Tech Tips

Does Your R985 Blow Oil Out The Breather?

A Case Study: The Aero Recip 24-7 AOG Service

Aircraft:  DHC-2 Beaver with Pratt & Whitney R-985 engine

The Defect:

Excessive oil leak from the engine’s magneto drives, and has abnormal pressure in the oil tank


On a busy June weekend, the operator’s PRM is unable to determine why the magneto drives are leaking. All engine instruments and performance are normal. Customer and pilot safety being the priority, the aircraft is grounded until the reason for of the leak and pressurized oil tank is confirmed and repaired.

Taking Action:

After attempting unsuccessfully to contact another engine shop, the PRM contacts Aero Recip on its 24-7 AOG line. Aero Recip answers immediately and connects the PRM with Aero Recip Technical Advisor, Wayne Cathers.


After careful analysis of the situation, Wayne directs the operator’s PRM to check for a cork in the rear case. “We pulled the fuel pump and drive off and reached in behind to find the port – that allows the oil out – blocked by the generator drive cork seal.  A cork is located. It had become lodged in the rear case drain port, impeding oil flow, causing the rear case to fill with oil.”

Root Cause:

The OEM installed generator is commonly replaced with an alternator. In these cases, the alternator drive spline can push the cork seal out of the bottom of the accessory drive gear, which causes the cork to fall into the rear case, potentially migrating to the outlet drain port.  “We converted the generator to an alternator drive on the R-985 a year earlier,” the PRM explains, “which pushed out the cork.  After about 300 hours the cork became lodged in the drain hole and oil built up in the rear case.”

Sleeping Again:

On confirming the cause of the leak, according to the PRM, “Wayne’s instructions really helped.  After removing the cork, we reinstalled the fuel pump and drive, tested the engine and returned the aircraft to service.  “We’re definitely happy with AeroRecip’s knowledge and support. It meant we could sleep at night knowing everything was running smoothly again.”



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