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MG MGA - Crankcase Ventilation - K&N Filters

Chaps, The previous owner of my 1600 mkll fitted K&N air filters, and unfortunately I didn
A Bennett


Why do you think that air inside the filter element is higher than atmospheric pressure? Surely suction from the carburettors and resistance from the filters would result in a lower than atmospheric pressure?
The tube should be fitted into the filter to prevent grit being drawn into the engine by the draught tube. You will not pressurise the crankcase as the draught tube will keep the pressure down.
With your alloy rocker cover, it should have a gap for air to enter the cap, and the cap should have some wire gauze in it that is wetted with oil to act as a filter.

Mick Anderson

Adrian, if you search the archives you will find chapter and verse on this subject about a year ago from many contributors especially Barney.
J H Cole


I believe the design intent is that the draft tube puts a vaccuum to the crackcase, causing air to be drawn from the upper vent tube. Unfortunately, the theory and practice do not talk to each other 100% of the time. Evidence of this is the oily air filters.

I have run an aluminum valve cover with repro S&H air cleaners for the last 7 years. I do not believe that my car leaks more than anybody else's. but I do use quite a bit of oil and I did have a gasket leak at the valve cover to the head. There is a very small hole in the valve cover cap, presumably to allow for the air to be drawn in by the vent tube action mentioned. I believe that it is not nearly large enough to be effective. And I continually collected a bit of oil in the ridges of the valve cover indicating that it was trying to mimick the original setup by pushing a bit of oil out the cap.

This year, I have changed over to the original Volkes filter housings with K&N filter elements. I have drilled and tapped the aluminum valve cover in the same location as the original and added a fitting for the original style hose to connect to the air filter housing. I have not driven the car to see if the oil at the top of the valve cover has disappeared.

For your setup, I think that you may drip oil from the open end of your valve cover vent tube. This is based on my observation of oil somehow, under some situations, coming out the cap. In your setup, the oil may find its way to the exhaust manifold. Rather than trying to use the vent hose, you may be better off trying to find a small filter directly attached to the valve cover fitting, if you can find one small enough.

I have attached a photo of my current setup.


C Schaefer

Forgot the photo.

C Schaefer


I was just checking through the archives for the information when John posted his comment.

Barney put me right about 18 months ago, despite my early protestations about how the ventilation system works. See here:

Steve Gyles

Many thanks chaps. Wish I'd found the thread Steve indicated before posting - would have saved you guys some trouble!!! Very comprehensive exchange of views, very clearly explained - I am now confident that I know what should be going on. In my case, I think I will attach some form of gauze filter to the free end of the vent hose until I decide whether to fit an alloy rocker cover (I wanted to do this because mine is a bit mis-shapen, and also to perhaps quieten the valve train a bit, although this isn't key since I quite like engines to sound like engines!). If I do fit one, then the vent hose will go and I will ensure that the oil filler cap is properly vented with an oiled gauze filter. If I don't fit one, I will attach the vent hose to the air filter in some way. What an asset this BB is! Adrian B.
A Bennett

Hi Chuck,

It is true that the draught tube at the side of the engine draws air from the crankcase and this is replenished with filtered air, through the air filter and then through the valve cover.
The oily deposit that you see inside the filter, near the tube connector, is caused by hot, rising fumes after the engine is shut off and the draught tube not working.
This can be seen if, after a hard run, you shut the engine down and remove the tube from the valve cover.

Mick Anderson

This thread was discussed between 26/05/2008 and 27/05/2008

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