The MiniPit, FC Edition

Discussion in 'Some Assembly Required' started by GR0NK, Jul 18, 2016.

  1. GR0NK

    GR0NK Got some screws loose!

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    So this is something I've wanted to do for a long time. I've had some Chinese pitbikes for a decade now and they get beaten like they owe us money... :lol2





    They just keep on running and running. The one in the videos actually STILL has the fish oil in the crankcase that it came with! These little horizontal Honda clones are so good I think some of them are actually better than the original Honda motors they are copied (or licensed) from. :nod

    However, while (some of) the motors are fairly robust, the frames and the rest of the components are pretty crappy. Swingarm bearing/bushings lasted a year at most, the steering stem on one of my pitbikes had excessive play in it but as soon as you tightened the steering nut it would start to bind, the rear shocks are nothing more than a spring with zero dampening and the forks aren't much better, and the seat/bodywork is simply atrocious to sit on. I'll take a stock KTM seat anyday! :lol3

    So what do you do with the motor when the frame is completely trashed? You find a mini roller of course and build a MiniPit. :evil

    Queue in the 91 Kawasaki KX80 roller I bought last year just for this purpose...

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    I do have all the plastics and everything is there except for the motor, carburetor and the electronics. The only issue with it is the rear hub which is a write-off as it is missing the mounting tabs for the brake disc. I'll see if I can find a complete wheel or even a good hub since the hoop is still good.

    Combining a four-stroke horizontal motor with a mini frame is hardly anything new since Kawasaki basically already makes them in the form of a KLX110. The glaring difference however is that the KLX bikes are trail/play bikes and have trail/play suspension. By using an actual mini you get better suspension right off the bat. In this case it might not be all that good since it is a 91 with conventional forks. The plus is that it has disc brakes front and rear. :smile6

    The great thing about fitting a pitbike motor into a mini is that if you smoke it someday, you can get another one, even new, for really cheap. Or if you use a much newer frame with good suspension bits you can buy some seriously powerful motors that will really make one of these things rip. The mounting points are pretty much identical for all the Honda clone motors so if one fits, they all will. :thumb



    Sean :freaky
    #1
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  2. GR0NK

    GR0NK Got some screws loose!

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    Right, so looking at the engine cradle, it is obvious there is no room for a horizontal motor without cutting up the frame.

    [​IMG]

    So let's get to it shall we?

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    No going back now. :evil

    One thing I didn't notice until today was this...

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    Since I won't be needing the radiator it's not a concern. :rofl

    Oops, need to cut some MOAR! :D

    [​IMG]

    In the background you can see a 125cc Loncin motor. It runs strong but that's not the one I plan to put into the MiniPit. That Loncin motor is reserved for another little project. :evil



    Sean :brow
    #2
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  3. FR700

    FR700 Heckler ™©®℗

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    Let's shall.


    Oh you're such a tease.

    :lurk



    .
    #3
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  4. GR0NK

    GR0NK Got some screws loose!

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    Yes.

    First thing is to get the CS sprocket as close as possible to the swingarm pivot. Originally the swingarm bolt passed through the KX80 motor and acted as the inner spacer. I was prepared to make a deal with the devil but to my surprise it looked like the motor would be in the right place if I hogged out the rear motor mount to let the pivot bolt through. So that's what I did...

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    I'll need to make a few spacers and that's it.

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    Countershaft sprocket might be sitting just a bit high but I can live with that. Chain sliders are cheap.:thumb Speaking of which, the original chain slider had worn away so it was easy to line up the sprocket with the grooves left by the chain. At least it lines up perfectly.

    Looks promising... (just mocked up)

    [​IMG]


    Sean :brow
    #4
  5. GR0NK

    GR0NK Got some screws loose!

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    I'm sure I mentioned this somewhere before, but it's worth mentioning again. I hate making kickstands! :dirtdog
    Seems so trivial, but it always ends up taking more time than it should. And I even started with a pre-made one from an old pitbike frame I have in the parts pile.

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    I got lucky again as this 91 still has some vestigial brackets left over from previous years.

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    It appears the subframe once bolted to this bracket but is now a soild and integral part of the frame. Tapped out the holes as I decided this was where the kickstand will go.

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    And by the magic of the Internets you see the final product in mere seconds when in actuality it took several hours... Fawkin' kickstands... :rolleyes

    [​IMG]




    Sean :brow
    #5
  6. GR0NK

    GR0NK Got some screws loose!

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    I'll have to figure something out for the intake tract as the fuel petcock is rather close to the carb inlet.

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    Otherwise the tank clears everything.

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    The front fender will need to be trimmed as well since it slightly interferes with the cylinder head.

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    Sean :bmwrider
    #6
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  7. Fast Idle

    Fast Idle Since the Sixties

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    #7
  8. nuggets

    nuggets It's all my fault...

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    :lurk:wave
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  9. GR0NK

    GR0NK Got some screws loose!

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  10. downtavern

    downtavern Pants Flyer

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    There are also many different shapes, styles, and lengths of the cast manifold for the horizontal Honda motors if you want a more factory look.

    This all looks awesome!:lurk
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  11. Scubawerx

    Scubawerx Scubawerx

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    What a cool idea for the Chinese bikes. Let's see what you come up with. Nice photos.
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  12. GR0NK

    GR0NK Got some screws loose!

