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  1. #1
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    May 2007
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    Default Engine leak down

    What's acceptable leak down per cylinder on 23 degree 406 sbc 12.5:1 compression. Going thru some oil and breathers not covered. Checked all 8 and ranged from 10% lowest to 28% highest on couple cylinders. Normal was 15 to 18 majority cylinders. Wet sump on alky if really matters with 2 barrel. Could it be rings or valve guides or?? Thanks for info and assistance

  2. #2
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    May 2007
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    Kansas
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    Quote Originally Posted by Jim11h View Post
    What's acceptable leak down per cylinder on 23 degree 406 sbc 12.5:1 compression. Going thru some oil and breathers not covered. Checked all 8 and ranged from 10% lowest to 28% highest on couple cylinders. Normal was 15 to 18 majority cylinders. Wet sump on alky if really matters with 2 barrel. Could it be rings or valve guides or?? Thanks for info and assistance
    When you do the leak down, listen 3 places and it will tell you where it's leaking.

    1. Carb = Intake valve leaking
    2. Header = exhaust valve leaking
    3. Breathers = Rings

    Was the engine cold?
    Was each cylinder close to TDC?

  3. #3
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    May 2007
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    Knew was more info needed. Yes cold and yes TDC. Tried hearing and once thought heard the big exhaust tube low note on right side and another time guy on other side said he thought heard thru intake.

    So if valves should we pull or think can get last 3 nights? Valves stainless and turn 7k or less.
    Last edited by Jim11h; 09-25-2020 at 07:49 AM.

  4. #4
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    Kansas
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    Assuming it's just a leak and not something serious (burnt piston, broken valve spring or etc) and just leaking (worn rings, bore, or valve job not sealing): Yes, it should be fine to run for several races.

    Depending on what's leaking (like valves) at RPM the time it has to leak compression out is so short it can't leak as bad as you would think and kill power that much. Most of the power loss is noticed at low RPM's. I've seen NHRA stocker engine's where the have to push the piston to valve clearance so they can run the cam they want, that bent valves slightly on the dyno and were only down 15 to 20 HP from where they should have been.

    You can pull the headers and put your hand over the exhaust port to seal it and you will feel how bad the exhaust is leaking, same if you remover the carb and seal it will something like saran wrap and see if it pushes the wrap up with much force. You can also remove the rockers or tap the rocker/valve with a rubber mallet and see if the leakage changes. If it does then you may have loose guides not letting the valve stay centered on seat and seal like it should or has valve seats beat out of it. Just tap hard enough to open the valve slightly and re-seat it's self a couple of times and see if it changes the leakage %

    Typically I would like to see the engines have 1 to 3% leakage once broken in (cold), but most guys/builders they will never get that and many new engines may be closer to 5 to 10%. So it really depends on where it started, but I would typically run an engine to close to 20% unless it just felt down on power and I needed the power. Then it also depends on where the leak is, if some valve leakage then the 20% I'm good with but if all the rings then it needs looked at.

    Just my 2 cents worth
    Last edited by billetbirdcage; 09-25-2020 at 03:17 PM.

  5. #5
    Join Date
    Sep 2009
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    681

    Default

    20 percent or more isn't unusual on a .030 GM block if it's not been honed with a torque plate. I'd send it.
    A teaspoon of engine oil in the cylinders can tell you a lot about the rings. If the leak down is better with a oil can squirt in the spark plug hole than that a good indicator of a ring issue.

  6. #6
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    Appreciate the help guys. One person was worried valve guides bad and dropping valve since don't recall last time these heads freshened

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