6. Internal engine oil leak on Lycoming engines. With Lycoming engines you need to check main bearing clearance and rear plug in center of crankshaft for leaks. Remove propeller governor. There is a passage that goes down to the center of the front main bearing. The big hole indexes to the oil transfer tube which goes to prop governor. Take a differential pressure tester and with a rubber end push against passage. Apply 40 pounds in and as oil blows out of bearing you will see the secondary gauge come down. After 15 or 20 seconds you want to see 5 pounds or more. If the bearings will not hold 5 pounds you have an unacceptable leak. This is a quick and easy test to do after engine assembly at overhaul.