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How to Make Your Own FAA Spec Leakdown Tester

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  • How to Make Your Own FAA Spec Leakdown Tester

    A local EVO owner recently asked me to assist him in conducting a leakdown test. When he arrived, his leakdown tester was a Chinese/Ebay/Harbor Freight unit that proved to be completely worthless. Not having replaced my leakdown tester since it was destroyed by Hurricane Katrina, I checked around and found units ranging from $25-$100+, with none of them mentioning any standard or specification in their product descriptions. When one measures leakage, it must be measured with respect to a standard that is engineered into the tester itself. Unless the tester is constructed to a specific standard, there is no way of knowing what it's telling us. This is like a chassis dyno generating a chart, but with numbers that don't reflect hp or any other known standard of measurement. Being dissatisfied with this, I decided I could do better by making my own, while saving a few bucks at the same time.

    Anyone who knows anything about the FAA (Federal Aviation Administration) knows the FAA doesn't screw around when it comes to engine maintenance standards. The FAA established the standard for leakdown testing, and that requires a 1mm (.040") orifice be placed between the two gauges. In other words, when pressure is applied (e.g. 100psi), what we are looking for is leakage in the cylinder as compared to the airflow through a 1mm orifice at that pressure.

    (2) 100psi 2.0"- 2.25"1/8 NPT gauges (Ebay) - don't get larger than 2.25" or there may not be enough space to fit them!

    (1) 1/4" NPT air pressure regulator (100psi capable) - Ebay

    (1) 1/4" NPT male Quick Connect fitting

    (1) 1/4" NPT female Quick Connect fitting

    (1) 1/8" NPT plug (for regulator gauge port)

    (2) 1/8" NPT male to 1/4" NPT female adapters

    (2) 1/8" NPT male/male unions

    (2) 1/8" NPT 'T' fittings (all female)

    (1) 1/8" NPT union (1-3/4" long)

    (1) 1mm (.040") precision drill bit. This can be found on Ebay, and possibly through Dremel tool parts listings.

    (1) 14mm spark plug hose with male Quick Connect fitting - if you have a compression tester, you may be able to use that one. Otherwise, check Ebay and auto parts outlets.

    The fittings and such are available from better hardware stores, Home Depot, Ebay, etc.

    Assemble everything as shown in the photo, with the exception of the long 1/8" NPT union. Carefully apply one wrap of Teflon tape to all threads.

    Now, to make the critical feature, the 1mm (.040") orifice ...

    This is easy to do with a bit of JB Weld "JB-KWIK" epoxy.

    Mix a small amount of the epoxy. Take the 1/8" union and insert one end into the mixed epoxy, getting enough inside to create a plug and seal the tube.

    Be sure to carefully wipe any excess from the threads. Let the epoxy set for ~30 min, or until it becomes fairly hard.

    Using the 1mm (.040") drill bit, carefully drill a hole through the epoxy plug. If you don't have a Dremel tool handy, you can even do this with your fingers if you're careful. Be sure to check that the hole remains intact after the epoxy is completely dry.

    Again, if you have a compression tester with a removable spark plug hose, you're already set.

    All things considered, this cost me ~$30 in parts, and I can be assured that I have a tester that gives me readings that make sense, and are relative to leakdown readings as described and discussed in professional terms.
    2003 Mitsu EVO VIII - 2.0L / 600+whp
    1988 BMW M3 turbo - Work in progress. . .
    1986 SVO Mustang - Work in progress. . .

  • #2
    Great write up Ted.
    The E30 M3 Renewed ~ Rebuilt ~ Re-engineered

    E30 M3 Rebuild Thread


    • #3
      Very cool. Thanks for the write-up!



      • #4
        thanks for sharing Ted.
        So how does one carry out a leak down test? I can't get my head round why there are two gauges. I was under the impression that you have to apply pressure to a cylinder, close the air supply and watch the pressure go down (or hopefully not)
        so what am i missing?

        Goodbye M3, you served me well.


        • #5
          Uwe, I'll make a writeup for a leakdown test separately. The basic rationale for the two gauges is as follows:

          Gauge #1 is on the pressure side and therefore reads the regulated input pressure, which should be ~6.8 bar or 100 psi.

          A 1mm orifice separates Gauge #1 and Gauge #2. Air flows through this orifice from the pressure side to the cylinder side.

          Gauge #2 is on the cylinder side of the orifice and therefore reads the pressure in the cylinder. If the cylinder leaks air faster than air flows across the small orifice between Gauge #1 and Gauge #2, there is a pressure drop that is reflected in Gauge #2. The percentage difference between the two gauges reflects the percent leakage as per FAA standards.
          2003 Mitsu EVO VIII - 2.0L / 600+whp
          1988 BMW M3 turbo - Work in progress. . .
          1986 SVO Mustang - Work in progress. . .


          • #6
            To do a leakdown test, you bring the piston to TDC on the compression stroke - so both valves are closed. If you're off by 2 or 3 degrees, the air pressure entering the cylinder will push the piston down, so you have to be sure you're at TDC, or you have to lock the crankshaft in place somehow.
            Connect your tester to the cylinder and to an air supply. Using the regulator, adjust the pressure on the 1st gauge to 100 PSI. Read the pressure on the second gauge. That's the air pressure inside the cylinder. Subtract that reading from 100 and you have your leakage percent.
            Testers come marked in % on the second gauge so you don't have to do the math. I have'nt seen one in awhile, but I thinDek they have a second knob to zero out the second gauge.

            Good thinking on that design Ted.
            Last edited by jimmyC; 03-19-2010, 07:28 AM.


            • #7
              Nice one, Ted, thanks!

              "Objects in mirror are losing..."


              • #8
                ok thanks.

                Goodbye M3, you served me well.


                • #9
                  Very nice write up. Now I need to build one.
                  Several E30 M3's.

                  More than I need but not as many as I want....


                  • #10
                    Basic question I know, but what is a "good" result in a leak down test. As in:-

                    Good = ??%
                    Acceptable = ?%
                    Bad = ?%


                    • #11
                      <5% = Excellent
                      5-10% = Good
                      10-20% = Leaky/Worn
                      >20% = Poor
                      2003 Mitsu EVO VIII - 2.0L / 600+whp
                      1988 BMW M3 turbo - Work in progress. . .
                      1986 SVO Mustang - Work in progress. . .


                      • #12
                        Very nice writeup, thank Ted



                        • #13
                          Nice job, Ted.


                          • #14
                            just getting the % leakdown is not the end of the info you can get from this test. while the cyl is pressurized you can listen in the exhaust, intake and through dipstick tube to tell if the leak is in an exhaust valve, intake valve or piston rings.
                            75 M2 Zinno
                            88 M3 Henna


                            • #15
                              I was looking around for a leakdown gauge and came across this product that is certified to FAA standard. It costs twice as much as the homemade version but an alternative nonetheless.


                              Thanks for the writeup, Ted.