the latest Rotax Installation Manual on pdf page 146 or 75–00–00/Page 24 January 01/2021 says this:

”furnishing the line with a bypass opening before the cowling outlet”

i have 2 questions and I would love to hear your thoughts.

1) seems that the vent hose from the overflow bottle cap is not used even if the manual states that is required. Why?

2) not an expert here so this question could very simple but, how the bypass opening looks like? Images are more than welcome.

Thanks for your comments 


  • Re: Overflow bottle - Bypass opening

    by » 12 months ago

    The bypass opening in the vent hose is presented by Rotax as one of three options for protecting that hose from freezing shut.  It is assumed that any portion of the hose that is outside of the cowl could freeze, so cutting a small notch in the hose just before it exits the cowl would allow for pressure relief in the event the hose end freezes shut.

    If the vent hose plugged completely it’s possible that the overflow bottle could become over pressured and rupture, even during normal cooling system venting.  Instead of the notch, you could also just stop the hose short of exiting the cowl, where it’s typically warm and would not freeze.  The overflow bottle is (and must be) vented even without the vent hose installed. The addition of the vent hose is not required, but using it directs any steam (contaminated with antifreeze) out of the engine compartment.  

    Thank you said by: Carlos Quijano

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