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Briggs engine, cam failure and cure

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  • #16
    Briggs engine, cam failure and cure

    One thing I forgot to mention:-

    When the billet rods and high compression pistons arrived I weighed them fully assembled, the big end first on a digital scales set dead level, then the piston and rings again level.

    Aim was to find the amount of reciprocating weight (piston, rings, small end and upper rod, and rotating weight (big end, racing bolts and lower rod)

    A V-Twin has a single counterweighted crank shared between the two cylinders at 90; it counterbalances each in turn so there is no secondary imbalance at all like you get in a single or parallel twin.

    So the balance is 50% of total reciprocating lumps, and all of the rotating masses; on mine it worked like this:-

    BALANCE REPORT, BILLET RODS AND A.R.T. PISTONS FITTED

    Stroke is 70mm so weight is at 35mm radius

    1) Rings 16 grams, Piston 304 grams, Big end 111 grams

    2) Rings 16 grams, Piston 304 grams, Big end 112 grams

    Total Rotating mass = 223 grams x 100% 223
    Total Reciprocating mass = 640 grams x 50% 320
    Counterweight total 543
    Add oil 004
    Total Counterweight mass required at 35mm radius 547grams

    I was able to get a steel doughnut made and cut into two halves to assemble round the crankpin.

    It was soon found n increase was needed, luckily you can buy 10mm Tungsten Carbide rod on eBay, guy posts from Israel

    By drilling out cast iron at 7.1 grams per cc, and inserting Tungsten Carb rod into them at 14.5 grams/cc the crank could be balanced on two knife edges

    Crank was 58 grams out of balance, no wonder in the early stages ancillary items kept working loose...;-)

    Afterwards it was smoother although firing pulses still could be felt, things stopped shaking.

    Cheers

    Kev
    G-KEVA
    BMAA 5696

    "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

    R.J. Mitchell :- Designer of the Supermarine Spitfire

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    • #17
      Briggs engine, cam failure and cure

      Kev, have you considered cryogenic treatment on the cam. Some better engine men than me seem to swear by it.

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      • #18
        Briggs engine, cam failure and cure

        Thanks Tom
        Am trying to make upgrades simple as a fair number are doing this route now, some flying, some building, bunging a steel bar down the cam is easiest route.

        The other discovery, that Precision Cams do a profile grind to suit 4000-4500 rpm and with a steel core reinforcement already fitted, came too late as I'd already ordered repair parts from Performance V-Twins, but that's the route that seems best for future.

        Mike Edwards at Precision sells a 30/30 grind for $250, advising that it suits my rev range

        Kev
        G-KEVA
        BMAA 5696

        "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

        R.J. Mitchell :- Designer of the Supermarine Spitfire

        Comment


        • #19
          Briggs engine, cam failure and cure

          Balanced the crank last night, the results were interesting

          Performance V-Twins now have run out of the cast ART pistons which were from Taiwan and are supplying Arias forged ones, they are engineering catnip ;-)

          BALANCE REPORT, BILLET RODS & FORGED ARIAS PISTONS FITTED

          1) Rings and Piston 308 grams, Big end 107 grams (was 320 and 111)

          Total Rotating mass = 214 grams x 100%
          Total Reciprocating mass = 616 grams x 50% 308
          Counterweight total 522 (was 543)
          Add oil 004
          Total Counterweight mass required at 35mm radius 526grams

          What was really nice was that instead of having to drill and fit Tungsten Carbide weights in the iron counterweights, I ended up reducing them by drilling to static balance

          So MK11 engine may be even easier to build, with a lower power-band. We'll see.

          Cheers

          Kev
          G-KEVA
          BMAA 5696

          "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

          R.J. Mitchell :- Designer of the Supermarine Spitfire

          Comment


          • #20
            Briggs engine, cam failure and cure

            Some photos of the new forged Arias Pistons and billet rods

            Camshaft opened out from 10mm to 11mm, reamed gently and 11mm silver Steel high Tensile bar inserted, the break is on the old cam, I haven't repaired the broken one...;-)

            Kev

            Attached files
            G-KEVA
            BMAA 5696

            "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

            R.J. Mitchell :- Designer of the Supermarine Spitfire

            Comment

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