57 Big Twin Flywheel

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  • April 9, 2018 at 4:42 pm #9595
    Mumbles
    Participant

      This 57 Big Twin is being converted to electric start using a flywheel from a slightly newer Lark but I’m having issues getting the flywheel to bed down on the crankshaft. It doesn’t want to go the last 1/8" or so. It stops and rocks around with about half the needed threads for the nut exposed.

      Both keyways are clean and true and the key fits into both of them OK. This motor has the long key and I’ve tried it with the key set multiple ways but it still wants to hang up. Shortening the key might work but I’m wondering if I’m not overlooking something as it seems the keyway in the flywheel might not be deep enough? 😕

      April 9, 2018 at 6:04 pm #73688
      frankr
      Participant

        US Member

        I’d have to do the research, but I believe you have the wrong key for the flywheel / shaft. They changed the key height (not length).

        Later…

        April 9, 2018 at 6:09 pm #73689
        Mumbles
        Participant

          OK, I think I found the problem. After digging out the old Lark crank, I compared it to a 57-58 Big Twin crank and the problem is obvious. Two different style keyways! It’s now time to do some fitting and filing!

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          April 9, 2018 at 6:14 pm #73690
          frankr
          Participant

            US Member

            There was a mid-year change in 1959. Was 302602, late models were 306253. Then there was another change to 307480. I believe that was the short (length) one.

            April 9, 2018 at 9:14 pm #73704
            garry-in-michigan
            Participant

              Lifetime Member

              The older Big Twins with the larger key torque the flywheel nut to 60 to 65 Foot pounds of torque. The newer Big Twins with the smaller woodruff key have a slightly different taper in the hub and rely on the taper to grip the crank more securely. These require 100 to 105 foot pounds of torque. Lapping them together will even the taper and make them less secure. If you MUST lap them, try to take more material out of the upper part of the taper . . . 🙄

              April 9, 2018 at 9:43 pm #73707
              Mumbles
              Participant

                So Garry, if I use this Lark flywheel, it requires the higher torque? The key has been filed so the flywheel fits a lot better now but now I’m thinking I might have a ’58 Big Twin electric start flywheel somewhere with a damaged taper. If I can use it, I should be able to use a longer key and lower torque value just like the original. Either that or swap the crank. 🙄

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