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  1. #1

    2000 GP800 engine case leak down test questions

    I've just done a leak down test on the case after having a hole in the case welded closed, a piston failed and a piece got between the case and crank, and doing a top end rebuild with new pistons, etc.

    I've not been able to find a definitive reference on what is an acceptable rate of leakage.

    I filled the case to 10psi, and here is the pressure after set time periods...

    10min = 6.7 psi
    20min = 5.6 psi
    30min = 4.7 psi

    I've I identified the source of air leakage and that is the rear seal right around the crank shaft, this was not replace in the rebuild, the crank seals appeared to be in good shape.
    I need to know if that is a place that should be 100% leak free or if with this rate of leakage things should be ok or not.

    Thanks for any and all help.

    Click image for larger version. 

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  2. #2
    Three things:

    1. If the crank busted a hole in the case, there could be a lot of wrong in this situation. Did you get the crank fixed or renewed? Also; I was un-aware cases could be fixed this way. I think we're going to need more details on this like if it was a clean break or a crack vs something much more.

    2. If you had the cases and crank apart, you should have replaced the crank seals - they are the most notorious problems for these engines. With those, its better to be safe than sorry. Since you've found a leak - replace the seals. I presume you used assembly lube in your rebuild process. After the lube burns off the seals may leak worse unless the 2 stroke oil helps seal it back up. Just a thought.

    3. Your pressure loss looks pretty good to me. Did you time it a second time after the 30 minutes? I've heard theres some passageways and crevises that fill after a few minutes - like if you are running this test with your reeds installed they may be bleading the pressure on you.

  3. #3
    Hey BTW, I like how you plugged the intake side - that is about a $2 piece of gasket material. I'll have to try that.

    How did you seal off the exhaust side? same way?

  4. #4
    Quote Originally Posted by ZeroClient View Post
    Three things:

    1. If the crank busted a hole in the case, there could be a lot of wrong in this situation. Did you get the crank fixed or renewed? Also; I was un-aware cases could be fixed this way. I think we're going to need more details on this like if it was a clean break or a crack vs something much more.
    It was a pretty clean dime sized hole in the upper case behind the starter, I had a shop weld the hole closed. I'd seen on threads on here where people had much bigger holes welded closed and they seemed to be ok.


    2. If you had the cases and crank apart, you should have replaced the crank seals - they are the most notorious problems for these engines. With those, its better to be safe than sorry. Since you've found a leak - replace the seals. I presume you used assembly lube in your rebuild process. After the lube burns off the seals may leak worse unless the 2 stroke oil helps seal it back up. Just a thought.
    Argh. I'll test some more and see how it goes, gonna be a pain to pull the bottom off but if I have to...

    3. Your pressure loss looks pretty good to me. Did you time it a second time after the 30 minutes? I've heard theres some passageways and crevises that fill after a few minutes - like if you are running this test with your reeds installed they may be bleading the pressure on you.
    Those are from the second time, I filled it let it come down and refilled it. No reeds installed.

    How did you seal off the exhaust side? same way?
    Yeah the same way the rubber mat and blue rtv silicone.

  5. #5
    boudin's Avatar
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    I would not worry much about ~4psi/10 min from the crank seals. It's not uncommon.

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