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.

Re: Rear Main Bearing Oil Behavior
 

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.


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Re: Rear Main Bearing Oil Behavior
 

One very important factor is crankcase pressure from blow by past the rings , this is certainly more important than what is going on in the bell housing.

Re: Rear Main Bearing Oil Behavior
 

I have wondered at times about stopping the drain tube above the oil level but have never thought about pressure in the bell housing. Everything I have read says it would make thgings worse but we've all heard about Fake News!! There would not be much pressure build up in the bell housing (has it ever been measured?) due to leakage and what pressure is developed is only by chance. That is, nothing is designed to build up pressure in there. If it were, I suspect it would also slow or stop oil leaking from the gearbox from around the selector shafts.
A very interesting theory. Now, how to pressurise the bell housing and test it.
Calling for volunteers.;)

Re: Rear Main Bearing Oil Behavior
 

I reringed my model a engine and now have a rear main leak. Any thoughts on how long it takes for rings to seat? I know it's a loaded question.. I had a couple thou clearance about 1' down between the piston and bore on all 4 cylinders and put new rings on the original pistons (.125 overbore andd don't know of bigger pistons out there). Once i got it running it would leak 7-8 drops per minute at idle. It's down to ~ 4 drops per minute after 325 miles of driving.

Re: Rear Main Bearing Oil Behavior
 

I found most of the leak happens once one stops, ( the sump fills up with its oil ) How many have seen how much oil is in the sump when engine running ? the end of the drain pipe is out of the oil I believe , We had a local who put windows & a light so you could see inside, There is sweet fanny oil in the sump when its running, lots went & put another litre in , Most of the oil is up in the engine, Now for the feed back ,Keep the ideas coming in & yes changed a non vented f/wheel plate foe a vented one twice & leak stopped, also last week guy complained his new engine with mod seal pissing oil out, Dip shit had put rubber cap over filler pipe,????? Wonder why it leaked,??

Re: Rear Main Bearing Oil Behavior
 

I believe there is a very good reason why Henry designed the rear main drain tube to be submerged in oil. On a recent tour, one Model A had an external oil filter that had a small leak. The owner never checked the oil. After about several hundred miles, enough oil had leaked out to the point that the tube was no longer submerged in the oil in the pan. Oil then started to leak from the rear main. So much that a small puddle would appear under the rear area of the engine when the car stopped. The rear main did not leak before.


After adding 3 1/2 quarts of oil, the rear main leak disappeared.


As long as the oil level was kept between the add and full mark on the dip stick, there was no rear main leak.


I have tubes in all my engines and do not have problems with rear main leaks.


Again, this is just my experience and opinion, obviously, there are other opinions.


Chris W.

Re: Rear Main Bearing Oil Behavior
 

I just had a rear bearing cap fail. Upon taking it apart it had a shortened drain tub. Most of the thrust was gone on the rear and about 20% of the bearing surface. Crankcase back-
pressure could come in to the question, also 8% mile and a half grade on every trip to town. Is going down hill worse than going up, is the oil in valve chamber to low to feed the rear tube to bearing? I put it back together with a stock drain tube, 3/8 in.

Re: Rear Main Bearing Oil Behavior
 

When I bought my coupe the engine had been rebuilt with only 100 miles on it. The oil leak from the bell housing was very bad. I dropped the pan and discovered the drain tube was a pipe nipple that was too short to reach the oil level. I replaced it with the proper drain tube and the leak stopped. All of the mathematics above are a little beyond me, but I got the correct result. I believe the drain tube needs to be in the oil. John

Re: Rear Main Bearing Oil Behavior
 

About ten years ago a nice 1930 Model A came into the shop with a small rear main leak.
I dropped the pan. Bearings were a little loose and no rear main drain tube. Did not adjust the bearings. Thought I found the problem. Installed drain pipe, installed pan with new gaskets. Started engine and the leak was worse. Removed pan, removed oil drain, reinstalled pan with new gaskets. Started engine slight leak as before.

Advised the customer that engine had to be removed to adjust the bearings or to rebuild the engine. Customer decided on rebuild while out our car due to other problems.

Fresh rebuild no leaks. Ten years later slight leak. Maybe Bob has something going.

Re: Rear Main Bearing Oil Behavior
 

Many good points are made above. Most eye opening to me is that I did not consider the effect of crankcase pressure due to piston motion. Hmmm. More to follow.

