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  • Leaky Valve Test Chamber

    It seems that in spite of all the care I took when machining and assembling my Atkinson engine, I have a leaky intake valve. Perhaps not leaking badly enough to prevent the engine from starting, but leaking, none the less. As I go through all the possible causes for the engine not starting and try to eliminate them, one thing popped up. Its a miserable chore to pull the head off the Atkinson once its all assembled, so I needed a way to test for leaking valves BEFORE assembling the head to the engine. So---I made a pressure chamber. This is simply a round peice of aluminum the same diameter as the cylinder, with the same bolt pattern as the cylinder, and an internal cavity which I can pump air into from my compressor. It is turned down to a diameter that fits into the recess in the cylinder head, and the internal chamber is big enough in diameter to miss the heads of the valves. I put a ring of gasket material between the test chamber and the cyl head, then carefully snugged down the 4 cylinder head bolts. Now I can pressurize the chamber with my compressor, and it immediately shows up a leaky valve. You can hear it hissing at the carb throat or the exhaust manifold. I am probably going to have to make a new intake valve cage ---maybe even a new valve. Ah well, bad on me. I thought that the pressure chamber idea might help some other poor sod who is attempting to build a 4 cycle engine.---Brian



    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

  • #2
    If this isn't the problem solved at least it is one more eliminated!!

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    • #3
      You do know valves get lapped in?
      Last edited by Mcostello; 09-22-2012, 10:36 PM. Reason: spelling
      mark costello-Low speed steel

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      • #4
        Originally posted by Mcostello View Post
        You do know valves get lapped in?
        Some do, but it is not necessary if cut or ground right. I prefer not to lap myself.
        James

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        • #5
          Originally posted by J. Randall View Post
          I prefer not to lap myself.
          James
          It's comforting to hear that --- I mean even if you could it just wouldn't be right,

          Brian, why not just inject compressed air at tdc of the compression stroke through the spark plug hole?
          it's called a leakdown test and regardless of how much is getting past your piston you can listen to the intake side and exhaust pipe for blow-bye...

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          • #6
            Originally posted by A.K. Boomer View Post
            Brian, why not just inject compressed air at tdc of the compression stroke through the spark plug hole?
            it's called a leakdown test and regardless of how much is getting past your piston you can listen to the intake side and exhaust pipe for blow-bye...
            Blowby, the same reason many professional shops have a rig similar to the OP's for testing valves and injector sleeve sealing off engine. Its possible to have a pretty bad valve - seat seal yet not notice via the method youre describing.
            "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."

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            • #7
              I was not convinced that the "special tool" that the OP made and used previously in a prior but recent thread was the answer to lapping-in the valves as there should be no radial or tangental thrust on the lapping rod/tool and the valve and ultimately the valve guide/s.

              All thrust on the vavle should be axial only.

              The thrust on/by a gear is normal to the pressure point along the pitch line (ie at 20 deg - or 18 or 17.5 etc) which is off-set from the centre line by and amount equal to the pressure angle.

              Any eccentricity between the axis of the drill driving the lapping tool and the valve axis adds to the problem (and the outcome).

              It is no accident that the traditional valve lapping is performed with balanced radial forces via the palms of the hands with also regulated a balanced axial force to actually lap the valve seat but to raise and lower the valve by its stem but located by the vavle guide.

              My guess is that the valve seats are ovoid instead of round/circular and do not seat correctly but will leak air.

              Any air-by-pass at the valves will impede the in-flow of the fuel mix and by-pass of the piston and rings will vent to the crank-case. Both instances/by-passes will also reduce the cylinder compression and will reduce combustion efficiency.

              My guess is that it now back to square one and re-facing and re-lapping the valve and cylinder head seats.

              I really hope it all works out as well as hoped and planned for.

              For some reason I still have doubts about the induction and exhaust flows.

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              • #8
                You don't have doubts now still, do you?

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                • #9
                  Are the seats square with the valve guide? Need a nice Neway cutter and the proper arbor.

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                  • #10
                    When the engine was assempled, wouldn't a simple automotive type compression tester screwed in place of the sparkplug have revealed this leak?

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                    • #11
                      Originally posted by Rosco-P View Post
                      When the engine was assempled, wouldn't a simple automotive type compression tester screwed in place of the sparkplug have revealed this leak?
                      This fixture allows him to work the next engine in-process rather than as a break-fix. I think he's looking forward to the next project more than worrying over the current one.

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                      • #12
                        I really do hope that Brian gets it all sorted out as I am sure he will be feeling better for it.

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                        • #13
                          Originally posted by A.K. Boomer View Post
                          It's comforting to hear that --- I mean even if you could it just wouldn't be right,

                          Brian, why not just inject compressed air at tdc of the compression stroke through the spark plug hole?
                          it's called a leakdown test and regardless of how much is getting past your piston you can listen to the intake side and exhaust pipe for blow-bye...
                          Used to do that with a piece of tape and some talc, put some talc in the port, tape the port shut and pressurize, leaky valves will either blow the tape off or produce a puff of talc 'smoke'

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                          • #14
                            To achieve the same test, I leave paraffin in the inverted combustion chamber over night, and if the ports are dry and the liquid is still mostly there in the morning, its sealing properly.

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                            • #15
                              Originally posted by justanengineer View Post
                              Blowby, the same reason many professional shops have a rig similar to the OP's for testing valves and injector sleeve sealing off engine. Its possible to have a pretty bad valve - seat seal yet not notice via the method youre describing.


                              Not the way it works, you could easily hear 1/10 of 1% in a leakdown test coming from a valve - there's no mistaking it.
                              all that's needed is a heater hose placed up into the exhaust pipe or intake manifold with the throttle plate open and the other side to your ear, it should be dead quite - Iv ran hundreds of leakdowns,

                              they are so precise that if you ever do catch any noise you have to pop the valve cover and give the suspect valve (or valves) a quick tap with a small fiber mallet - generally 99% of the time the noise instantly goes away,
                              the reason being is this; ---- by simply removing the spark plugs you drop small pieces of carbon into the combustion chamber, the first tested cylinder usually ends up good if you start with the cylinder that already had it's valves closed - but it's the valves that were in the open state hanging down below the plugs level that will give you these results, as you rotate the engine by hand to do a test on these valves they will wedge the tiny particulates between the valve and the seat - now you pour the compressed air to it and as miniscule as these pieces of carbon are they will make the valve immediately sound bad no matter how little - a quick tap with a fiber mallet on the rocker arms end and walla - the sound is TOTALLY gone, and when I say TOTALLY GONE I mean ZERO blow bye..

                              if it's an engine that has a difficult valve cover to remove then you bring each cylinder up to tdc of the compression stroke and then remove it's plug, maybe harder to turn over by hand but you avoid dropping the carbon onto a valves sealing surface when its exposed...

                              It's impossible to have a "pretty bad valve" and not be able to hear it.
                              I am a professional shop and have worked in many - and none have had a separate "rig" for testing valves like your taking about - it would be totally impractical to remove a cylinder head just to test valves... there's also no reason to bench test them after machining and either prussion blue testing or lapping and verifying contact 360 degree's on both seat and valve.

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