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Thread: Blow by

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    Default Blow by


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    I have very little mechanical knowledge so excuse if this is dumb question.
    I recently bought a GN125 which needed an engine rebuild. I’ve done about 300km on it since then, only riding it moderately, for the “seating-in” of the rings. I undid the oil cap the other day while the engine was running and there’s a lot of pressure in the crank case (blow-by I assume). Does this mean the rings didn’t seat properly or is this normal during the seating period? Or is it ever normal for there to be pressure in the crankcase.

    Thanks

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    At Work Doc's Avatar
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    Default Re: Blow by

    It's normal for there to be some crankcase pressure on a four stroke - especially singles as you don't have any other pistons moving in opposite direction to 'offset' whichever piston is on its downward stroke. It only becomes a problem if the crankcase breather becomes blocked.
    If the bike's running OK and you're not burning oil I wouldn't worry too much about it at this point.

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    Should Get Out More a_morti's Avatar
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    Default Re: Blow by

    Quote Originally Posted by Doc View Post
    It's normal for there to be some crankcase pressure on a four stroke - especially singles as you don't have any other pistons moving in opposite direction to 'offset' whichever piston is on its downward stroke. It only becomes a problem if the crankcase breather becomes blocked.
    If the bike's running OK and you're not burning oil I wouldn't worry too much about it at this point.
    :Plusone:

    When the single piston goes down, it adds pressure in the crankcase.

    If you've fixed it and it's working, stop fixing it before it stops working!

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    Default Re: Blow by

    Has one on't cross beams gone awt askew on't treadle?

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    Default Re: Blow by

    Quote Originally Posted by MsignorMartinez View Post
    Has one on't cross beams gone awt askew on't treadle?
    May have done but the swagger compensating clinker should sort that.

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    Should Get Out More RiceBurner's Avatar
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    Default Re: Blow by

    Quote Originally Posted by Samuelson81 View Post
    I have very little mechanical knowledge so excuse if this is dumb question.
    I recently bought a GN125 which needed an engine rebuild. I’ve done about 300km on it since then, only riding it moderately, for the “seating-in” of the rings. I undid the oil cap the other day while the engine was running and there’s a lot of pressure in the crank case (blow-by I assume). Does this mean the rings didn’t seat properly or is this normal during the seating period? Or is it ever normal for there to be pressure in the crankcase.

    Thanks

    You undid the oil cap and ran the engine?

    Did you get covered in a fine oil mist?

    Yes there would be considerable pressure within the engine cases blowing oil about, also exacerbated by the threshing motion of the engine innards.

    If you do it again - please remember not to breathe in.

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    Default Re: Blow by

    If your worried just do a compression test .

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    Should Get Out More Tomcat's Avatar
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    Default Re: Blow by

    All engines 'breathe' to some extent. On a single it's quite major because as the piston comes down it pushes the same amount of air as the swept volume of the cylinder into the crankcase. The crankcase breather pipe allows this air to vent to atmosphere, because it would otherwise pressurise the crankcase and cause oil leaks. As the piston comes back up, it sucks air back into the cases in the reverse action. It does mean the breather system on a single is rather 'busy' but it's completely normal and nothing to worry about.

    A 3 or 4 cylinder engine is a different matter because as you have one piston descending you have another rising, and air thus flows from the underside of one piston to another. This makes the airflow in the crankcase rather busy but there should be little gas coming out of the breather!

    In your case the short answer is, don't worry about it. What you have is completely normal and only if the engine starts using a lot of oil or smoking should you suspect piston ring problems.

    PS while you're on a learning curve there are no dumb questions. It's better to ask and get advice rather than assume something and bugger it up.

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