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    Thanks, I've been wanting to do this for a long time now. If this one turns out not too bad I would like to get a performance Honda clone motor and install it into a contemporary mini frame, like a 2005+, although it would be harder to find and certainly more expensive. :thumb



    Sean :bmwrider
    #12
  13. GR0NK

    GR0NK Got some screws loose!

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    Ok so with the swingarm bolt done it was time to get some brackets for the upper mount. An old Chinese pitbike frame donated those.

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    Had to bend and tweak them a bit but they worked great.

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    Can't weld through powdercoating...

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    Still my welding needs more practice...

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    Sean :brow
    #13
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  14. GR0NK

    GR0NK Got some screws loose!

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    Typical pitbike frames have a large single backbone with no downtube and the motor is held only by the two rear motor mounts. The upper tubes of the perimeter frame are too small for that. I'm sure it will break with just the weight of an adult on the bike so I need to make downtubes and I would also like to have at least one forward motor mount. This way the motor is not only solid in the frame but is also actually a stressed member that adds rigidity to the whole design.

    This part was very time consuming. First I mocked up the downtubes using 3/4" electrical conduit. Much cheaper and easier to bend until I get the shape right.

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    I had some 1"x1" square structural steel so I cut some sections to act as the lower engine cradle. The original frame was pretty hammered and flattened under the engine so I decided to cut it away.

    Before:

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    After:

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    The square tubing is 1/8" wall so it won't buckle under hard impacts even on solid objects like rock.

    Also, at first I was thinking about making a removable engine cradle as that would alleviate having to be careful about where the various tubes and brackets will go. But the more I thought about it, the more I just wanted it solid for reliability and strength.

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    That block you see was supposed to tie in the downtubes but it created a lack of room, especially if I need to use an intake adapter to angle the carb off to one side.

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    I later removed this and the tubes tie in directly with the frame. Simpler, less metal and less weight.
    As you can see in the pictures I tried to retain the aluminum skidplate that was on it. It's not very straight but it does a good job of protecting the starter motor located underneath. After further though I decided to toss it and I'll make something else.

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    After finalising the shape of the tubes I made them out of 3/4" steel tubing. It was thicker wall than what the frame was originally made with but I was still able to bend it with my EMT bender.

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    Then I made a last minute change without considering the consequences...
    I chopped the square tubing back to allow the downtubes to come down at a better angle but failed to think about the spark plug and the exhaust.

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    There was no room to get to the spark plug and there was no way a stock header would clear the tubes. I decided it would be easier to alter the downtube on the exhaust side rather than have to fabricate a new header pipe.

    Here is the final result. It's not symmetrical with the other side but it is just as strong.

    [​IMG]



    Sean :brow
    #14
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  15. GR0NK

    GR0NK Got some screws loose!

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    I also wanted a lower engine mount. At first I thought one of the factory brackets from the original downtube would work...

    [​IMG]

    And it would have , except that with this stupid little addition I could no longer extract the motor. :baldy

    In fact, the most time consuming part of building this thing has by far been removing and installing the engine repeatedly to make sure it can be done every time I make a change.

    The final bracket needed to be removable and out-of-the-way.

    [​IMG]

    This particular engine has a couple bosses that extend from the electric starter housing so I used one. If a different motor is chosen later on it may or may not have them. It's something I'll worry about if I ever have to swap it.



    Sean :bmwrider
    #15
  16. Simosez

    Simosez go ride your bike.

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    Nice. I have an 02 KX85 in the fleet. The frame and linkages look remarkably similar. I bet a USD front would bolt straight on. You also have a big wheel option too.
    #16
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  17. GR0NK

    GR0NK Got some screws loose!

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    The rear wheel I got with the bike has a broken hub; the entire disc mounting surface is ground away. It actually looks like the bearing broke and they kept riding until it was ground off completely.

    [​IMG]

    So I found a complete wheel on Ebay and it appears that it is a big 16" instead of the 14" little wheel. The front is a 17" so if it looks goofy or sits like a stinkbug I'll find a big front also. :nod



    Sean :beer
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  18. GR0NK

    GR0NK Got some screws loose!

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    Horizontal Honda engines (and clones) have the the exhaust coming out on the right hand side while the 91 KX80 has the stock exhaust running down the left. The airbox has an indent to give clearance to the stock pipe but that doesn't help me here. For obvious reasons I want the exhaust as close to the frame as possible so I cut a slot out of the airbox for clearance. Not sure if I'm going to keep the airbox at all but I cleaned it up because I'm pretty sure I'll at least use the rear part with the mud guard.

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    Clearance slot...

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    Adding some length...

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    Final placement of the muffler...

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    Great, let's weld. :-)

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    Hung and painted.

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    Sean :brow
    #18
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  19. GR0NK

    GR0NK Got some screws loose!

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    Next was cutting the CS cover to fit.

    Had to cut twice. :D

    [​IMG]

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    Sean :brow
    #19
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  20. GR0NK

    GR0NK Got some screws loose!

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    In case you hadn't seen it in the last picture...

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    ...this is gonna be a problem. :baldy



    Sean :brow
    #20
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