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Finding TDC with vacuum?

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  • Finding TDC with vacuum?

    You all are some mechanical geniuses... Has anyone ever had any luck positioning a piston to TDC by applying a vacuum to the spark plug hole while the valves are closed? I just had the thought to try it, now that I'm running p-mags that need to time at tdc instead of 25 degrees.

  • #2
    That sounds interesting. I was just thinking about a skinny screwdriver in the hole and wait until it reverses direction.
    ​Christopher Owens, EAA #808438
    Project "Expedition"
    Bearhawk 4-Place Scratch Built, Plans #991
    Bearhawk Patrol Scratch Built, Plans #P313
    Germantown, Wisconsin, USA

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    • #3
      Interesting,
      I get the concept, from a theory sure it will work. Un intended consequences most likely not worth it.
      Need a lot of vac capacity because rings, valves will leak a lot.
      Connecting rods are compressive devices not tension. Play at TDC will be large because friction is high and the pull in the rod low and the restraint stop is well .......a vacuum.
      not much there.
      And eventually just as you think you have it the intake valve acts a relief.

      The flip side is put compressed air in a cylinder
      180 out and drive it to BDC. Unless the cam is already lifting..........

      Easy is never simple....

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      • #4
        The screwdriver in the spark plug hole will get you close. The only way I know of to be exact is to remove enough cowling/baffling/nose bowl to see the flywheel timing mark in relation to the case split.

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        • #5
          I've been using the piston stop pin method, which requires writing calculations on the hangar door.

          Next time I might try the vacuum in parallel and see if it works or produces reliable results .

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          • #6
            Different strokes , I don't buy all this. I do agree with the timing marks as you remove the gear lash. One could tape their cell phone to the prop and get it dead on. I remove all plug wires and one set of plugs top or bottom , safety first. Stinger

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            • #7
              you can count how many teeth on the ring gear to see how many degrees per tooth. I seem to remember on mine it was around 2.4 deg/tooth. Then simply move back from the 25deg mark on the flywheel and mark again. I have also used a digital protractor taped to a plastic qt paint pail taped to spinner. Set the engine to 25deg or whatever your known timing mark is, zero the protector then turn -25 deg and you are at TDC.
              John Snapp (Started build in Denver, CO) Now KAWO -Arlington Washington Bearhawk Patrol - Plans #255 Working on skinning the left wing! -Ribs : DONE -Spars: DONE, Left wing assembly's: DONE., Top skins : DONE YouTube Videos on my building of patrol :https://m.youtube.com/user/n3uw

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              • #8
                I use the digital angle finder taped to the prop, probably works the same way as the phone would, but then I wouldn't be able to answer all of those incoming calls if the phone was attached to the prop! It's nice to not need to worry about the 25 degrees part anymore, with the p-mags.

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                • stinger
                  stinger commented
                  Editing a comment
                  You're too popular. Stinger

              • #9
                Last year at Johnson Creek I had a no start condition ( dual P Mags) I used the screwdriver in the spark plug hole to find TDC and reset timing. That worked!

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                • #10
                  Right around top and bottom dead center, the piston motion becomes insensitive to crank rotation, all the way down to zero at TDC. So even if vacuum gets you close to top dead center, theoretically it will never get you there, and there could be a significant error in the crank angle. It could could still get you close, just be aware.
                  On the small unmanned Two Strokes I work on, we screw a timing wheel into the crankshaft with a pointer at approx tdc, and a Caliper in the spark plug hole to dial it in. You can get pretty accurate timing with that. I have no idea if that would work on these big 4 strokes...
                  Mark
                  Scratch building Patrol #275
                  Hood River, OR

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                  • #11
                    This is why the screwdriver/coffee stir stick method is not ideal. I'm just curious to measure the results of both methods, perhaps including application of the vacuum then testing the play in crank rotation.

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                    • #12
                      Yeah, I reckon there are quite a few degrees of rotation there at the top, isn’t there?
                      ​Christopher Owens, EAA #808438
                      Project "Expedition"
                      Bearhawk 4-Place Scratch Built, Plans #991
                      Bearhawk Patrol Scratch Built, Plans #P313
                      Germantown, Wisconsin, USA

                      Comment


                      • #13
                        Correct. So actually, when I zero the pointer on the timing wheel, I don't reference the extreme piston position, but say 0.500 before and after that, coming at the top from both directions to account for mechanical lash. If the pointer reading on the timing wheel btdc is the same as atdc for the same piston position, it is zero at exactly TDC.
                        Mark
                        Scratch building Patrol #275
                        Hood River, OR

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                        • Bdflies
                          Bdflies commented
                          Editing a comment
                          Well folks, Mark gets the prize!

                          Bill

                        • Chewie
                          Chewie commented
                          Editing a comment
                          Yeah sorry, we get paid to overthink things at work, it's hard to turn that off!

                        • Bdflies
                          Bdflies commented
                          Editing a comment
                          Not overthinking at all. Just a great description of a precise process.

                          Bill

                      • #14
                        I was talking about the digital protractor method above. This is the full tool to set engine timing, The first step is t9 find TDC with it then move to the need degrees.


                        AIRCRAFT ENGINE TIMING KIT (WITH MAG SYNCHRONIZER) - The Rite-System consists of two essential aircraft maintenance tools in one kit. The first is Mag-Rite, which is used to time magnetos with digital accuracy. And the second is Fly-Rite: a digital protractor. Includes LED Mag Synchronizer.
                        John Snapp (Started build in Denver, CO) Now KAWO -Arlington Washington Bearhawk Patrol - Plans #255 Working on skinning the left wing! -Ribs : DONE -Spars: DONE, Left wing assembly's: DONE., Top skins : DONE YouTube Videos on my building of patrol :https://m.youtube.com/user/n3uw

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