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Thread: Compression Tester Orifice

  1. #11

    Join Date
    Mar 2021
    Kitplanes back in October 2017 had a good article on making your own orifice. The author couldn't get his Harbor Freight leakdown tester to work on his small bore Jabiru engine. The HF automotive L/D tester had a 0.080" orifice. He had a drawing of an orifice that looked like a carburetor jet with a 60 degree bevel at both ends of 0.25" long piece. He mentioned the requirement for less than a 5" diameter bore is a 0.060" orifice but didn't state where is reference for this was. Then he mentioned he used 0.040" diameter orifice for his small bore Jabiru.
    I recall I read somewhere the orifice size is determined by the cubic inches of the cylinder but sorry I don't know where I read that now.

    Anyways for my Rotax tests I just bought a ATS L/D tester designated for the Rotax 900 series motors and the catalogue listed a 0.040" orifice for that tester. One thing we learned in Rotax maintenance school is most leak down testers have a check valve in the sparkplug coupling. We found a used Rotax motor had no leakdown with that check valve and then made realistic leakdown numbers once the check valve was removed. So after I removed my check valve from my coupling my Rotax 912 ULS with only 48 hours run time still had 85#/85# all cylinders. But I can verify with the tester valves the orifice is working as soon as I uncouple the tester hose the air bleeds right down which it didn't do with the check valve installed.

    FYI I used the ATS L/D tester instructions and it made it a breeze to do each cylinder. Just use the regulator to add less than 20psi and crank using the prop until the cylinder side (L/D) gauge comes up to match the supply pressure gauge - then your at TDC ready to crank up the supply pressure to 80psi or 85psi or whatever you use. Read the cylinder side gauge and thats it.

    Hope this can help ya.

  2. #12
    Eric Page's Avatar
    Join Date
    Jul 2011
    Toledo, WA
    Quote Originally Posted by cardo0 View Post
    ...crank up the supply pressure to 80psi or 85psi or whatever you use.
    Just a note that Rotax specifies using a pressure of 6 bar for testing, which equates to 87 psi. This is often missed by mechanics/operators who are used to the Lycoming and Continental world where 80 psi is the norm.
    Eric Page
    Building Kitfox Series 5
    Member: EAA, AOPA, ALPA
    ATP: MEL / Comm: SEL, Glider / ATCS: CTO
    Map of Landings

  3. #13

    Join Date
    Mar 2021
    Well that makes it a conflict when my manufacture annual inspection calls out 85psi. I guess I could do it twice but with zero leakdown I don't see the need. But to quote the Rotax Maintenance Manual Line "Now put constant pressure, between 5.5-6 bar (80–87 psi) on the line and take readings at pressure gauge".

    I have to say thanks for the lookup because now I also read in the MML the actual orifice description and I'll quote that here too: Orifice jet*, of 1 mm (0.04 in) inner diameter and 3 mm (0.12 in) length. * or equivalent e.g. orifice diameter 0.040 in., long 0.0250 in., 60 degree approach angle according to AC43.13, latest issue.
    And I hope this will help someone else too.

  4. #14

    Join Date
    Mar 2021


    Oh, oh, oh! And if you google that AC43.13 reference you can find the FAA specification for a leakdown tester orifice on Section 8-14, page 8-7 (page 386 0f 646). And I'll quote those two paragraphs here: (1) For an engine cylinder having less than a 5.00-inch bore; 0.040-inch orifice diameter; .250 inch long; and a 60-degree approach angle. (2) For an engine cylinder with 5.00 inch bore and over: 0.060 inch orifice diameter, .250 inch long, 60 degree approach angle.

    ​, we done it!

  5. #15

    Join Date
    Nov 2011
    Hood River, OR
    Gentlemen, please accept my apologies for my lack of clarity. There are orifices and there are orifices. There is the orifice that is in the flow when the compression test is being performed. The size of this orifice is either .040" or .060" depending on bore size. These sizes were determined shortly after WW2 and not the subject of my question. Although how these sizes were determined would be interesting to know. My question concerned the orifice that Continental calls a "Master Orifice". Continental calls the first or in use orifice a "restrictor orifice". The little that I can find says the master orifice size also is .040",but I am not sure what orifice they are talking about. Incidentally as I understand it the use of master orifice to determine "allowable" leakage is only applicable to Continental engines. In any event, the question is, how was the size determined.

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