Rear Main Bearing Oil Flow Study

[Bob Bidonde]

Sometime ago, I mentioned that I was doing a study of the oil flow through the rear main bearing. Recently, I came to a conclusion with a theory I am sharing with you in the attached slides. The last slide sums my theory as to why oil leaks to the ground.
The slides are JPEGS. Agree or disagree, I hope you find my slides entertaining and food for thought.

[nkaminar]

Bob, excellent artwork. I agree that it is a balance between the oil moving out of the rear of the bearing and the drain back into the pan. It seems to be a delicate balance and works OK if the bearing is not pushing out excessive oil. The Ford engineers should have tested the system with an engine set up with excessive clearances.

[jeepguy1948]

Why would they do that LoL, Henry wanted to sell more cars. They did (I believe) increase the size of the drain on the later cars didn’t they? Bob, I think that you are investigating the problem that I hear about 5 times as frequently than any other (including me personally). Thanks for the good work!

[Will N]

I'd also add that as the engine wears and the clearances in the rear main increase, the amount of oil in the space between the bearing and crank increases, and the amount of oil working its way back to the slinger to drain back via the tube overwhelms the capacity of the tube. Adding to this is that if the bearing clearances have gone out of tolerance, it's also more than likely that the piston rings are worn, allowing blowby into the crankcase, increasing crankcase pressure which would work against the oil draining from the rear main via the tube. In fact, the oil drain tube would become a path to vent the crankcase pressure rather than a path to drain the oil.

[alexiskai]

Originally Posted by Will N

Adding to this is that if the bearing clearances have gone out of tolerance, it's also more than likely that the piston rings are worn, allowing blowby into the crankcase, increasing crankcase pressure which would work against the oil draining from the rear main via the tube. In fact, the oil drain tube would become a path to vent the crankcase pressure rather than a path to drain the oil.

Doesn't the oil filler tube vent the crankcase? Isn't that why some folks put those hoses on their oil fillers, because they have blowby and it's causing oil to spray out the cap? Seems like you couldn't build up much pressure as long as the tube is functioning as a relief valve.

[rotorwrench]

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Crankcase pressure was designed to vent out the oil filler but pressure takes any pathway of low resistance. Internal combustion engines have compression leakage from day one but it tapers back some as the rings fully seat. As the engine wears, the clearances slowly increase which creates more blow by and journal bearing leak down rate.

Ford engineers knew that the rear main was going to have some leak down and some compression leakage from the start so they wanted to make sure that the rear oil slinger cavity would not get overwhelmed which would insure the least amount of leakage they could accomplish. The drain tube extending down into the sump oil was the best way to do that.

The early Ford designs all seeped a bit of oil but they were trying to limit that the best way they could with the technology they were utilizing at the time. The oil just flowed down there by gravity feed so this was only marginally better than the model T. If you wanted a car that leaked less then you bought a Lincoln and paid the price for it.

[jack backer]

The hydrodynamic principle is a constant, .001 oil clearance for each inch of crankshaft journal diameter. Exceed that and the oil won’t bear the load of the crankshaft journal, you won’t create the pressure required to “float” the journal. Ford designed the labyrinth seal to leak if the clearance is exceeded. If ford engineers designed the drain back function to carry more oil than leaked by the bearing at .0015 clearance there would be no tattle tale oil leak indication of bearing wear..

The previous owner of my engine added a crankcase breather to the valve cover above the rear main bearing oil sump..I question the need for it, but left it cause it can’t hurt.

[Kurt in NJ]

another factor is the labyrinth fit, that very thick shim pack so you can adjust for wear forever also increases the gap between the slinger and the cap, I used that fit to determine the shim pack thickness, .004 of shims made that round and concentric to the crankshaft on my car, still no leaking out of the rear main after 35 years—- perhaps a drop in the middle of the night, but no marking of territory.
Another interesting experiment would be the direction of oil flow in a running engine that has been modified with oil line to the bottom of the center main cap.

[hazelhoff]

Bob, amazing ! I can only applaud ! Thank you for the thinking and the wonderful illustrations.

