. I think PR1422 is fine with its +120C upper limit and it absolutely does work on the bolts. What works on the edge of the crankcase, I don’t know. Lots of people have struggled with crankcase edge leaks. The most problematic spot seems to be the top edge – because the mating surfaces are so narrow. But it is also the most accessible. One method is to attach a vacuum pump to the breather pipe while applying the sealant
However I don’t see that a leak from the crankcase can have an operational (i.e. oil consumption) significance given the presence of the massive breather pipe hole. IMHO most of the crankcase oil vapour must come out through the breather. And you can lose a lot of oil – say 1 quart per hour – via the breather if the oil control rings are bunged-up, and I have proved this.
Once our C152 drops below 4 quarts it burns hardly anything.
What is the max level for the C152?
As I posted earlier, it would be really interesting to get some data on this, but still I think anybody doing this over long trips would need real balls because if something happens and you do get a real leak, you will have much less time before the engine seizes
Peter wrote:
However I don’t see that a leak from the crankcase can have an operational (i.e. oil consumption) significance
I’m sorry you weren’t with me last summer as I diverted into Bergen with oil all over the cowling and wing, and streaming back over the flap and into the slipstream.
Though I must agree that the actual measurable loss was rather small. A little oil can go a long, long way
Can you post detail, Timothy, i.e. what actually happened and where the leak occurred?
PA31 with two 400 hour engines (of completely different provenance.)
I was on a longish trip around Scandinavia and had noticed over a number of flights that a little oil was appearing around the filler area, but I put it down to spillage.
The last leg of the journey was from Ålesund to Cambridge on airways at FL120. This was the highest I had flown since the outbound flight, as the rest has all been low level VFR.
Soon after top of climb, a large amount of oil appeared over the top of the cowling, streaming back to the trailing edge and spreading out over the flaps.
I diverted into Bergen, where an engineer did a full inspection, starting with having a fire engine blast away all the oil. We could not find a source of the oil, which did not leak at full power on the ground.
I decided to fly home low level, round the coasts of Norway, Denmark, Holland and Belgium. By the time I got to Cambridge there was again a lot of oil all over the cowling and wing.
I returned to Biggin, my maintenance base, where they could again find no fault and thought it might be a leaking filler tube or dipstick. These were changed to no avail, and eventually, with the help of talcum powder, it was traced to the join between the crankcase halves.
The engine was sent away to a reputable rebuilder who found that the lugs between the crankcase halves were not interference fit, and they were fretting, destroying the silk thread seal, which itself had been very badly done, passing through bolt holes rather than around. . They ordered a remanufactured crank case. However, they also discovered a large number of other serious faults in the engine which demonstrated that the previous overhaul had been done very badly indeed.
The engine was rebuilt and returned to service and, after four hours of service, had another, unrelated major failure, which was fixed quickly and under warranty by the rebuilders, requiring yet a further new crankcase! But that is another story.
The engine has done 60 hours since then and seems to be fine.
