is my Cylinder too short

Discussion in '2-Stroke Engines' started by Grandpa Herman, Sep 11, 2011.

  1. Grandpa Herman

    Grandpa Herman New Member

    When I release the clutch handle, and roll the bike in order to turn the engine, the piston hits the head cover. This means that the engine is physically incapable of spinning. Duh, right.

    I started with a 2010 Grubee GT5. The studs ripped out, i screwed up the repair. I decided, for many different reasons, to purchase a new bottom end. I got the 66cc type from Kingsmotorbikes. It is awesome. Good heft, more solid than GT5. It has 8mm studs for cylinder. Good.

    With the new bottom end and the GT5 top (I used the GT5 piston as well) I had the problem which is the topic of this missive.

    I was then advised by Kingsmotorbikes (over the telephone) to purchase their $10.00 Super Rat 66cc head cover. I did that. They said it had more clearance. It did not work.

    I think I need to purchase a piston to match it. What I mean is, I think the GT5 arm is a little longer than the arm on the new bottom end. I hope I don't have to split the case in order to swap the arms.

    Has anyone else gone through this?

  2. ddesens

    ddesens Member

    Could you maybe just double up on the head gasket?
  3. motorpsycho

    motorpsycho Active Member

    it's not a head cover, it's the actual head itself.
    Are you positive that the piston is hitting the head itself and not just hitting the spark plug?
    Normally if the piston comes up high enough to hit the head, it will destroy the electrode on the spark plug.
    if the piston is contacting the head, the stroke is way too long for the cylinder and the head for some reason.(a missing gasket possibly)
    you did put a head gasket on right?
    you did put on the gasket that seals the cylinder to the bottom end right?

    A 49 c.c. bore is 40 mm and the stroke is 38 mm.
    A 66 c.c. bore is 47 mm, and the stroke is 38 mm.
    so, no matter which parts you have, the stroke is the same for 66 and 49 c.c.'s.
    A 49 c.c. piston and rings will be too small for a 66 c.c. cylinder, but the stroke will be the same.
    I'm thinking that you may have left out a gasket, specifically the one that seals the cylinder to the bottom end or the head gasket.
    You can run these engines with no head gasket (using copper gasket sealer in it's place) or a thinner headgasket, so i don't think the head gasket is the issue.
    Typically there is plenty of clearance between the top of the piston and the head.
    Last edited: Sep 11, 2011
  4. HeadSmess

    HeadSmess Well-Known Member

    look for ibedayanks thread. gt5 jug and piston. all questions answered.

    the piston con pin is slightly lower or higher depending on which one you have.

    apparently cylinders are the same but they cant be if youre sticking one onto a 66crankcase. cus the 66 has m8 studs that are further apart than the m6's on a 48... :confused: so how you did that without resorting to a drill, iunno!

    anyways. its .175 of an inch or 3-4 mm difference in that piston pin location...

    and then having changed piston heights will affect all the port timings...:eek:
  5. IbedaYank

    IbedaYank Member

    this is the piston you need...
    I need the piston with the writpin hole closer to the top of the piston, about .870 from the center of the wrist pin hole to the top of the piston.

    #2 Piston Set - 80cc/66cc [#2-24,25-GB80-F2-12-F2-19]
    Quantity 1 x $15.00 = $15.00

    contact joshua kirby-smith <>
    he will set you up with the right parts

    and its .120 difference in the wristpins
  6. Grandpa Herman

    Grandpa Herman New Member

    Motorpsycho, be assured that I have the gaskets on correctly.

    HeadSmess, I believe that you are on the right track. I seem to recall that a phone rep for Kings mentioned the height of the pistion's connection point. I am checking your post suggestion now.
    Last edited: Sep 12, 2011
  7. Grandpa Herman

    Grandpa Herman New Member

    Ibedayank, I am inclined to seriously consider you suggestion. Are those distance measurements in inches?
  8. IbedaYank

    IbedaYank Member

    GH yes I copied what they sent me from the shipping verified email
    so that should be the right part numbers
    measured with a dial caliper
    Last edited: Sep 12, 2011
  9. motorpsycho

    motorpsycho Active Member

    sounds like you are on the right track.
    I had to ask about the gaskets because sometimes you never know.
  10. Grandpa Herman

    Grandpa Herman New Member

    MotoredPsycho, too true. I have read some head shakers on this forum.