70cc HT just blew at 3,200 miles

Discussion in 'General Questions' started by Simonator, Sep 28, 2008.

  1. Simonator

    Simonator Guest

    EDIT: It blew at 3,320 miles. I don't know why i typed 3,200 miles

    yep she seized up on me and i skidded to a stop. Good thing I was only going about 20 mph. I have a brand new one thats ready to go on. i was hoping to break 4,000 miles. Oh well. Its just chinese junk.
    Last edited by a moderator: Sep 28, 2008

  2. sparky

    sparky Active Member

    AANNDDD... *This* is why I feel like you've gotta have a freewheel on the engine drive chain. Locking up the wheel is not good.
  3. skyl4rk

    skyl4rk Guest


    happytime RIP
  4. arceeguy

    arceeguy Active Member

    Should motorcycles have a freewheel on the engine drive chain?
  5. Simonator

    Simonator Guest

    motorcycle motors dont seize up every 3,000 miles lol
  6. terrence

    terrence Member

    Congrats, you got 2820 miles more than mine. 500 miles and "ka-bluwie"
  7. arceeguy

    arceeguy Active Member

    Yup - you're right. A motorcycle is more likely to tolerate moving around a 270lb person for more than 3,000 miles. lol
  8. s_beaudry

    s_beaudry Member

    Guessing that you paid $200 for your HT kit....

    3,320 miles divided by $200 equals 6 cents per miles riden!

    Not a bad deal actually!
  9. Simonator

    Simonator Guest

    Yep... and it will cost me even less on my new motor. I paid $75 for a whole new kit :grin: A friend of mine sold it to me dirt cheap :p
  10. arceeguy

    arceeguy Active Member

    So did you do a postmortem on your engine yet?
    Pictures of the destruction would be cool. Everyone loves a train wreck you know!
  11. Simonator

    Simonator Guest

    The only damage I know of right now is the piston siezed in the cylinder. The crankshaft still wants to turn. I might take some pics when I disassemble it.
  12. Simonator

    Simonator Guest

    I just installed my new motor. She's a runner. Yay!
  13. Simonator

    Simonator Guest

    how did you blow yours at 500 miles??
  14. ZnsaneRyder

    ZnsaneRyder Member

    WTF?!? The piston is locked, but the crank still tries to turn? WOW you must have a LOT of play on your piston rod from bad wear!

    What was the engine doing the last moments before it locked up? Maybe the engine is still salvageable or repairable. Do they sell new piston/rod replacements?
  15. Simonator

    Simonator Guest

    The day the engine locked up it was lacking alot of power. that is the only warning sign I got. I did not notice any unusual sounds.

    I just disassembled the engine. The crank is fine. Either the wrist pin roller bearing is completely shot, or the rod wore out. I am not sure which. There is EXCESS EXCESS play. It's ridiculous. That is obviously why it seized. I kept this engine very well oiled. I can salvage it. It requires a new cylider, piston, bearing (obviously), and possibly rod. I might rebuild it for something to do. The question is, is it worth it? If I buy parts from dax it will cost me: $50 for the cylinder, $20 for the piston, $11 for the rings, $10 for the bearing, and $9 for the head gasket. The rod isn't even offered. This brings the total to $100 SO FAR. I guess its not realy worth it. I can get a used engine for around $100 on egay
    Last edited by a moderator: Oct 1, 2008
  16. arceeguy

    arceeguy Active Member

    The rod is part of the crankshaft. The crank is two pieces that are pressed together, along with the rod, to form one assembly.

    Maybe the top roller bearing let loose and the shrapnel got caught in the ports, locking up the engine.
  17. Simonator

    Simonator Guest

    that would explain the excessive play...

    I just took apart my other broken HT as well. I wanted to take a close look at the wrist pin and roller bearing assembly. These roller bearings aren't all they cracked up to be. I noticed the rollers in the bearing left some nice deep indentations on the wrist pin. Hmmm
    I am not real familiar with small engines like alot of you guys are. This is all new to me. I have only worked on car engines. Here is what I don't understand. Why is there a roller bearing on the wrist pin anyway? Why isn't the wrist pin pressed into the rod and spun freely in the piston like car engines? is that just the way 2 strokes are because there is no oil pump?
  18. Simonator

    Simonator Guest

    Another question. Is the roller bearing supposed to be pressed and or fit tightly in the rod? When i removed the piston it the bearing slid out with ease.
  19. Simonator

    Simonator Guest

    yep, wrist pin roller bearing did let loose. That is why the engine seized. Another one bites the dust....
  20. arceeguy

    arceeguy Active Member

    Engines can use pressed in wrist pins (pressed into the piston, not the rod) or "free floating" wrist pins with keepers. The roller bearing is employed because of the scant lubrication available in a typical two stroke. It requires very little lubrication to function, and the open cage design allows more of the oil/fuel mist in the crankcase to contact the load bearing surfaces. The disadvantage is that the cage holding the rollers will break the moment you hit the "critical RPM" point, so they aren't tolerant to over-revving. In your case, with 3000 miles, maybe the bearing was substandard and failed, maybe it was weakened from a "high speed blast" down a hill and failed under "normal" use. Who knows?