I will send you an article I wrote a couple of years ago. Basically this article is focussing on the need for having oil grooves …. maybe we could be of mutual support …

[Bob Bidonde]

I have pictorial evidence that brand new Model A's in the dealer's show room leaked oil (see the drip pans), so it also leaks when the bearing clearance is within specification.
I agree that as the bearing clearance increases, hydrodynamic oil pressure decreases, so the crankshaft runs off center and eventually spins in contact with the babbitt.
I am glad you enjoy my study.

[jack backer]

There are a few reasons for leaks in that area shown in the picture, one of the leaks was designed by engineering. The transmission fork shafts weren’t sealed, allowing seepage to run down the clutch housing and lubricate the wishbone ball.

The increased oil clearance caused by bearing wear will over come the ability of the bearing drain, although I have no proof it was intentionally designed in to indicate bearing wear. I do know a correctly assembled and maintained engine won’t leak from the rear of the rear main bearing.

[WHN]

Originally Posted by Bob Bidonde

I have pictorial evidence that brand new Model A's in the dealer's show room leaked oil (see the drip pans), so it also leaks when the bearing clearance is within specification.
I agree that as the bearing clearance increases, hydrodynamic oil pressure decreases, so the crankshaft runs off center and eventually spins in contact with the babbitt.
I am glad you enjoy my study.

Great picture. I have not been able in my 60 plus years owning Model A’s to get one to stop dripping oil. Not a lot, but they seem to like to mark there spot.

Nice job. Keep up the interesting work.

[Bob Bidonde]

Here is a cliche I was told by an old timer mechanic back in the 1960s when I was driving my newly acquired first Model "A". "If it ain't leakin' its out of oil!"
I remember seeing the dark stain down the middle of roads from engine oil leaks.

[jack backer]

Sunrise Highway.. bleached concrete on the lane tire tracks, 2 foot of the center jet black from oil. Computerization of machining and modern sealants dried it all up, most folks today see a spot and think something’s wrong.

[GeneBob]

All,
I saw a suggestion somewhere that when you have the drain tube out, file a small notch into the back side of the tube where the threads are located. Just a couple threads deep, to ensure that the tube does not restrict the return flow if you thread it in a turn too far. Some of these parts have been threaded in several times and may be a little worn.

[Richard Knight]

Look at any of the hundreds of factory photos of the New Ford in books that have been published over the years. Most all have oil spots under the car. Go to your local food market, home depot, whatever look at the parking lot, how many oil spots do you see ?, how many model As are in the lot. Probably none but there are plenty of oil stains. Modern cars leak. Virtually all model As leak, some very little some leave a trail. There is good info in this thread. The evolution of the drain back pipe is a 5/16 pipe increased to 3/8s. Bs are 3/8s, early V8s a short tube. And then no tube with just a large hole in the cap. Crankcase compression combine with bearing clearance are the factors that determine oil leakage. Pressure in the air space is the same in the oil.

[JoeCB]

Interesting discussion and thank Bob for the research and graphics.
Nowhere here mentioned the possible influence of spirally shaped oil grooves in the rear bearing. The conventual form is sort of a "loop" , anyone ever tried a slow sprial, driving oil toward the pan end? I hear that this has been tried on the pulley hub.
Joe B

[hazelhoff]

Most of your cars have oil grooves. Mine doesn’t because the machinist thought it was inappropriate to put them in ! Lucky me, he understood hydrodynamics, I didn’t at that point in time. Oil grooves will destroy your oil pressure caused by the crank, because they cut right through the ‘pressure bulb’.
That being said, my lateral clearance was over 0.0015. Drop of oil every 5 seconds. I repaired this ‘axial’ clearance to some 0.001. Drop of oil every minute. This is not bad at all, assuming my ‘non grooved’ Model A has a higher oil pressure than the grooved ones …

[hazelhoff]

If you ‘must’ … limit yourself to putting in oil grooves in the upper half only ! Connecting the oil intake to the lateral ‘wells’ halfway down. That will be safe for the oil pressure. I have a very interesting article containing Britisch scientific lab experimental studies on the effects of oil grooves on oil pressure/bearing wear …

Any body interested: pm me !

[jack backer]

If the clearance is correct the hydrodynamic principle will occur on all non grooved surfaces. The grooves provide a break and help to flush debris from the bearing