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Old 01-05-2021, 04:32 PM   #1
Cape Codder
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Default Cause of Blow By

What is the cause of blow by in an engine?

The left side of the block in my car always has a mist of oil on it. I did the sock trick but many have said that is not a good idea. I have thought of trying to buy the aftermarket metal hose which attaches to the oil filler pipe, BUT that is just a "BAND AID" in my eyes.

So back to the start, "What is the cause of blow by in an engine?"
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Old 01-05-2021, 04:40 PM   #2
Synchro909
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Default Re: Cause of Blow By

There is a mist of oil in the cranks case while the engine is running. It lubricates everything that is not positively lubricated, like cam shaft, cylinder walls etc. Worn piston rings or ring that have lost their tension (maybe over heated) and/or worn bores allow too much exhaust gas past them and into the crank case, forcing the oil mist out of the breather. That is why you get the oily film everywhere. One way of dealing with it is to feed it back into the inlet tract and burning it but that doesn't deal with the cause of the issue. That can only be dealt with by a new set of rings (after making sure the bore is parallel) or a rebore with new pistons and rings. The first option is somewhat temporary.
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Old 01-05-2021, 05:16 PM   #3
Mulletwagon
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Default Re: Cause of Blow By

A first step would be a compression check to assess ring performance. Maybe try some Seafoam if case the rings are just stuck. Let us know how it goes.
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Old 01-05-2021, 05:17 PM   #4
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Default Re: Cause of Blow By

Look at the ford service bulletins, there have been several changes in the designs. Is your cap venting properly ? if it pushed down to far it will not vent
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Old 01-05-2021, 05:28 PM   #5
Kurt in NJ
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Default Re: Cause of Blow By

I got to watch it happen with my smiths compresser. I had the oil level to the middle of the letter "F", the owners instruction book for the car says "up to the f", once it expelled the excess oil it stopped
Are you getting smoke clouds or just oil drops, with the splash system if the oil level is above the dipper tray too much oil will get splashed around --- lowering the oil level will reduce that cause, if the rings are worn there will be more blow by
The model B has a larger diameter oil fill tube to reduce the problem, and if you look in the service bulletins there's several versions that were used to reduce the problem, , if you look at old pictures of roads there were oil slicks down the center of the road because most all cars leaked some
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Old 01-05-2021, 05:37 PM   #6
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Default Re: Cause of Blow By

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Quote:
Originally Posted by Cape Codder View Post
What is the cause of blow by in an engine?
Cape, To answer your question, It is all in the name, 'Blow by' the rings. There will always be a certain amount of blow-by, the rings can not seal 100%. If it is excessive, sorry to say the only cure is new rings/rebore.

If not excessive I would install one of the metal breather hoses to route under the car instead of the sock. The breather hose is actually a good thing as it acts as a road draft tube to draw the bad byproducts of combustion from your crankcase, acids, moisture and unburned hydrocarbons.
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Last edited by Ruth; 01-05-2021 at 08:28 PM.
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Old 01-05-2021, 08:04 PM   #7
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Default Re: Cause of Blow By

[mention]Cape Codder [/mention] I had similar concerns with my Tudor. I recently purchased a spark plug adapter so I can perform a compression test but haven’t gotten to it yet, as my list keeps growing. I am on Cape Cod as well if you’d like to borrow it !


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Old 01-05-2021, 08:15 PM   #8
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Default Re: Cause of Blow By

Synchro909 - I'm talking about an engine with like 14,300+ miles on it. Are you saying that the rings are getting worn already OR should I not expect it to get much more mileage on it before rebuild? The engine has never over heated i.e. radiator has never boiled over so that being said can the rings overheat as you suggest?

Mulletwagon- Recently had head off due to seepage on left side of block. Replaced head with a new Copper Gasket. Did a compression check after the head was installed and all cylinders were in the upper 60's.

Kurt in NJ - I don't have any smoke clouds just a light coating of oil on the side of the block! the Model B Oil Filler tube sounds good BUT not to solve the problem.

Ruth - So maybe it will have to be the "metal breather hose" fix!

Thanks to all who responded to my question!
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Old 01-05-2021, 08:23 PM   #9
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Default Re: Cause of Blow By

As stated above there will always be some amount of blow by. It becomes a matter of just how much one is willing to live with.

I have not used one but to me a sock is one of the least desirable ways to minimize blow by.

Give this a try. (yes another band aid fix but...) Place a smaller sized PLASTIC not metal brillo type dish scrubber pad inside of your filler tube. Make sure the cap is not pushed down too far and your oil level is not too high. Midway between the full and low mark on your dip stick is good.

Also make sure you dip stick is fully seating into engine block so you are getting an accurate reading.
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Old 01-05-2021, 09:15 PM   #10
Jack Shaft
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Default Re: Cause of Blow By

Model A pistons use wide rings,they don't seal as well as late model car engines.Compression passing the rings also means oil is reaching the combustion chamber,not necessarily a bad thing.Its normal for these engines to use some oil.One other tip to control crankcase pressure other than the above reccomendations is to put a second road draft tube in the upper rear of the valve cover.This assists the oil fill tube venting as well as eliminating any pressure in the valve chamber,which can cause rear main bearing leakage
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Old 01-05-2021, 09:45 PM   #11
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Default Re: Cause of Blow By

There will always be some amount of blow-by past the rings.
They are never seated end-to-end to prevent 100% of blow-by. It's normal.
Also normal is that the crankcase is a torrent of oil and oil mist splashing all of the engine internals.

Given that the pressure from blow-by has to exit the crankcase via the oil filler tube, there will always be some slight amount of oil mist that is carried with it. The oil level in the oil pan doesn't have much to do with blow-by, as that oil is below the dipper tray.

In later vehicles, PCV valves were installed to eliminate the crank pressure and route the oil vapor back into the combustion chamber.

For a Model A, the best (IMO) solution is the breather hose, routed below the engine splash shields. At any speed, the oil vapor is routed outside of the engine area, and at driving speeds, a slight vacuum should be created.
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Old 01-05-2021, 10:29 PM   #12
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Default Re: Cause of Blow By

I think it is possible that the Model A oil breather cap does not "coelesce" the oil mist enough and it escapes out the breather under normal crankcase pressure. I had a 1951 Plymouth that had a "push in" oil breather /fill cap with a coelescing mesh in it, and did not have any problems with blow by. I looked on the INTERNET and there are a number of push in oil breather caps which may work. Some are nice "period" looking. I may check a few of these out come spring.
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Old 01-06-2021, 02:02 AM   #13
Richard in Anaheim CA
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Default Re: Cause of Blow By

Cape Codder,
You say you tried the sock trick and was told it isn't a good idea.
You got a lot of advice and what is causing the blow by. I don't think that is what you are asking.

Yes it is important that the crankcase must breath so never stick a pot scrubber, "sock"or anything in the oil filler tube. Make sure the cap allows the crankcase fumes to escape. To keep the mess off your engine and under hood, there is nothing better than a gym sock over the fill tube and cap which you throw away and replace when is saturated.

That minimizes the mess but of course doesn't fix the problem. You can still go on tours and enjoy your Model A while you track down the source of the excess blow by.

Richard
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Old 01-06-2021, 08:23 AM   #14
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Default Re: Cause of Blow By

I’ve been using a Fumaze flexible metal breather tube in place of the oil filler cap for over 50 years; it does a nice job of directing the crankcase fumes away from the engine compartment so the engine stays clean. These Fumaze breather tubes are regularly available on the auction site.

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