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Thread: Leakdown test (long read)

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    Jedi Warrior roscoe's Avatar
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    Leakdown test (long read)

    download.pngQuick tip for anyone who is inclined to buy or already has a differential pressure cylinder leak down tester. Unless you have a kit that comes with the required 14 mm spark plug bore adapter you will need to make one. I've had a tester for the last 40 years but never got around to checking my compression with it until recently, just never had the adapter as the aircraft engines I got the tester for all have 18 mm plugs.

    My direct compression test gage bit the dust ( or should I say turned to dust) and led me to believe I was in dire need of engine repair so I figured it was time to use the leak down tester. I stole the spark plug out of the gas powered leaf blower that was given to me but never used ( what is the point of moving leaves around anyway?). I figured it would be easy to gut the plug by drilling it out and then just weld a male air fitting to the end of the shell. I don't know what is in those things that is so hard ( besides a bit of ceramic) but after smoking a few bits I finally got it reamed out and snapped off the shell electrode. Seeing a half used tube of JB Weld on the bench I opted to weld with that instead of trying for an airtight fitting with the mig welder on such a small part. Worked like a charm.

    I was pleasantly surprised to record 76/80 for 3 cylinders and 78/80 for one . That is a 5% leak down and it was small enough that I heard no hissing from the exhaust pipe, air cleaners or crankcase via listening at the oil filler on the valve cover (of course the hearing isn't quite what it used to be). Bonus was the JB Weld was plenty strong enough and I didn't lose an eye during the process. The test gage set is available from about $30 to $300 and I think the differences are in the gages and the quality of the built in regulator. For those who aren't familiar, I've included the diagram showing the concept. The inexpensive sets certainly work and you can always put good gages in them. If you Google the size of the standard orifice it would not be hard to make a tester from scratch. For all the nervous Nellies out there it gives great peace of mind if you get good numbers and is an excellent diagnostic tool if you have marginal numbers. Standard for pulling an aircraft engine cylinder was 60 to 65 psi over 80 but there can be things you can do on the spot that may raise the reading, like whacking a valve stem to better seat a valve.

    A couple of useful tips : An aircraft engine has a great big lever on it called a propeller, which you use to rock the crank back and forth to get right on TDC, which you need to do so the piston doesn't move, which would in turn cause the valves to open as during the end of a compression stroke. You can load the cylinder with 20 or 30 psi of air pressure while you securely hold the prop and then move in the direction of rotation until the downstream gage peaks and the prop doesn't want to move on its own; there you are right on tdc. People have died or been severely injured by letting go of the prop at the wrong time when they were in it's path You can't do that with a car engine and believe me you don't want to try like by putting a wrench on the crank nut....you WILL get hurt. What you can do instead is get the piston on the compression up stroke by hooking up the test set and again, running 25 or 30 psi into the cylinder. By beeping that great little button on the back of your starter solenoid you can bump crank the engine ( all spark plugs removed) a few times until you see the down stream gage hold pressure and then you know you are at tdc and there is no movement. It takes several tries but once you get the hang of it it just takes a few seconds for each cylinder. No TDC indicator or large wrench required. And yes, there are some projects I'm trying to avoid today....
    Last edited by roscoe; 12-06-2018 at 08:27 PM.

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    John Turney  (12-06-2018)

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    Yoda Randy Forbes's Avatar
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    Re: Leakdown test (long read)

    While I'd love to read your post, potentially to learn something new or make a contribution, I won't because the lack of paragraph breaks makes it too much effort.

    **** shame, as I miss out on a lot of vette's posts too for the same reason.
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    Jedi Warrior roscoe's Avatar
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    Re: Leakdown test (long read)

    Odd,
    I put a paragraph break in about 2/3 of the way down where it says "A couple of useful tips" but it eliminated the space when it posted.

    À chacun son goût !
    Jon Robbins
    1956BN-2 (do it all yourself, you'll be glad you did)

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    Re: Leakdown test (long read)

    Quote Originally Posted by roscoe View Post
    Odd,
    I put a paragraph break in about 2/3 of the way down where it says "A couple of useful tips" but it eliminated the space when it posted.

