Oil drain tube

How does the drain tube work? Considering there must be flow, then why is the bottom end of the drain tube submerged in crankcase oil? I am thinking that the oil drain pipe is not necessary. Here's my theory.

For flow to occur in the drain pipe, the squeeze-out oil pressure developed in the rear main bearing must be sufficient to force oil through the drain pipe and to displace the crankcase oil in the submerged end of the pipe.

Why submerge the bottom of the drain pipe? Crankcase atmospheric pressure from the motion of the pistons plus blow-by will act on the oil in the drain pipe whether or not the pipe is submerged. By submerging the end of the drain pipe, flow in the pipe is impeded.

Oil flow out of the rear main bearing will find the pathways of least resistance, so will oil spill into the flywheel housing if and or when the drain pipe backs-up? I think yes it does.

If the drain pipe is removed from the bearing cap, will oil drain into the crankcase? I say yes it will, perhaps more so, and less oil will leak into the flywheel housing.
 
Bob,
Most of the time the crankcase is under positive pressure from any bit of blow-by that is present. If you subject the oil trying to flow from the rear main to any amount of back-pressure, you are going to slow its flow. The oil tube sealed to the rear cap and submerged in the oil in the pan stops this pressure from slowing the flow. The pipe also creates a bit of "draw" on the rear main cap as the pipe is vertical and the oil entering the top of the pipe falls/flows to the level of the oil in the pan.
I have fixed more than a few rear main leaks by installing or replacing a missing, short or extremely loose drain tube.
Good Day!
 
I don't think a straight tube that is long enough will fit. It will hit the pan where the pan starts to slope forward.

I'm surprised no one has mentioned, it is a good idea to file a "C" grove in the thread area on the side facing forward to insure the tube isn't threaded in too far. Then do whatever you do to lock the tube in place. If your are worried about a leak from that area, put a little fillet of JB weld around the pipe.

Richard
Anaheim CA
 
The breather in the oil filler pipe on the left side of the engine will release any crankcase pressure, so blow-by pressure in the crankcase is not a problem unless the filler pipe or cap is plugged up. I think that the oil from the rear bearing simply drains through the tube by gravity.

Whether or not the end of the tube is under the surface of the oil when the engine is stopped does not matter. When the engine is running, the splash system will agitate the oil enough so that there will be no “normal oil level” in the pan.
However, the drain tube will deflect oil and prevent it from being splashed back up into the rear main bearing drain port. If the pipe were missing or too short that would create a condition where backsplash could occur.
I think.
:)
 
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Dave,
The pressure in the crankcase would act on the crankcase oil as well and tend to push it up the drain pipe. So it doesn't matter if the drain pipe is open to the crankcase atmosphere or it is submerged.
 
Right, the pipe has to go into the oil. That is where it's ment to be, if it don't it can and does lead to oil leaking out the back into the clutch housing. Someone posted above, that fitting this tube correctly when one was missing or to short, did correct the oil leak.
Thems the facts.
I do understand your thinking about pressure above the oil level will act on the oil also, your thinking then has the oil being pushed back up the tube, as it can't displace the oil in the pan.
The way I see it is, think of the crank case pressure , the pressure above the oil will indead effect the oil below the level. We know that, the difference here is its not really normal pressure that's the problem, normal pressure would vent via the oil fill tube. What's going on in the crank case is a whole mess of air being pulled and pushed by the pistons, whizzed around stupid by the crank and rods and having oil splashed and thrown around by all that business. That mess does push oil back up the drain tube if it's to short or missing. The relative peace and stillness below the oil level prevents less of a restriction to oil flow down the tube, than that living hell going on above it.
That's my opinion, makes sence to me that way. But we must keep in mind, opinions aside, it does work properly with the tube below the oil.
If adding a tube of the right length don't fix the leak, theres something else that also needs fixing.
Martin.
 
I installed the new oil drain tube in the rear main today. Put permatex on both pan and block and bolted everything back up. The leak out of the bell housing has stopped!!! I want to thank everyone who gave their 2 cents on this problem. It has been a fantastic day. Thanks again, John
 
Bob, the theory behind the oil tube is kind of like the water pump on a Model T, it doesn't have one. Sounds bad but Model t's never had a water pump and in good shape won't overheat in the hottest weather. The oil tube in an A completes the flow from the sump to the valve galley to the pan. Oil being used from the bottom of the pan creates a slight suction from the drop in level when the oil is pumped to the valve chamber. This suction creates a slight pull at the tube which is submerged in the oil in the pan. This completes the circulation of oil. If you don't think your motor won't leak without the tube, just remove it and see. Ask me how I know this.

