I have been chasing oil leaks and blown main seals on a freshly overhauled and modified 912. newly installed in a different aircraft. Whiling working on the return line we noticed it is only about 7/16 ID and my kit fox is 1/2 inch. Fox runs great no leaks in 12 years it is factory built and I have found they really do their homework. The fittings  are also smaller.

could this cause a higher crank pressure especially on this high HP engine?

why does one 912 have a 1/2 hose and fittings then another have 7/16? What is he correct size?




  • Re: Oil return line

    by » one year ago

    Hello Fred

    You need to do the oil system pressure test as shown in the installation manual.  If your crankcase pressure is too high then oil seals will weep.

    The installation manual also shows the correct size for lines on the system.  I suggest a 13mm ID line (½ inch) at the least.  Be sure that your fittings are not too small also.  The most common problem for high crankcase pressure is a restricted return line from the crankcase to the oil tank.  





    Thank you said by: Fred Northup

  • Re: Oil return line

    by » one year ago

    We have Checked the pressure well within limits on the ground so maybe under load it is higher? Replacing the seal then recheck.

    any other reason to push the main seal out.?

    the banjo has a smaller nipple to match the smaller hose same with the tank fitting.


    thank you for input




  • Re: Oil return line

    by » one year ago

    Oil pressure above limits. HIGH OIL PRESSURE

  • Re: Oil return line

    by » one year ago

    What manual and page that in? 

  • Re: Oil return line

    by » one year ago

    Operators Manual page 2-2

    Operating Limits

    Oil pressure

    Max. 7 bar (102 psi)

    Min. 0.8 bar (12 psi) (below 3500 rpm) 1.5 bar (22 psi)1

    Normal 2.0 to 5.0 bar (27–73 psi) (above 3500 rpm) 1.5 to 5.0 bar (22 psi-73 psi) 1

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