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Old 01-13-2019, 02:28 PM   #2
BRENT in 10-uh-C
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Default Re: Rear Main Bearing Oil Behavior

Originally Posted by Bob Bidonde View Post
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.
  1. Oil feeds into the rear main bearing by gravity in the Model "A" and by pump-pressure in the Model "B".
  2. Pressures inside of the rear main bearing reach thousands of PSI due to squeeze-loading from the crankshaft.
  3. Oil squeezes out at both ends of the rear main bearing. Squeeze-out oil at the front of the bearing (d)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.
  4. 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.
  5. 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.
  6. 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.

I have read this 4 - 5 times and am still struggling to find anything to really disprove your theory however, there is one or two points that probably need to be re-discussed. For the purpose here, let's just discuss the Model-A application to keep it simple(r).

#1 While the tube feeding oil from the valve chamber area is approximately the same size as the drain tube, all of the oil in the tube is used at the same time. I am not going to do the calculation here but to basically round off the numbers, let's assume the tube feeding into the main journal is 0.375". One would need to calculate in the oil transfer groove to the wells, but the key is how much clearance is between the ells and the crankshaft journal pin? I think #2 factors into this.

#2 I think by theory, the micro-scratches of the crankshaft can/could create some pressure however it is much like the Model-A water pump which is not a positive pressure displacement pump. There really is not anything "capping" the oil to create a pressure. Therefore IMO the micro-scratches and the crankshaft's rotation is really just acting as a transfer pump and not a pressure pump.

#3 I agree.

#4 While in theory this makes sense, the difference is that as soon as the oil backs-up in the tube, the weight of the "backed up" oil begins to create downward pressure which will cause it to continue to flow. Calculations can be made to determine the level the oil in the drain tube will be above the surface level of the an oil, but I would anticipate the oil would only be an inch or so above the level of the oil in the pan. As more oil is back-up in the tube, theoretically there should be more head pressure to push the oil out into the pan.

#5 I agree.

#6 The oil pipe, -while submerged, still has head pressure from above that will force the oil to the level at which the oil is outside of the drain pipe.

Where all of this theory changes is by the clearances between the oil inlet and the journal pin. If the clearance is around 0.00125" - 0.00150", the flow is restricted by a greater amount than if the clearance were at 0.002" - 0.003". At some point (-that would need to be calculated) the inlet oil volume exceed the hydraulic flow of the passage way within the cap and the tube. IMO, modify the cap like you have shown in your picture for the additional oil flow from the slinger area, and you can have additional inlet flow (-caused either by pressure or clearance.). Good job on your theory and cap mods.


BRENT in 10-uh-C
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