This just shows that what we think shouldn't work sometimes is the only way it will work. Like the thermosyphon of a Model T.
 
James Rogers said:
Bob, the theory behind the oil tube is kind of like the water pump on a Model T, it doesn't have one. Sounds bad but Model t's never had a water pump and in good shape won't overheat in the hottest weather. The oil tube in an A completes the flow from the sump to the valve galley to the pan. Oil being used from the bottom of the pan creates a slight suction from the drop in level when the oil is pumped to the valve chamber. This suction creates a slight pull at the tube which is submerged in the oil in the pan. This completes the circulation of oil. If you don't think your motor won't leak without the tube, just remove it and see. Ask me how I know this.

This just shows that what we think shouldn't work sometimes is the only way it will work. Like the thermosyphon of a Model T.
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That suction if it exists wouldn't amount to a hill of beans in my opinion. Oil is constantly falling back into the pan to maintain its level.
 
edmondclinton said:
That suction if it exists wouldn't amount to a hill of beans in my opinion. Oil is constantly falling back into the pan to maintain its level.
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Do like James said take the tube out and try it. please report back, we might all learn some thing new. I,m with James on this one.
 
While I have worked on cars for 50 years, my knowledge of Model A's is limited. I know that with the short oil drain tube in the rear main there was a hell of a leak. With the correct tube which is in the oil, the leak stopped. At this point, I don't care about the pressures in the crankcase. I solved the leak and I'll let the rest of you decide what works and what doesn't. Again, thanks for the help. John
 
29spcoupe said:
While I have worked on cars for 50 years, my knowledge of Model A's is limited. I know that with the short oil drain tube in the rear main there was a hell of a leak. With the correct tube which is in the oil, the leak stopped. At this point, I don't care about the pressures in the crankcase. I solved the leak and I'll let the rest of you decide what works and what doesn't. Again, thanks for the help. John
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That is the correct answer, decide for yourself. All I know is, I had one to fall off once and had the most embarrassing oil leak I ever had. Replaced the tube and welded it in and no more leak.
I would also like to add that the tube and main cap thread is not a 3/8-24. It is a 3/8-32 special thread.
 
I'm evaluating my '31 Model A engine for a freshening/rebuild and found this thread that's related to a problem I found. My engine didn't have a drain tube on the rear main cap, and yes, it did have a significant oil leak at the rear of the engine. Not only was the tube missing, but the cast iron is broken out about 50% of the way around what would be the female threaded area where the tube threads into the cap . It's broken out to the extent that about 25-35% of the edge of the galley plug is exposed. The babbitt in the cap is in good enough condition that I feel it can be used as-is with some slight re-shimming, so I don't want to do a lot of pre-heating of the cap, as is often done when welding cast iron, that might compromise the babbitt. I don't really care if I can ever remove the tube so I'm wondering if I could do tack welds to install the drain tube to not overheat the cap, let it cool, and repeat as needed to secure the drain tube in place? Suggestions?


Thanks


Lynn
 
I had purchased a rebuilt engine from a shop in N.C.I wound up with a terrible oil leak at the rear of the engine.I dropped the pan and found a short home made tube,replaced it with a original tube and that corrected the oil leak.
 
Tom Wesenberg said:
Yes, you can MIG weld it. You could stuff a wet rag into the babbit and do short welds so it doesn't get too hot.
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Good plan described by Tom.
FWIW: You do need this tube...no matter what others say in this thread. The rear main will most often leak without a tube long enough to be submerged into the oil. I have repaired too many leaking rear mains that have had missing or short tubes to know this. The theory as to why it works is not important....just that it will.

When I have a condition as you describe but maybe not to the missing material extent you describe, I often seal the pipe and galley plug to the cap with slow set JB-Weld after securing the pipe to the cap with spot welds. If you use the JB-Weld, be sure to de-grease the cap, pipe and plug area with a good solvent. The JB-Weld will also add to the support provided by the welds.
I may have an open engine with the pipe sealed like this, if so, I will take a photo and attach it later today or early tomorrow.
Good Day!

www.durableperformance.net
 
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The drain tube works like a boat in the current of a river. The oil does the same form the tube it flows with the current. Which will pull oil out of the rear main.
 
Thanks for the pointers everyone. I feared it might be a bigger problem than it would seem it really is. Have MIG ready to go.....and some JB Weld to add that extra bit of strength and sealing to the joint.


I appreciate the timely replies.


Lynn
 
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