    À chacun son goût !
    I powered through it, but but there's ways to make technical writing more inviting
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    Re: Leakdown test (long read)

    Randy,
    I have taken your tip ( and edited the post) and will keep it in mind. I do tend to get carried away. I've published quite a few trade journal articles (helicopter maintenance) and my only complaint about editors is when they don't do their job. So, thank you.
    Jon Robbins
    1956BN-2 (do it all yourself, you'll be glad you did)

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    Yoda Randy Forbes's Avatar
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    Re: Leakdown test (long read)

    Examples of homemade & store-bought leak-down testers:

    I made this one probably thirty (>30) years ago, and it's served me well...



    ... until I got suspicious about an engine blowing smoke whose problem eluded me (eventually found the #6 bore .006" out of round AT THE BOTTOM) so I bought this set only to get the same readings!



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    Yoda Randy Forbes's Avatar
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    Re: Leakdown test (long read)

    Quote Originally Posted by roscoe View Post
    Randy,
    I have taken your tip ( and edited the post) and will keep it in mind. I do tend to get carried away. I've published quite a few trade journal articles (helicopter maintenance) and my only complaint about editors is when they don't do their job. So, thank you.
    Thank YOU!

    I am willing to accept it's MY problem, but putting those breaks in there gives you a chance to pause (absorbing the just read content) without losing your place in the piece.

    And I get it, I've read manuals that you literally have to be sitting on a stool to keep from nodding off!
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    Yoda steveg's Avatar
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    Re: Leakdown test (long read)

    Thanks guys for the paragraph discussion! In High School one of the teachers said "if it's a new thought, time for a new paragraph."
    Steve Gerow
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    Maker of most complete Big Healey rear disc kit
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    Yoda John Turney's Avatar
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    Re: Leakdown test (long read)

    The paragraph breaks definitely help. If there are none, I have to highlight the text a line at a time to keep track of where I am.
    John, BN4

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    Yoda Michael Oritt's Avatar
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    Re: Leakdown test (long read)

    Jon--

    What do you have against using a wrench--or better a socket and breaker bar--to bring a cylinder onto TDC or BDC? Unfortunately this is not logistically possible on my Healey but it is on both of the Elvas and I find it very easy to simply bar the engine over to where I want it, then lock it up in gear and block the wheels.

    Best--Michael Oritt
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    Jedi Warrior roscoe's Avatar
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    Re: Leakdown test (long read)

    Nothing wrong at all, I just can't imagine doing that with the Healey. I bet if you try it as I described you will find that doing it in neutral and not having to reposition your wrench may be easier. I dont have to put it in and out of gear to go through the cylinders either, or block the wheel for that matter. You have pretty much eliminated the safety issue of a flying wrench by being in gear and chocking a rear wheel but I'll bet my way is a bit faster.
    Jon Robbins
    1956BN-2 (do it all yourself, you'll be glad you did)

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    Re: Leakdown test (long read)

    Quote Originally Posted by steveg View Post
    Thanks guys for the paragraph discussion! In High School one of the teachers said "if it's a new thought, time for a new paragraph."
    Since we've already veered off-topic:

    When I was in tech I had to read a lot of dense technical manuals. The best ones, I felt, all used the same format:

    - Main topics got a chapter (sometimes with a brief summary at the beginning, helping you decide if you needed to read the chapter at all)
    - Within a chapter, subtopics got a section, usually with larger font, bold-face headers
    - Within a subtopic, the first sentence of each paragraph summarized the paragraph; following sentences in the paragraph went into detail
    - Subtopics were usually not nested more than one or two levels

    With this, you could skim the book--or a pertinent chapter--by skimming the chapter and subtopic headers to get an overall 'feel,' then go back and skim again including the first sentence of each paragraph. If you needed to dig deeper you went back and read pertinent paragraphs word-for-word. Paragraphs were seldom more than a couple inches 'tall,' in eleven or twelve point font. I prefer sans serif fonts, though " According to most studies, sans serif fonts are more difficult to read" (https://www.google.com/url?sa=t&rct=...cUABTe5AyZune1).

    It's not like reading a story or novel, where word-for-word is appropriate. If I tried to read a tech manual like that, I usually forgot the first part of a chapter before I finished it. If the manual was written by a non-English native speaker, well, good luck.

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