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Crankcase question


J A Boggan's Avatar
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09-03-02, 03:48 AM   #1  
J A Boggan
Crankcase question

Would someone explain (in detail) just why it is so important to keep to keep your crankcase "sealed". For example, if you remove your dip stick (for whatever reason), why will the engine start blowing oil? I'm having a hard time visualizing just what is happening.

 
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09-03-02, 06:17 AM   #2  
mikejmerritt
Hello J A Boggan, look at it this way, when an engine is running the oil creates a vapor that is floating so to speak in the crankcase. Without some kind of negative draw in the crankcase the piston will draw this vapor with it when it moves on its upstrokes. When this happens a certain amount of oil can get by the rings because they are designed to seal on the combustion chamber side of the piston not the crankcase side. On the flip side of this, blowby that gets by the rings (some is normal) is drawn out of the crankcase by the same negative pressure helping to keep the oil from being contaminated and thinned down by slight amounts of unburned fuel. Where does this oil/blowby residue go?.....Its trapped in the PVC valve filter on small engines. Thats why cleaning the valve cover is so important when tuning up but is most often overlooked because they don't completely stop up often but at some point they will and it will lead one to assume the engine is on its last leg due to smoking. I've had to many very happy customers come in with smoking engines on their riding mowers that assume due to the awful blue smoke that the engine would have to be shot. The fact is, an engine that is able to process and burn that much oil and keep running can't have poor compression. The larger HP Briggs seem very prone to this anomoly and its unbelievable how much blue smoke they can produce.
I hope this helps some and is far from all there is to say so others jump in with some thoughts......Mike

 
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09-03-02, 08:10 PM   #3  
J A Boggan
Thanks Mike,
For your detailed response. It really helps to the "why" of different processes.

 
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