since 1940

Crankshaft seal leaks oil on Continental or Lycoming aircraft engine

See also 

Oil Leak at Crankshaft Nose Seal

Lycoming Aircraft Engine Nose Seal Leakage. Crank seal leaking

Make sure that the oil leak is coming from the crankshaft oil seal rather than at the crankcase parting surface. It is more common for the leak to be at the seal and this has resulted in multiple seals being replaced before it was found that the crankcase was leaking.

1. Crankcase pressurization. Normal Lycoming aircraft engine crankcase pressures range from 1-1.5 inches of water pressure in climb, zero inches in cruise, and 0.5 to 1.5 in descent. Lycoming specifications allow up to 5 inches of water maximum. However, no type certificated engine installations reach this level of pressurization.

An easy method of measuring crankcase pressures is to hook up an airspeed indicator to the crankcase, with the hose from the meter installed over the dipstick tube. The airspeed indicator would indicate 100 mph for a pressure of 5 inches of water or 50 mph for 2.5 inches of water.

High crankcase pressures can be caused by:

• Blowby of combustion gas past the ring belt
• Frozen or blocked breather vent

B. Turbocharged engine pressurizing the crankcase through a damaged guide boss. A loose valve guide that has enlarged the guide boss creates a gas path from the turbocharger back into the rocker box. Grab the exhaust valves and try to move them. If the valve and guide moves in the boss, then the guide is loose in the cylinder head causing a gas path from the exhaust port to the crankcase via the rocker box and push rod housing.

C. Loose oil filler cap gasket.

D. Air leakage past the nose seal.

E. Blocked or frozen engine breather tube. Moisture is expelled from the engine crankcase through the breather tube which often extends through the bottom of the engine cowling into the air stream. In freezing conditions, this moisture may freeze and continue a buildup of ice until the tube is completely blocked. A "whistle slot" (small hole in tube) should be provided in a warm area near the engine. See Piper Service Letter No. 370, "Modification to Engine Breather line for certain PA-24 aircraft. See Gulfstream American (Rockwell) Service Bulletin 112-50 "Engine breather Tube Modification."

F. Piper Apache. Clogged breather. Breather exit next to exhaust to keep it from freezing. Over the years oil carburizes and blocks the breather. Engine crankcase pressurizes, oil bleeds out of seals and nose seal leaks.

2. Poor seal installation.

3. Inadequate clearance between crankshaft oil slinger and crankcase. Continued problems with nose seals leaking on Lycoming engines may be caused by the fit between the crankcase and crankshaft. If you have inspected the crankshaft seal to confirm the proper type, installation, crankshaft surface finish, crankcase pressurization, but still have a seal problem, then it's time to start looking elsewhere. Buying an exchange or welded crankcase and replacement crankshaft means the crankshaft may not be mated properly to the crankcase. Three subtle engine conditions may make it impossible to seal the crankshaft, These are:

A. Excessive clearance between the main crankshaft bearing and crankshaft journal which allows more oil to leak out of the bearing shell, thereby delivering more oil to the crankshaft oil slinger.

B. A bent oil slinger. As the crankshaft rotates, the slinger wobbles, thereby becoming an efficient oil pump.

4. Not enough clearance between the front slinger face of the crankshaft and the crankshaft nose seal retainer in Lycoming aircraft engine. Lack of clearance will trap oil and sling it forward into the seal. If you have any of these conditions, it requires complete engine removal and disassembly to correct. How do you inspect for these conditions without disassembling the engine? With the nose seal removed, pull the crankshaft forward. The crankshaft thrust face will now be against the crankcase thrust face. There should now be 2 to 7 thousandths clearance between the crankshaft slinger and the nose seal retainer. Slip a piece of paper in behind the nose seal casting on the crankcase and see if it fits between the casting and the crankshaft slinger. If it does not fit, you do not have the proper clearance. Rotate the crankshaft, making sure it stays forward and see if the clearance oscillates. If it does, the slinger is bent and will pump oil into the seal. If you do not have at least 2 thousandths (preferably not fewer than 5 thousandths) clearance, how do you change it?

The Lycoming aircraft engine overhaul manual explains how to grind the face of the oil slinger until you get the proper clearance. Any time you inspect a crankshaft for run-out, also inspect the crankshaft slinger and check the clearance between the slinger and crankcase when you lay the crankshaft into the case during re-assembly.

5. Bent crankshaft oil slinger.

6. Scored crankshaft. For Lycoming engines see Service Instruction 1111 for crankshaft oil seal surface reconditioning procedures.

7. Faulty seal.

8. Engine breather tube froze over causing the crankcase to pressurize.

9. Continental GTSIO520 C s/n 150001 thru 150869. GTSIO520 D s/n 188001 thru 188754 check to see of crankcase modification to provide additional oil drainage behind the oil seal was ever performed. Reference Continental Service Bulletin M70-3.

10. Crack in crankshaft. This type of crack usually shows up 50 to 100 hours after a propeller strike. Wipe the shaft with a rag and then rotate the propeller a couple of revolutions. Look closely at propeller shaft for a oil leakage that will look like a black hair on the crankshaft - this is your crack.

11. The crankcase oil seal boss is oblong causing the seal to flatten. This can be caused by a crankcase that has been resurfaced. Any removal of metal on the crankcase parting surfaces changes the dimension of the oil seal boss. You can check the boss dimensions against the dimensional limits that should be in the engine Table of Limits section of the overhaul manual. It is also possible to check the boss for an out-of-round condition. An out-of-round boss will force the seal into also go out-of-round thereby preventing an adequate oil seal around the crankshaft.

12. Frozen or blocked passages in air/oil separator.

Aeronca on floats

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