This is my take on the Tom Thumb minibike, (somewhat updated).
I did this not because I needed a minibike, but because I was interested in TIG welding and fabrication, and I wanted to really "old school" it and make as much as possible myself. . I can MIG weld to some degree, but for TIG welding, I was only somewhat successful. I can actually do a stack of dimes on lap welds or but welds of sheets of metal on a table but three dimensions of shapes in the air will still take a lot of experience. THEN I read that one of the hardest things for even an experienced TIG welder to do is small-diameter pipe..."a grinder and paint make me the welder I ain't". The results may not look that good but I'm fairly certain that they are strong.
I bent the pipe (yes, pipe, 5/8" ID) with a Harbor Freight pipe bender. Since this has a lot of trouble going >90 degrees, the pipes were joined to produce >90 bends except the two at the back which I BARELY did and that was after about a 100% waste factor on the pipe and a lot of finnese. Without a jig, making these parts accurate and symmetrical is a LOT of work without the proper ($$$$-$$$$$) equipment!
The wheels/tires are HF also, cheap, and I bought four to come up with the two bearings on each side per wheel. Then I discovered like everyone else that the sheet metal bearings are no good and I replaced them with real bearings. The tires are supposed to be lousy also - so, buying these was probably a false economy. I made the hub/gear adapters (those, I MIG'ed).
I took some liberties with the design; the wheels are larger and bigger (5"); the frame is at least 1.5" wider because that's how the 49CC engine worked out. I made the handlebars taller in the hopes that it would be easier for me to ride (I'm ~6' tall). ALso, I'm (somewhat) wider than the very spindly kid who originally built this in 1970!
I polished the front cover of the engine to some degree. I'm running it at 16:1 for break-in and it's pretty smoky, I'll probably go to 25:1 later (with modern 2-stroke oil, this is supposed to be safe - comments, anyone?
I really wanted to mount the brake (yes, I wanted a brake) INSIDE the frame perimeter but this worked out to be about 1/4" beyond possible, so it wound up on the back. I really wanted to make sure it would stop and this was probably overkill.
The gas tank is made of PVC fittings. I made the seat and had a car shop upholster it. That cost too much but I was too far into it at that point...
The throttle cable and brake cables were shortened. Lots of good YouTube videos on how to do this with silver solder. I couldn't believe that this could be done but it did work, first crack. I even measured correctly. Good thing, I had only one set and exactly three ferrules!
Getting a muffler for this is tricky. I am using a box-type muffler now with a shield to deflect the exhaust from my foot! This may not be my final take on the muffler.
I am paranoid about getting the chain wrapped up in something so there is a chain guard made of aluminum on the left side, with mounting tabs for it on the frame (not shown here).
I BARELY fit on this, I don't know what to do with my feet; i can barely make the foot pegs work for me. My initial ride showed me that the acceleration is pretty brutal (it's geared at maybe 20:1). I'm glad it has a good brake. It is scary to start it because it will do an instant wheelie with no one on it and any throttle; starting it while you are sitting on it is basically impossible, at least with my 60+-year-old physique!