Had a question:

Would blow by decrease when you have a valve job? Also would blow by increase if there was a valve clover gasket leak?

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1981 240D
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Most of the combustion gases escape via the rings on the compression stroke when the valves are closed, so a valve job will affect blowby minimally..........unless the clearance to the guides is huge.

Blowby is the same..........independent of the valve cover gasket leak. Without the leak, you can observe all the gas escape via the breather hose. With the leak, some of the gas escapes via the gasket..........so the appearance of blowby may be reduced.

Not to mention that a bad valve cover gasket will leak a lot and be very noticeable!