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Old 01-13-2019, 01:26 PM   #1
Bob Bidonde
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Join Date: May 2010
Location: Long Island, NY
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Smile Rear Main Bearing Oil Behavior

This is my theory regarding oil leakage from the rear main bearings of Model "A" and Model "B" engines. This theory is a bit long, so please bear with me.
  • Oil feeds into the rear main bearing by gravity in the Model "A" and by pump-pressure in the Model "B".
  • Pressures inside of the rear main bearing reach thousands of PSI due to squeeze-loading from the crankshaft.
  • Oil squeezes out at both ends of the rear main bearing. Squeeze-out oil at the front of the bearing rains into the oil pan. Squeeze-out oil at the rear of the bearing slings into a bearing trough, down a drain hole by gravity and through a drain passage in the bearing cap to the oil drain tube.
  • Oil gravity drains into the tube, but the end of the tube is submerged in crankcase oil, so oil backs up in the tube and bearing trough which overflows into the flywheel housing.
  • In the flywheel housing, there is a cotton absorption pad which drains into into the radius rod ball, and oil eventually drips onto the ground.
  • The cause of oil flow into the flywheel housing is the submerged drainpipe exit.
The idea that there is sufficient suction acting on the submerged drain tube exit due to the oil pump intake is not reasonable because the pump intake is too far away from the tube.

It is reasonable thinking that as the bearing clearance increases from wear, the oil squeeze-out and overflow into the flywheel housing increases.

It is reasonable that as the end play of the crankshaft increases with thrust bearing wear, oil squeeze-out and overflow into the flywheel housing increases.

Here's the evidence behind my theory. I am currently running a Model "B" engine that has an extra oil drain in its rear main bearing. This drain exits above the crankcase oil level. Bearing oil leakage onto the ground has been almost none. However, some oil does get to the ground, and it comes from the transmission.


The 1st generation throw-out bearing access cover does not have vent louvers, so air pressure is produced in the flywheel housing by the spinning flywheel. The 2nd generation cover has louvers to vent the air pressure buildup. By using a cover without vents, the pressure in the flywheel cavity would act to significantly reduce any oil leakages.


Okay, amuse me. Fire away at my theory.
Attached Images
File Type: jpg Model B Engine Rear Main Bearing Drain 243KB.jpg (65.4 KB, 246 views)
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