Avstar Aircraft of Washington, Inc.

10415 172nd St. E., Hangar A1
Puyallup, WA  98374
office (253)770-9964
or (253)770-0120
email:  avstarair@att.net

01/31/2009 Ask Mike! Archive Page
Oil leaks on B55 Baron with TCM IO-470L's.

Hey Mike,

Oil leaks on a B55 Baron with twin IO470L's.  It seems the motors don't burn it, they seem to leak it and we are forever chasing the leaks.

It seems the oil is around the filler cap and the dip stick tube and the cowl flap area.  Last week it was at the tach drive seal.  Previous to that it was the drain plug.

But it does seem to make sense if you wipe the area for oil, it will find it's way to those areas.

The engines run great, start great, idle great and run cool, but the cowl flap area always seems to have oil around it.  If you run the engines for long periods the oil will end up on the underside area of the flaps.

The engine calls for 12 quarts of oil, but when the plane was purchased the owner advised that both engines would spit it out till they got to 9 quarts, then would stay and he is correct.

At the annual they filled to 12 and made a huge mess!

Any input?

Leaky oil mess!

Hi Frank,

You didn't mention time since overhaul on the engines.  On some installations, after about 800 hours and/or 10 years, they may start leaking from regular parting surfaces due to thermal cycling - the gaskets just tend to wear out.

What you did tell me:  oil on cowl flap (I heard breather), oil on wing flap (again, I heard breather), oil at dipstick connection (a poor, low-pressure connection) and oil at fill cap (again, poor, low pressure connection).

You have too much crankcase pressure.  Normally an engine will toss the top quart or so.  These are blowing off the top 3 quarts?  Definitely too much pressure inside the engine.

Runs great and idles great?  Doubtful it is a compression issues (ring blow-by), but would inspect for this (compression check, borescope inspection).  My thoughts go to the crankshaft oil seals.  In flight, there is pretty good pressure on the top of the engine if the baffles and baffle seals are doing their job.  This can actually pressurize the crankcase through the crankshaft oil seal, without much (if any) evidence of an oil leak in this area.  This excess pressure will cause oil to appear at all points discussed.  Let me know what you find.

Gear Green,

Thank you for the reply!

Time on engines:  have to check the dates, I'm thinking 92's

LE .......1152 SMOH
RE........1001 SMOH

Not sure if they are 1500 TBO or 1800?  I'm thinking 1500.  Can you better describe where these crankshaft oil seals are?

Thanks for the input!

Always three green!

Hi Frank;

1500 Hour TBO, fine print also reads 12 years, but if you are part 91, if all is looking OK, shouldn't be an issue.

These seals are directly behind the crankshaft flange in the front of the crankcase.  Prop has to come off and there is generally a retainer bolted to the front of the crankcase over the seal.

If these are the original oil seals with 16 years on them, I'd bet this is your problem.



I missed something in the translation of why it could be an oil seal.  As the idea around an oil seal is to keep the oil in, which the seal is doing?  You mentioned baffles and seals, in my mind, baffles are for direct oil and seals are for keeping in a specific spot.

Pressure in a crankcase is handled by the breather tube, something like a car and a PVC valve?

Still very interested!


No problem, Frank

When the aircraft is in cruse, there is 180 MPH 'wind' into the front of the cowl.  A ram-air effect.  The engine baffles and baffle seals direct the airflow through the cylinder fins and oil cooler to take away the engine heat.  That much we know.  However, there is approximately a 5.5 - 6" HG difference between the upper portion of the (outside of the) engine and the lower portion; this is what creates the airflow.  Six inches doesn't sound like much, but it is 3 PSI (roughly).  Add this 3 PSI airstream into the crankcase where there is a natural pressure from moving components, as well as oil splashing and misting from parts flailing about at 2500 RPM, and the airflow, from intake at the crankshaft oil seal, to it's exit at the breather or poor, low pressure connection (dipstick, oil fill cap) will 'blow' the oil out of the engine.

The crankshaft oil seal lip that rides on the crankshaft will loose it's tension after several years; this allows that air pressure to pressurize the crankcase.  I have seen these lips get so bad they have rotted off the main body of the seal, yet no evidence of an oil leak in this area due to the ram-air pressure in the cowl.

So, yes; the breather tube 'handles' the crankcase pressure, but you cannot install an air/oil separator with enough volume to counteract either a crankshaft oil seal that is allowing engine internal pressurization, or excessive ring blow-by.

An engine that is sound doesn't even need an air/oil separator, unless it is there to support a wet vacuum pump.

Does that help?



I have you loud and clear and I do understand!

So, is there another way to tell if the seal has failed, other than just replacing it?


Hi Frank;

If the lip is off the seal, you may see it with a small mirror while the prop is slowly being turned.  Otherwise, you may be able to use low pressure (I'd suggest no more than 2 or 3 PSI) air into the engine and soap for air movement in the area.  As indicated before, 16 years is pretty good calendar time on these seals.