2c leakdown test
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Thread: 2c leakdown test

  1. #1
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    2c leakdown test

    Hi
    I'm thinking of trying to test my crankcases, seals etc with a pressure test of some sort. I;ve been on you tube and reckon i can construct some thing, but I would be interesed to know what anyone else has done. Also I read that you can't test each cylinder separately , you have to do them both, but if thats the case, how do you test the seal between the two? (Do they call it labarynth seal)

    Any info would be much appreciated,

    Regards

    Mark

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    Errol Kowald
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    I built this. It works fine
    hooroo, Errol www.engenia.com.au [136,351 km - and counting .....]

  3. #3
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    Blimey, that’s the first article on leak down testing I’ve seen that actually makes engineering sense.

    Manufacturers of leak down test rigs tend to write complete crap, as do many authors of ‘technical’ articles. It seems to have become commonplace nowadays for the second (downstream) gauge of a leak down tester to be calibrated in % which is very misleading. It seems to have led a lot of technical writers to believe that only a percentage of the air flowing into the cylinder is actually leaking out. I’ve read so-called technical articles that actually say that. Such an assertion is utter nonsense as a moment’s thought will reveal. Air is not magically created or destroyed inside the cylinder, so when the gauges have settled into a steady state, the flow of air in must be the same as the leakage out. Therefore the leakage must always be 100%. The % reading on the dial is meaningless in any engineering context. The gauge may as well be calibrated in furlongs or Volts. Forget %, it’s just a number.

    The gauge is actually measuring pressure, so it would be more helpful if it was in KPa or psi, the same as the first gauge. What the two gauges actually tell you is the pressure drop across the orifice plate that’s positioned between them. And that’s an indication of the flow rate of air (an orifice plate is a common device for measuring flow of fluid or gas). For a given orifice size, you can determine the flow rate by calculation, or by looking up the pressure drop on a chart. The flow rate is generally expressed in litres/second or kg/hour, and that’s the quantity that actually means something.

    Even then, it’s a subjective measurement. It’s really only useful to compare individual cylinders of the same size using the same test apparatus at the same initial pressure setting on the regulator. You can’t do a meaningful comparison between gauges or between different sized engines. If someone tells you that your engine should have 4.3% leakage, they're talking though their arse.

    Sorry for the rant, I've recently had my fill of people talking bullshit about leak down tests.


    As for Mark’s initial query, it is possible to do a leak down test on your 2-stroke engine.

    Testing the leakage past the pistons is easy, just stick the tester in the spark plug hole in the normal way, but make sure the piston is positioned at or near TDC so the rings are above all the ports.

    To test the crankcases (one side at a time), you’ll need to blank off the exhaust port and close the intake port. That means turning the crank until the piston is at BDC. Connecting the leakdown tester to the spark plug hole will then pressurise the space in the crankcase via the transfer ports. I reckon you could expect quite a bit of leakage though, because the seal between piston and intake port (or reed or rotary valve on some engines) isn’t perfect. You’ll also get leakage across the labyrinth seal into the other crankcase half. To get a better idea of the leakage through the labyrinth seal you might want to consider removing the carby and blanking off the intake manifold to close that avenue of leakage. Then you’re only testing the crankshaft seals.


    Cheers,
    Cam








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  5. #4
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    Thanks for your replies.
    I was intending to block the exhaust port, take off the carb and make up something to pump some air in through the inlet manifold, testing one cylinder at a time, but Sprocket is suggesting that I'll lose pressure through the labarynth seal, so will only be able to test both cylinders at the same time. Its really the crank seals I want to test. Any explanation would be gratefully received
    Ta

    Mark

  6. #5
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    Cam Douglas
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    You can test one side at a time. With the carb inlet an the exhaust port blanked of, you will be measuring the leakage through the crank seals only. I would expect the labyrinth seal to leak more than the outer crank seals, but don't know for sure. It'll be interesting to see what your test reveals.

  7. #6
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    I have a Motion Pro test I bought used.
    I works quite well. Easy to hear leaks- and see if any issue.

    https://www.motionpro.com/product/08-0071

    I’ve had numerous vintage 2-strokes that had bad crank seals. This verifies if they are ok or not, and then you can ascertain which is bad.

    Mark

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