Small drive gear

Discussion in 'Frame Mounted Engines' started by fatboy67, Jun 14, 2008.

  1. fatboy67

    fatboy67 Member

    Hi to all. I have had to replace the little drive gear that drives the flywheel.
    This is on a ht 48cc motor. Ok I have searched and haven't been able to find any thing that is helpfull. Anyway I have allready taken the old one off, the trouble I'm having is putting the new one on. These things are a tapered fit, but mine would only go on half way. I tried gently filing the inside of the gear.
    I then tried gently tapping it on with a copper hammer this seemed to work at first but then it cracked.
    I think that maybe with these cheap motors you can't expect to buy a replacement part and expect it to fit all motors. Has anyone had much trouble taking this part off and on.

  2. We need pics,yo.
  3. srdavo

    srdavo Active Member

    since your new gear cracked, I'm wondering if your "moon" key might have been misaligned on the crankshaft or the gear?
    check the keyway on the shaft for damage.

    I'm still running my 1st happytime engine, from 2004. A few months ago, the bolt, holding the small gear, loosened up enough to allow the gear to slide loose....injuring the moon-key and the keyway on he crankshaft.

    Not having the patience, eyesite, steady hand file the keyway to fit (& not willing to spend more money on this old engine), I fixed it, anyway.

    Not a recomended fix!

    since this engine was basically done, I had nothing to I put the small gear on the shaft, without the moon-key. I pounded the gear on with a hammer & a block of wood.
    then I tack-welded the gear to the shaft, in 2 spots....where the bolt goes.

    300 miles later....still running!
  4. Ghost0

    Ghost0 Guest

    Anytime you are working with a tapered shaft you should heat up the part that you are installing. If you heat the gear up before putting it on, not only will it fit better but it reduces the chances that it will crack.
  5. fatboy67

    fatboy67 Member

    Hi again
    When talking about if the woodruff key may have been missalined, well I tried to fit it first with out the woodruff key and it was still as tight as.
    Hey Ghost thanks for your reply. I thought of heating it before I put it on but I think once it cooled and contracted it more than likely would have cracked. I think that may be this part I brought may have not been machined correctly.
    These motors are as cheap as and I think you get what you pay for.
  6. Ghost0

    Ghost0 Guest

    Actually it probably wouldn't have cracked. Here is a quick metallurgy class. Wonder why some steel is very hard and some is pretty soft? We will forget about the actual elemental analysis, carbon ratios etc and concentrate on heat treating. If you take 2 identical pieces of steel and heat them both to around 1000 degrees, one you will let cool slowly until it reaches room temperature and the other you cooled it off as fast as humanly possible. The one you cooled slowly will be very soft, the one you cooled quickly will be very hard. So my point is that if you heated up that gear to install it and let it cool slowly you would have softened (annealed) it so it would have been a little softer than when you started and not as brittle so it probably wouldn't have cracked.
  7. fatboy67

    fatboy67 Member

    about heat

    Firstly thanks for all your replies Ghost as I really do appreciate your help. Yes the steel will be a bit softer. I am 43 and been a boilermaker for nearly all of my working life, but I have had stuff before in the past when even with heat it was hard to make fit and then after it cooled it was to much for that type of steel and has split or cracked. NOW I did not try heating this so maybe it might not have cracked. I just thought judging by how tight it was more than likely it would have. I think that the part was machined wrong to start with and I should have just sent it back. When I replaced the flywheel this goes on in the same way and it went on EASY/////////////. With the small drive gear I was just to impatient and in the back of my mind I know I was doing the wrong thin by forcing it, just wanted to get the bike going AS I have just finished fabricating some new engine mounts(from stainless) that rap around the frame. They are insulated by 6mm rubber. This rubber is meant to be acid resistant, so hopefully the petrol will not rot it.