Re: Rear Main Bearing Oil Behavior
 

I donno, I thought the pump pumped about a gallon ( 4 qts) a minute...so at 5 quarts in the sump, the drain is likely out of the oil alot. Just wondering.

B.

Re: Rear Main Bearing Oil Behavior
 

Crankcase pressure should be neutral unless there is blowby.

Re: Rear Main Bearing Oil Behavior
 

When main bearing clearance begins to increase, the ability of the tube in the rear main to drain the oil back into the pan can become overwhelmed. First order of business should be to adjust the bearing clearance .At the same time the oil hole in the valve chamber above the rear main is in most cases too large and drains more oil onto the rear main than is needed. So with the excess clearance the oil ends up being slung into the bell housing. The simple fix is to adjust the clearances in the bearings and at the same time use a 1inch long piece of 1/4 in. tube with a flare on 1 end, and drop it in the oil drain hole over the rear main in the valve chamber. The rear main will still receive adequate oil and the tube will stay in place by gravity. I was told this by a professional engine builder who helped me with my oil leak into the bell housing. We did the above and I have been without leaks ever since. Happy Motoring!

Re: Rear Main Bearing Oil Behavior
 

https://i.imgur.com/s208QVN.png?1

This is an oil flow illustration with a pressurized engine,factor the picture without pressure as in a gravity feed system and you can see that hydrodynamic force does not pressurize the oil.

https://i.imgur.com/OTV26Ni.jpg?1

the previous owner of my engine ran a vent tube from the top rear of the valve cover.This vent tube prevents positive or negative pressure from building in the valve chamber directly above the rear main feed tube..I personally have doubts about the need for it,but left it for it causes no harm.

Oil leak at the radius ball?sure it isnt your shifter shafts leaking?

Re: Rear Main Bearing Oil Behavior
 

Wow, is that illustration written by someone who is human:) I can read a few English words, the rest is totally beyond my comprehension. I will take your word for it what it all means.

Re: Rear Main Bearing Oil Behavior
 

Hello,
a hydrodynamic lubricant film is created in every relative movement. It does not be a matter if the oil slides under pressure into the bearing or if the oil gets into it by spraying or dip in. This makes the adhesion / cohesion, to get in the bearing parts. Because the oil amount is extremely low, the oil film thickness is only a tight micro inch.

Re: Rear Main Bearing Oil Behavior
 

The above graphic and explanation is well written and provides more detail than the average Model A owner needs, or can understand. It appears to have come from an Engineering 101 book. When most people start getting oil into the bell housing and on their clutch they want a fix for the problem. The above explanation did not provide a solution to the problem. While there can be numerous reasons for the oil getting in there, the most likely reason is excessive bearing clearance. There again, returning the bearing clearance to the .0015 specification stated in Les Andrews book will go a long way to solving the problem. As I also stated, if you will put the small piece of tube in the hole in the valve chamber to slow down the flow, you can help prevent the slinger groove and drain tube from becoming overwhelmed.

Re: Rear Main Bearing Oil Behavior
 

Just to insure my comment is not taken out of context it was meant. My post was not a criticism or complaint. I am sometimes blown away by the depth of knowledge of members of this forum.

Re: Rear Main Bearing Oil Behavior
 

I'm not sure I 'buy into' this theory. I can't help but wonder how that "professional" determined that??

I would think the restricting the oil flow to decrease volume is a band-aid for masking a problem hidden elsewhere When you consider the crankshaft journal has approximately 0.00075" (- approximately ¾ of a thousandths of an inch) clearance between it and the babbitt when the total clearance is set at 0.0015", ...and that effectively restricts the oil from flowing freely down the supply tube. From there, if the passages inside the rear main cap are clear from the slinger area to the bottom of the drain tube, then any excess oil from the journal area should be able to drain freely.


I will share with you that many engine rebuilders do not take the time to remove the welch plug in the rear main cap and clean the passages due to the extra time it takes. Often times there is all kinds of dirt, sludge, babbitt,& debris that is in there even after the cap has been cleaned. All it takes is just a small amount of 'krud' in the passage to restrict the flow which can cause a rear main to overflow and leak. It probably takes an extra ½-hour of time to thoroughly clean a rear main cap, so when the budget is not there, this step generally gets omitted.