Vibration Cure

Discussion in 'Performance Mods' started by jaguar, Nov 5, 2012.

  1. jaguar

    jaguar Well-Known Member

    There's two main reasons why these engines vibrate so much. One is that the stock CDI fires way too advanced at any rpm above about 3000. Two is that the crank is not correctly balanced for the piston/wrist-pin. You can take the crank out and drill holes through the flywheels close to the conrod pivot point, or you can lighten the piston/wrist-pin. Standard wrist pins have a 5 or 6mm inner diameter. After I drilled some holes in my piston (which really is almost no change at all weight-wise since aluminum is so light) and had a machine shop drill out the pin to 7.5mm (and of course it had the Jaguar CDI to solve the timing problem) then the vibration was almost non-existent.
    Here's the dimensions of the stock wrist pins:
    48cc piston: 33mm x 10mm outer dia. x 5mm inner dia.
    55cc piston: 37mm x 10mm OD x 6mm ID
    60cc piston: 37mm x 10mm OD x 6mm ID

    Treatland has a wrist pin that will work perfect for the 48cc because it has a 7.5mm inner diameter.
    Yamaha QT50 33mm x 10mm x 7.5mm wrist pin $9
    https://www.treatland.tv/OEM-yamaha-QT50-YT60-10mm-wrist-pin-p/piston-pin-163300.htm

    I am waiting to hear back from pocketbikeparts.com on their wrist pins.

    Treatland also has a wrist pin bearing of good quality that will fit any of these engine sizes for $8 which you should get while you're at it:
    https://www.treatland.tv/olympia-needle-bearing-10-x-14-x-13-p/oly-needle-bearing-10x14x13.htm

    If anyone wants to measure their 66cc engine wrist pin and report the dimensions to me then I will look to find a lighter wrist pin to use as a substitute.
     
    Last edited: Nov 5, 2012

  2. Fabian

    Fabian Well-Known Member

    That's handy information to know Jaguar
     
  3. jaguar

    jaguar Well-Known Member

    pocketbikeparts emailed me and said these wrist pins have a 7mm inner diameter which makes them lighter weight than the stock ones.
    Generic 33mm x 10mm wrist pin $9 http://www.pocketbikeparts.com/Piston_Wrist_Pin_p/cgpr302445.htm
    Generic 36mm x 10mm wrist pin & bearing $13 http://www.pocketbikeparts.com/Piston_Wrist_Pin_Bearing_and_Clips_10mm_p/pbpn10mm.htm
    Generic 36mm x 10mm wrist pin $9 http://www.pocketbikeparts.com/Piston_Wrist_Pin_p/cgpr302446.htm

    1mm shorter than stock (37mm) is completely OK in case you're wondering.
     
  4. jaguar

    jaguar Well-Known Member

    Also, the vibration expends some engine power which slows things down. I just replaced the wrist pin (5.4mm hole) of my 60cc engine with the one I had drilled out to have a 7.5mm hole and got 2.1mph more top speed. A 2 stroke is all about harmony. Less harmony = less power.
     
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