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Fixing a Wonky Crankshaft

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  • Fixing a Wonky Crankshaft

    Enough time has gone by now, that I can start thinking about the wonky crankshaft on my opposed piston engine. Through trial and error I have weeded out the carburation issues, the flywheel issues, and now the only real issue remaining is the crankshaft. The engine runs great. The crankshaft seems to be good everywhere except for the portion that extends out past the front bearing. I built the crankshaft in pieces, and held the two ends in a jig for my final assembly, hoping that would yield a nice straight crank, but sadly that was not to be. There is considerable run-out as can be easily seen by looking at all the bobbing and weaving that aluminum hand-wheel on the front of the crankshaft is doing while the engine runs. It doesn't do that on the flywheel end---seems to run "true as a die" back there. I really don't want to build a new crankshaft. My plan is to cut the front off the crankshaft right beside the "throw" nearest to the front bearing, hold the flywheel end in my 3 jaw chuck, and bore the "throw" 13/32", then make up a new end which is .375" diameter over its full length except for the bit that extends through the 1/4" crank throw, which will be 13/32" diameter. I now have a tailstock chuck for my lathe with a bearing on it so that it can rotate freely. I am considering holding the flywheel end in the main chuck, holding the new end in the tailstock chuck, and silver soldering the final joint while the part is held perfectly aligned in the two chucks. There won't be enough heat transfer to damage either chuck, and if it doesn't work, I can then build a whole new crankshaft.-Nothing ventured --Nothing gained!!! To see what I am describing as "Wonky" click on the video link and watch that front aluminum wheel.

    Brian Rupnow
    Design engineer
    Barrie, Ontario, Canada

  • #2
    I know its a bit wobbly but it makes a lovely sound!, fixing the wobble might ruin it!
    You wouldn't varnish your Stradivarius would you lol
    I used to have an old dumper that had a 2 cyl lister engine, used to sound like that, damn thing would keep chugging away all day, sold it and bought a case 1845c to make life easier, bloody thing only lasted 6 months before the engine gave out, i have still got to pull it and rebuild it (with the help of the manual that Willy kindly sent, thanks)
    Keep up the good work your engines are brilliant
    Mark

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    • #3
      Thanks Mark. I'm very proud of the way that engine turned out, but you know how it is---No matter how well the engine works, the first thing everyone notices or comments on is the crankshaft. I hope to fix that.---Brian
      Brian Rupnow
      Design engineer
      Barrie, Ontario, Canada

      Comment


      • #4
        Not sure if it will translate to your crank, but we used to align pressed together motorcycle cranks and then tig weld the crank pins and con rod pins to the flywheels. Primarily the Suzuki 1100 racing engines. Without tig welding them the cases would split when the cranks warped... must have been a real thrill the first time...

        The old Honda single cylinder and twins were aligned by whacking them while suspended between centers on a lathe with a lead hammer until they ran true. No welding here, as the horsepower was not high enough to spin the pins in the flywheels... you do have to stop when it's all lined up :>)

        paul
        paul
        ARS W9PCS

        Esto Vigilans

        Remember, just because you can doesn't mean you should...
        but you may have to

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        • #5
          Mount the engine on the lathe bed, crank it up and turn the wonky crank to the next standard diameter.
          The difficult done right away. the impossible takes a little time.

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          • #6
            I do my own alignment on pressed up motorcycle cranks too. The process is basically to thump it like a neanderthal till it runs true.
            It is a put between centres, clock it up and mark the problem spot, dismount, over to bench over something solid and give it a good hearty *thwack* a few times, then repeat until happy on the gauges. That is assuming the webs are at true right angles to the spindle's axis and the only error is how they are pressed together, but if so, you can de-wonk your crank the same.

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            • #7
              Just tell the naysayers the wobbly bit on the end of the crank is a very precise amount eccentric to precisely balance the internal defriginator resonance.


              doug

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              • #8
                I designed for a small engine manufacturer. Crankshafts were pressed together with the con rod assembled on the throw and the rest of the parts pressed together. The pressed on parts were left a few thou fat and were then ground dead on.

                For your engine you could mount the engine on your lathe bed, shimmed to put the center line of the crankshaft approximately in line with the lathe spindle. Then, while the engine is running, you could toolpost grind the extended shaft true and then bush the disc on the end to fit.

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                • #9
                  Before I begin to tear everything apart, I decided it would probably be a good idea to measure just how much "Wonkiness" my crankshaft actually had. Measurements with my dial indicator show a total runout of 0.013" on the flywheel side, which means only 0.006" out of true center. This I can live with, because it really isn't all that noticeable---in fact I didn't see it at all with the engine running. However, on the front side of the engine where that knurled aluminum wheel is, there is a total runout of 0.038"--which is .019" out of true center---And that is very noticeable. I hope I will be able to repair the bad side of the crankshaft without introducing more "Wonk" to the good side.

                  Brian Rupnow
                  Design engineer
                  Barrie, Ontario, Canada

                  Comment


                  • #10
                    Jpfalt--That is a remarkably good idea.--If I had a toolpost grinder, I would do that.
                    Brian Rupnow
                    Design engineer
                    Barrie, Ontario, Canada

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                    • #11
                      Surely you have a dremel...

                      See, you do have a toolpost grinder--of sorts.

                      doug

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                      • #12
                        Brian, take your best running model engne and mount a grinding wheel on it. Then back that up to your engine you want to grind. Wal-la done! Not a good idea I suppose but it would be cool to see your engines doing "real" work!
                        Location: The Black Forest in Germany

                        How to become a millionaire: Start out with 10 million and take up machining as a hobby!

                        Comment


                        • #13
                          I use two toolpost grinders. One is a pneumatic die grinder with a rectangular bar, hose clamped to the side of the body. The second is an electric Horrible Fright extended die grinder with a bar that is hose clamped to the extended nose. They really work surprisingly well. I have a rubber sheet I throw over the carriage and bed to catch any grit and then clean it well with a solvent damp rag.

                          I used the electric setup last week to take about .015 off center out of chuck jaws on an old 1930's South bend lathe. After grinding, the chuck was repeatable within about .002" TIR.

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                          • #14
                            Dang--What a miserable little piggy to get the crankshaft out of. No worse than any other engine I suppose, but an hours work to get everything apart. the right hand end of the crankshaft is the end I will be replacing.
                            Last edited by brian Rupnow; 04-19-2014, 10:33 AM.
                            Brian Rupnow
                            Design engineer
                            Barrie, Ontario, Canada

                            Comment


                            • #15
                              So---Set up in the lathe, I measure .030" of total Wonk. That's .015" out of true center. The next trick will be to cut that bad end off and bore out the "throw" to 13/32"
                              Brian Rupnow
                              Design engineer
                              Barrie, Ontario, Canada

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