Breather cap discussion

OldMercs

OldMercs

Member
So I'm telling my grandson about crankcase ventilation in the 50's. Air through the breather cap, through the crankcase and out through the draft tube leaving an oily streak in the middle of the lane which I had to avoid when riding my motorcycle.

I took the breather cap from my car (292 c.i.) to show him how it filters the air going into the crankcase expecting to see something in the cap like coarse steel.wool.

My cap is empty; nothing in there. And I live on a dirt road, dusty dirt road in the summer.

I then pulled the cap of my parts car (256 c.i.) and it has a filter material like I was expecting in it. Then we started looking at the differences between the two caps. The one from the 256 is bigger and has an extension that fits in the oil fill tube. The 256 is on the top in the first picture and on the left in the other two.

Did breather caps change over the years? Is the filter element changeable or just buy a new cap? How did anyone get the filter element out of the cap I'm using?

This is possibly the most irrelevant question of the day but my 15 year old grandson needs answers.
 

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Not sure, but I don't think the material is replaceable. I used to just clean 'em with kerosene, or replace the whole cap if they were really gunked up.
 
I took the breather cap from my car (292 c.i.) to show him how it filters the air going into the crankcase expecting to see something in the cap like coarse steel.wool.

My cap is empty; nothing in there. And I live on a dirt road, dusty dirt road in the summer.
Click to expand...

The caps originally used hogs hair for filtering material. You were to submerge them in solvent every service interval, the same way you would service an oil bath air cleaner.

If no filtering media, it was most likely an aftermarket cap.

If you see one today that has a plastic foam filter media, run.

You remember those EDLEBROCK AIR CLEANERS that used an open foam element?

Made great vehicle fires.
 
OldMercs said:
So I'm telling my grandson about crankcase ventilation in the 50's. Air through the breather cap, through the crankcase and out through the draft tube leaving an oily streak in the middle of the lane which I had to avoid when riding my motorcycle.

I took the breather cap from my car (292 c.i.) to show him how it filters the air going into the crankcase expecting to see something in the cap like coarse steel.wool.

My cap is empty; nothing in there. And I live on a dirt road, dusty dirt road in the summer.

I then pulled the cap of my parts car (256 c.i.) and it has a filter material like I was expecting in it. Then we started looking at the differences between the two caps. The one from the 256 is bigger and has an extension that fits in the oil fill tube. The 256 is on the top in the first picture and on the left in the other two.

Did breather caps change over the years? Is the filter element changeable or just buy a new cap? How did anyone get the filter element out of the cap I'm using?

This is possibly the most irrelevant question of the day but my 15 year old grandson needs answers.
Click to expand...

No, the kid is curious and wants to learn. You are lucky.

You might consider having him inoculated for CHEV FEVER and have him carry a SHOT CARD. We don't want him the go GM or MOPAR ... :D

The open air filtered cap was used on both the ROAD DRAFT SYSTEM and the later early design PCV SYSTEM (OPEN SYSTEM). In about 1964, some states required the fresh air to be drawn from the air cleaner (CLOSED PVC SYSTEM), which in addition to tighter emissions controls provides cleaner air into the crankcase.

You need to source a new oil cap. Usually, the better quality will have a steel mesh as a filter media.
 
With those caps venting the crankcase, it makes me wonder why we even need the draft tube at all.
 
Here is the best definition of ROAD DRAFT I can find -

Crankcase Ventilation System - Road Draft Tube​

The first refinement in crankcase ventilation was the road draft tube, which is a pipe running from a high location contiguous to the crankcase (such as the side of the engine block, or the valve cover on an overhead valve engine) down to an open end facing down and located in the vehicle's slipstream. When the vehicle is moving, airflow across the open end of the tube creates a draft that pulls gases out of the crankcase. The high location of the engine end of the pipe minimizes liquid oil loss. An air inlet path to the crankcase, called the breather and often incorporated into the oil filler cap, meant that when a draft was generated at the tube, fresh air swept through the crankcase to clear out the blow-by gases.

The road draft tube, though simple, has shortcomings: it does not function when the vehicle is moving too slowly to create a draft, so postal and other slow-moving delivery vehicles tended to suffer rapid buildup of engine sludge due to poor crankcase ventilation. And non-road vehicles such as boats never generated a draft on the tube, no matter how fast they were going. The draft tube discharged the crankcase gases, composed largely of unburnt hydrocarbons, directly into the air. This created pollution as well as objectionable odors. Moreover, the draft tube could become clogged with snow or ice, in which case crankcase pressure would build and cause oil leaks and gasket failure.
Click to expand...

The breather cap supplies fresh filtered outside air to the crankcase (to eliminate gases and moisture as the result of combustion). The actual tube causes a draft (partial vacuum) within the crankcase to draw contaminated air out as the car moves.

If one uses just open breather caps (without draft tube or PCV), the gases will not be drawn out efficiently to completely evacuate the crankcase.

SOURCE - https://www.liquisearch.com/crankcase_ventilation_system/road_draft_tube
 
Thanks for that explanation. I learned something today!

I'm thinking back to my 55 Ford farm tractor (model 850). It had a vented cap with a wing nut on top so the cap could be dismantled and the steel mesh filter media removed, cleaned and replaced. It also had the tube extending down into the fill tube. But I don't remember a draft tube on that engine anywhere. Probably because it usually traveled at speeds less than 10 mph...often WAY less. I wonder how they ventilated the crankcase on that engine? I don't have the tractor anymore so I can't go look at it.
 
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