M20 Oil Separator Survey

[dfgreene61]

In your opinion, the M20 Oil Separator is:

 

1) The greatest thing since sliced bread.

2) Basically useless.

3) Potentially causes harm.

 

I've had one on three of the planes I've owned, and none of them would let you "fill" up the sump.  They spit up oil regardless.

 

My vote - 3

 

 

[BigTex]

My plane has one... That being said, I am concerned that it might potentially do some damage. As the great Mike Busch said about these devices... It's like taking a hose from you mouth to your butt.

[Hank]

Why are you trying to "fill the sump"? I've not heard of any Mooney that will keep more than 6½ quarts [out of 8] in the sump. I certainly don't try.

[Piloto]

I have the M20 oil separator and could not be happier with it. I fill to 8qts and get about 8-10 hours per quart (IO-360) on one hour flights. Absolutely no oil on the belly. The trick for good performace is on the installation.

 

1. Make sure the M20 inlet is at least one inch above the crankcase.

2. Mine drains to the bottom of the valve cover.

3. Make sure that the breather hose/tubing has no holes. Some mechanics drill holes on the tubing thinking about icing. But the reality is that you do not need them due to the engine temperatures. Any hole will impair the suctioning effect of the oil into the M20.

4. The breather outlet tube should extend beyond the cowl flap for better suctioning

 

The oil accumulated in the M20 will not return to the engine until shutdown due to crankcase pressure when running. It returns by gravity, so it is important that the return line has no tight bends.

 

José  

[N201MKTurbo]

I have an m20turbo separator. It was on the plane when I bought it. My plane uses about a quart in 25 hours. I've never ran this engine without it, so I don't know how much the separator helps. The best I got out of any of the three engines I had on my M20F was about a quart in 10 hours.

[DonMuncy]

This is one of the VERY FEW instances where I disagree with Mike Busch. I don't consider the oil mist to be bad stuff. If an air-oil separator will retain the oil and return it to the sump, that would be a good thing. Whether they are really effective, I don't know. But I don't see how they could be damaging to anything.

[Guest]

Within the engine and vent tube there is water vapour which used to be vented overboard. Now we collect it as well as oil mist in the separator can where it returns to liquid form and returns to the oil sump through the drain line. How do I know this to be true? I've installed an Andair water trap in the return line and regularly find water in it.

The goal used to be the fly the engine long enough and hot enough the vent moisture from within the engine overboard, not collect and return it to the engine.

I'd be curious to know how many engine with bad camshafts have oil separators.

Clarence

[aaronk25]

I think that if your run the engine at least weekly a oil separator doesn't do any harm, I'm not convinced it would accelerate corrosion even if not operated.

I installed the largest version of the airwolf oil seperater last summer and considered the m20 but went with air wolf due to it's larger size and because it uses vacuum pump exit air to help push back into engine and the size of the collection unit was larger.

That being said I've heard good reviews with the m20 but it seems the installation configuration is much more critical for successful operation.

[Danb]

#1

[PeytonM]

I don’t think you can even buy them anymore. At least the “M20” brand.  I’ve had it on my plane for >2000 hrs. Belly is clean, but I still don’t fill it to 8. I usually start w/6 plus CG and let it get down to under 5, then add a quart.
 

I’ve seen drips of oil on the left nose gear door. How/should you clean these: taking it off & running solvent thru it? I assume they don’t wear out. 

[A64Pilot]

It’s been a couple of decades ago, but I built a few copies of the M20, it worked OK, nothing great.

I believe the Airwolf is really a Walker Air Sep, which is or was the gold standard if yiu are running a Detroit 2 stroke in a boat, without one it’s nasty, anyway if it is a Walker it’s probably the best.

I built my own because I could and I’m cheap, plus I had an Airwolf wet vacuum pump, which really is a Pesco.

Oh and for I guess nearly the last 70 years or so every automobile has had an oil separator, many looked like a small can and connected to the top of the valve cover, often it was as simple as being full of steel wool or similar, but they worked.

I guess since PCV which I think may have been early 70’s? the separator part moved usually inside of the valve cover, but auto engines have had a form of separator for a very long time.

Oh, and if you don’t have a “whistle slot or hole in your crankcase vent, it can indeed freeze in cold weather, and if it does you find out when the crankcase seal is blown out and your windshield is covered in oil.

I believe you could connect the crankcase vent to the exhaust and that couldn’t freeze, and if done right it could even give a slight negative crankcase pressure, which would be desirable, but as that’s a modification to the TC, I can’t do it without a lot of work and likely $$.

I’ve heard one model of Mooney has done this but it needs cleaning regularly to prevent the hole from coking closed?

 

Edited August 15 by A64Pilot

[Gee Bee Aeroproducts]

Why return dirty oil to the engine

 

They only reason to use a aos is too lazy to top the motor.

 

If your blow by is that bad, then time to fix it.

