Rear Main Bearing Oil Flow Study

[nkaminar]

It is true that the hydrodynamic pressure is reduced at grooves and holes. But does that matter? Except for some exceptions, the Model A is not a race car.

A PCV (pollution control valve) that reduces the pressure in the crankcase will help to control oil leakage out the back bearing. Several owners have put these on their cars.

[Richard Knight]

Henke, is right with plenty of material to back him up. Lots of machinery rely on hydrodynamic principles to develop oil pressure without an oil pump. Such as crankshaft grinders. Automotive engines have plain bearings and abide by the laws of hydrodynamics.

[Richard Knight]

Henke, is right with plenty of material to back him up. Lots of machinery rely on hydrodynamic principles to develop oil pressure without an oil pump. Such as crankshaft grinders. Automotive engines have plain bearings and abide by the laws of hydrodynamics.

[alexiskai]

Originally Posted by nkaminar

A PCV (pollution control valve) that reduces the pressure in the crankcase will help to control oil leakage out the back bearing. Several owners have put these on their cars.

Are you talking about the oil breather kit? The hose that attaches at the top of the oil filler tube?

[40 Deluxe]

Originally Posted by alexiskai

Are you talking about the oil breather kit? The hose that attaches at the top of the oil filler tube?

No. "PCV" refers to a Positive Crankcase Ventilation valve, used on vehicles since about 1963. These valves permit intake manifold vacuum to draw fumes from the crankcase into the intake manifold to be burned in the cylinders. Fresh air is admitted into the crankcase through a hose from the air cleaner so there is a constant flow of clean fresh air through the crankcase. Thus there is usually a slight vacuum in the crankcase instead of blowby pressure so less oil will be pushed out past the rear main bearing.

[alexiskai]

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Originally Posted by 40 Deluxe

No. "PCV" refers to a Positive Crankcase Ventilation valve, used on vehicles since about 1963. These valves permit intake manifold vacuum to draw fumes from the crankcase into the intake manifold to be burned in the cylinders. Fresh air is admitted into the crankcase through a hose from the air cleaner so there is a constant flow of clean fresh air through the crankcase. Thus there is usually a slight vacuum in the crankcase instead of blowby pressure so less oil will be pushed out past the rear main bearing.

I know what a PCV is, but (a) that's not the PCV he claimed to be talking about and (b) I haven't heard of anyone literally retrofitting a PCV to the Model A engine. So I want to know whether he was referring to the oil breather kit. The breather kit is not a PCV since it does not route fumes back to the intake.

[Bob Bidonde]

Here is a list of some things that negatively affect hydrodynamic oil pressure in the engine's bearings:
1. Excessive bearing clearance due to wear;
2. Babbitt damage to due to fatigue cracks, chunks of babbitt broken-off;
3. Axial alignment of journal & babbitt;
4. Inadequate motor oil supply due to low oil level / sludge / failed oil pump / failed oil pump drive / failed drive hold-down spring, extra drain hole in bearing cap;
5. Excessively high bearing loads due to lugging the engine, RPM too high, clutch chatter;
6. Rough surface of babbitt / bearing journal;
7. Excessively high temperature due to overheating;
8. Breakdown of motor oil film strength & loss of additives;
9. Excessive debris in motor oil;
10. Coolant intrusion into motor oil.

[nkaminar]

Sorry, I was referring to the PCV that 40 Delux talked about. Maybe the name should be NCPV, Negative Crankcase Pressure Valve. The way they work is to open up when the intake manifold pressure is increased from the idle throttle position so that the car can idle. I guess it could be hooked to a hole in the valve cover and the cap on the filler tube be restricted to limit air intake to the crankcase.

[alexiskai]

Originally Posted by nkaminar

Sorry, I was referring to the PCV that 40 Delux talked about. Maybe the name should be NCPV, Negative Crankcase Pressure Valve. The way they work is to open up when the intake manifold pressure is increased from the idle throttle position so that the car can idle. I guess it could be hooked to a hole in the valve cover and the cap on the filler tube be restricted to limit air intake to the crankcase.

This came up a few years ago. The general sentiment seemed to be that it would not reduce oil leakage and might have undesired side effects.

https://fordbarn.com/forum/showthread.php?t=232846

[jack backer]

Thick ring pistons and a splash lubrication system don’t lend themselves well to positive crankcase ventilation. Plug fouling, stuck piston rings and heavy carbon build up will be the result. The A needs a cleaner combustion chamber, it has enough carbon, burning oil vapor will make it worse.

[Richard Knight]

None of those things happened to my B engine with a pcv system. It did help with oil leaking. Wherever the vacuum us applied such as the side cover the opening needs to be shielded so you are not drawing oil soaked air. The shield acts as a scavenger. The pcv valve I used was oriented vertically for it to function properly. I took thevacuum off of an antenna downdraft manifold.

[johnneilson]

Yeah, I run a PVC system on my coupe, none of those problems.
It does help to have a mechanically good condition to start off with.
Splash oil system has nothing to do with it, where do you think the oil from a pressurized system goes?

On the race car, it pulls negative crankcase pressure from 5 stages of scavenge pump on dry sump system. Needless to say the leaks are minor.

J

[hazelhoff]

Interesting to compare the Model A engine to Model T. T didn’t have an oil seal at all, no oil grooves either. So … Model A engines were ‘improved’ T- engines ?

[Flathead]

The T didn't need a rear seal because the engine and trans were directly connected and shared the same oil supply.

[hazelhoff]

Very true Flathead ! Sorry about that … still … no grooves …

[Richard Knight]

Bob listed 10 things that negatively affect hydrodynamic oil pressure. Number 11 or what would be #1 on my list would be oil grooves across the face of the bearing.

[[email protected]]

Originally Posted by 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.

[[email protected]]

thank you good post

[Gold Digger]

When I was 13 I had a model A strip-down, and all it had was radiator, engine, transmission, rear end and frame and wheels. Worn almost completely out. The back main got so bad it left a trail of oil everywhere I went and I was almost out of old used oil one day so i had the bright idea to plug the oil hole in the tappet bay. I cut a stick and pluged it up. About the time I almost had it buttoned up I thought why didn't I cut a flat place on the stick and let some oil down to the main. I tour back into it and fixed the stick and almost stopped the oil leak. I guess that was what they call "farmer fix" now days.

[JoeCB]

Gold Digger, good story... you could have just hung a bucket under it and "recycled" ... you would have been ahead of your time.

Joe B