piston stroke too much!

Discussion in '2-Stroke Engines' started by triride, Jun 20, 2015.

  1. triride

    triride New Member

    Hi guys,
    Had an engine lock up on my China girl 80cc a few weeks ago.
    On inspecting the damage,I found that the piston wrist pin/small end brg had collapsed and the piston was damaged although the cylinder bore was ok.
    I ordered a replacement piston kit & have just fit.
    I have now noticed that piston travel is too much and comes into contact with the head!
    On checking the big end brg before & after doing job by checking freeplay with con rod movement,all seems ok?
    I think maybe the crankshaft could be bent?
    Anyone had a similar issue?

    Cheers, Pete.
     

  2. butre

    butre Well-Known Member

    there are 2 different piston designs. I believe you got the type A when you needed the type B. You have an FM80 connecting rod and not a Z-L, unmarked, or ZAE50 rod, correct?
     
    Last edited by a moderator: Dec 15, 2015
    Timbone and triride like this.
  3. KCvale

    KCvale Motorized Bicycle Vendor

    Doubtful you bent the crank, did you put your cylinder bottom gasket in?
    Got 2?
     
    triride likes this.
  4. triride

    triride New Member

    Hi butre,
    Super information, did not know there where different variants!
    I will duly order the correct "B" type piston.
    Thanks, Pete.
     
  5. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    if you had of compared the old piston to the new one before installing the
    differences would have been obvious.

    The question is how did you miss that the new pistons wrist pin hole was lower?

    Also when I assemble my motors I turn them over using the nut on the magneto
    to check where the BDC and TDC are and to spread oil to the rings and bore before
    the head is installed.
     
  6. triride

    triride New Member

    Wrong piston!

    Yes your right MotorBicycleRacing, I should of checked both pistons before fitting it.
    I did not realise there where two different types of piston for the 80cc so "assumed" (something you should never do!) it to be the right one.
    No damage done as I turned it over by hand.
    Does anyone know where I can buy the correct replacement piston/cylinder body/head ?

    Thanks, Pete.
     
  7. jaguar

    jaguar Well-Known Member

    Google "motorized bicycle parts"
     
    triride likes this.
  8. triride

    triride New Member

    Thanks for the replies!

    I have just purchased a piston & cylinder from MZMiami.
    Cylinder is compatible with A or B type piston and piston is for a Skyhawk which I believe has the same piston as mine with higher wrist pin location.
    Will let you know how it goes when i've fit the parts :)

    Cheers, Pete.
     
  9. jaguar

    jaguar Well-Known Member

    that means the new cylinder is high to match a high piston (longer from wrist pin to piston top)
     
    triride likes this.
  10. triride

    triride New Member

    The piston that I need has it's wrist pin higher up the piston skirt 7mm from bottom of piston to bottom of pin.
    The wrong piston that I ordered had it's wrist pin lower down 5mm from bottom of piston to bottom of pin.
    Hope this makes sense, i'm enjoying learning about these engines and obviously will make mistakes along the way as we all do when starting out :)

    Cheers, Pete.
     
  11. crassius

    crassius Well-Known Member

    I don't stock those parts here, but have sent customers to mzmiami & all seemed to work out well.
     
    triride likes this.
  12. jaguar

    jaguar Well-Known Member

    I hope they match but looking at this statement it looks like they don't match:

    >Cylinder is compatible with A or B type piston and piston is for a Skyhawk which I believe has the same piston as mine with higher wrist pin location.
     
    triride likes this.
  13. triride

    triride New Member

    I've been in touch with vendor & they confirmed that piston & pot are correct for my engine.
    Why do you think they might be wrong,jaguar?
     
  14. triride

    triride New Member

    Just done a bit more research and now realise what you mean.
    Type A piston (Grubee SkyHawk) is what I have ordered & should be right & not Type B as earlier post suggested.
    Very confusing all these different combinations! :(
     
  15. jaguar

    jaguar Well-Known Member

    but you said the next cylinder is good for both types pistons which means it is higher but the incoming piston will be lower.
    No big deal, if the compression (cranking psi) is low then just have the head or cylinder lathed down to increase it.
     
    triride likes this.
  16. butre

    butre Well-Known Member

    if you don't have access to a lathe I've used a belt sander to deck the jug before
     
    triride likes this.
  17. triride

    triride New Member

    Hi Guys,
    Parts arrived and fit, all ok with size of barrel and piston.
    Took a while to start up, but managed a short test run today.
    Does the new barrel/piston need to be broke in like you would do with a new engine?
    As it lacks power on any sort of incline and has definately lost some power?
    Stroke is correct,checked it with head off before fitting head and pre-lubed .

    Cheers, Pete.
     
  18. butre

    butre Well-Known Member

    It does need to be broken in, yes
     
    triride likes this.
  19. triride

    triride New Member

    Cheers butre!
     
Loading...