 

For the beech tcm I stock three variations of the correct engine breather tubes in 304 stainless steel.

 

Next were duplicating the late Baron 55 that are nla in stainless steel.

Jig

cnc bend

cross check in jig

add ice hole

Polish on Bandor Buffer

Copy per sample for 70% in better lifetime material.

 

 

 

[varlajo]

On 2/13/2014 at 5:21 PM, BigTex said:

 It's like taking a hose from you mouth to your butt.

That's... graphic! 

[Will.iam]

On 8/15/2025 at 10:19 PM, Gee Bee Aeroproducts said:

Why return dirty oil to the engine

 

They only reason to use a aos is too lazy to top the motor.

 

If your blow by is that bad, then time to fix it.

 

For the beech tcm I stock three variations of the correct engine breather tubes in 304 stainless steel.

 

Next were duplicating the late Baron 55 that are nla in stainless steel.

Jig

cnc bend

cross check in jig

add ice hole

Polish on Bandor Buffer

Copy per sample for 70% in better lifetime material.

 

 

 

Yea i wonder why mooney installed oil separators on the brand new engines of 252’s from the factory?  And since it was certified with one from the factory, i don’t think i could remove it without a field approval. Since it’s always been on there i do not know how much it helps compared to not having one on there. 

[Gee Bee Aeroproducts]

Beechcraft updated there spec in 1972 to a dry pump from wet.

when owners go all electric or dry they do the swap in tubing 

There are four versions on singles 

Iam the only one that has provided the ability to Cnc to sample in 304 

[Will.iam]

I do not know what the difference between dry and wet pump and how does that effect whether there is a oil separator on the engine and is that available for mooney aircraft?

[N201MKTurbo]

38 minutes ago, Will.iam said:

I do not know what the difference between dry and wet pump and how does that effect whether there is an oil separator on the engine and is that available for mooney aircraft?

Wet air pumps have metal rotors and vanes and are lubricated with engine oil. They require an air oil separator to recover the oil from the pump exhaust. On some installations the exhaust is used to pressurize deice boots. You don’t want to fill your boots with oil. Wet pumps are more reliable than dry pumps, but are messier and the separators take some maintenance. In the 70s most manufacturers changed to dry pumps. This was supposed to be a big improvement. They were cleaner and less maintenance, but they were prone to sudden failure.

https://www.chiefaircraft.com/aircraft/vacuum-static-system/air-pump-wet.html

 

[Bartman]

We removed my oil separator in 2020 when we installed the overhauled engine.  I add at 5 and fill to 6.  I'm happy with a clean belly.

[Mufflerbearing]

On 8/18/2025 at 4:14 PM, Bartman said:

We removed my oil separator in 2020 when we installed the overhauled engine.  I add at 5 and fill to 6.  I'm happy with a clean belly.

Have you verified your dipstick to actual oil in the engine?  I just did and found that the dipstick reads one quart low from what I put in.  So, reading 6 quarts is in reality 7.  Just found it interesting.

[EricJ]

5 hours ago, Mufflerbearing said:

Have you verified your dipstick to actual oil in the engine?  I just did and found that the dipstick reads one quart low from what I put in.  So, reading 6 quarts is in reality 7.  Just found it interesting.

The dipstick just reads what's in the sump.    If your filter holds some oil, which happens sometimes, or there is otherwise oil suspended somewhere in the engine other than the sump, this can happen.   

[MikeOH]

7 hours ago, Mufflerbearing said:

Have you verified your dipstick to actual oil in the engine?  I just did and found that the dipstick reads one quart low from what I put in.  So, reading 6 quarts is in reality 7.  Just found it interesting.

I have heard (i.e. I don't have a cite) that there are different dipstick/tube possibilities...IOW, it is possible you have the wrong one installed and are getting an incorrect reading.

[A64Pilot]

The whole returning dirty oil to the engine is nonsense.

The oil mist / vapor that goes overboard is no different than the oil in the crankcase, it’s not dirtier.

Every modern automotive engine I can think of has a separator in its PCV system, because of course you don’t want the engine induction system getting liquid oil into it.

Automotive engines haven’t dumped blow by overboard in what maybe 60 years? 

I’ve seen no difference in oil analysis on engines running separators from ones that don’t. Don’t you think  Blackstone etc would have raised a flag years ago if there was?

Having said that neither of my current airplanes have them, I’m lazy and cheap, I just clean the oil off once in awhile with mineral spirits.

I know some aren’t even allowed to wash their airplanes it seems, if I were that guy I’d probably want one.

Some perfectly Airworthy engines blow oil out, some don’t. 

Wrt pumps are “better” in that they are extremely reliable, and last as long or longer than the engine, but they do pump out an oil mist in their exhaust, it’s not an excessive amount, you don’t have to run a separator but it will increase a greasy belly if you don’t.

Oh, wet pumps are bigger and much heavier and more expensive than dry ones.

Edited August 31 by A64Pilot