Chinese (2-stroke) Bicycle Motor - Bearing Part Numbers

Discussion in 'General Questions' started by Fabian, Oct 14, 2009.

  1. Fabian

    Fabian Well-Known Member

    G'day all

    I am in the process of rebuilding my original Chinese 2-stroke 69cc Bicycle Motor (engine number 2 though is still going strong at 20:1) as the bottom end needle roller bearing connecting the crankshaft pin has failed.
    Amazingly and even though the connecting rod can move in the vertical plane by 2mm (clunk, clunk, crikey that's a lot of play when moved about on the crank pin), the needle rollers are somehow still captive in the needle roller cage and give the appearance of looking ok.
    I don't understand how this is possible as the motor was rattling it's guts out and the small end needle roller bearing looks to be in perfect condition.
    Upon disassembly, a metalic mud film was found to be covering the crankcase area, so something in significant quantity has removed itself from the bottom end bearing and pin.

    Now that the motor has been stripped down, i want to replace all the bearings and seals, including connecting rod needle roller bearings with high quality items.
    Does anyone know a reputable manufacturer and a listing of part numbers and dimension specifications for replacement bearings.
    When the crankshaft is rebuilt, i want to get it balanced with the new piston for a super smooth and reliable engine.

    I'll post hi-res photos of the rebuild process.

    Cheers Fabian
     

  2. 210061741

    210061741 Guest

    Great idea.
    I want to do this to my 48 cc once it starts snowing.
    If you look at the bearings some are marked with a stamp.
    I'm not sure about the needle bearing.

    Heres my plan.
    You will need micrometers and a good set of Calipers.

    If you can measure the shaft dimensions and measure the diameter that the outer part of the bearing seats in, and measure the Depths of them as well.
    Or measure the bearings if still in ok shape.
    Any machinist of machine shop could get these measurements.

    These will all need to be in metric or converted from inch to metric.


    I can find compatable replacements.
    High quality bearings rated for a much higher load and RPM.
    Then we could reliably run the motor at a higher RPM without doing damage.
    Or have a motor that should last alot longer.

    Anyway i have seen some of the bike places sell bearings.
    Looks like DAX has alot.
    Not sure what brand they would be or what there specs would be but they go to be better than stock i would hope.

    So if you want simple by thoose.

    If you want the best. ALSO MORE EXPENSIVE

    Give me sizes or part# and i'll let you know what the options are.
     
  3. Fabian

    Fabian Well-Known Member

    Hi Rich

    I would be most grateful if you could give part numbers for the best quality bearings i can get my hands on.
    Once i get a karting shop to press apart the crankshaft and pull the bearings from the receiver pockets, i'll give you a list with dimensional specifications of all the bearings, unless someone has already been through this process and has them handy.

    Possibly the main limitation for safely increasing the rpm potential is the shocking crankshaft balance factor set by the Chinese manufacturers of bicycle engines.
    I would almost say they do not even set a balance factor into their crankshafts; as mine vibrates terribly above 3500 rpm (the first engine was no different).

    If rpm's could be safely increased to 5000 by high quality bearings and an optimisted crankshaft balance factor, stress on the bicycle frame would be reduced; fun factor increased, not to mention a more stress free ride, having to constantly be mindful of keeping the revs low.

    Fabian
     
    Last edited: Oct 14, 2009
  4. 210061741

    210061741 Guest

    Maniac Mechanic balances cranks i believe.

    Anyone ever try to balance there own?

    I wonder whats involved.

    Anyway i musta got lucky with my motor.

    Vibrates but not too bad.

    Never been limited by RPM.

    I run WOT all the time.

    On flat or slightly downhill roads only.

    With the SBP Expansion Chamber I'm sure to ride at 35 to 40 mph with my 36t sprocket. My motor actually likes higher RPM.

    It is tuned that way.

    But i have the 48cc seems like the cranks are more out of balance on the 60cc
     
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