Jerry Kieffer's fully functional scale Knucklehead.

via: rocket-garage via: craftsmanship museum

"Jerry Kieffer prefers to work from the real thing when building his scale models. In this case, he actually does own a 1947 Harley Davidson "Knucklehead" motorcycle that he completely restored. That means he can measure each and every part of the real bike to make the scaled down parts for his models. His plan was to model the entire motorcycle and have the motor run and all the systems work. He started with a 1/6 scale model and built the entire engine. It is completed and runs fine, but after thinking more about the project, he felt to model the entire bike at that scale would produce a finished model that would be too large for his liking. Rather than continuing on, he left the first engine as a running display model and started over from scratch in 1/8 scale. The second Harley is now under way and coming along nicely.

yes those are tiny spark plugs and yes the motor runs...

One of the ways Jerry challenges himself is to announce beforehand what he intends to accomplish. Sometimes he regrets this later when he runs into seemingly insurmountable problems with a build, but the pressure of coming through with what he has promised causes him to work at a problem until finding a solution rather than simply giving up. In this case he has stated that he wants to be able to start this tiny engine with a kick starter just like the real engine. He also wants the gearbox to function and the speedometer to work. These are some pretty incredible goals when you look at the small size of the bike and think about duplicating all the parts of a transmission or a speedometer and drive in 1/8 scale. Many of the parts are hard enough to make at full size, much less 8 times smaller. Keep in mind that when reducing in size, the part size goes down as a function of the cube root of the volume, not as a simple linear reduction. For example, the volume of a part at 1/2 size is actually 1/2 the length times 1/2 the width times 1/2 the height. That means the 1/2 size version of a 1" cube would be a cube .5 x .5 x .5 or .125 cubic inches—1/8 of a cubic inch. A1/8 size model of a 1 inch cube is .125 x .125 x .125 which equals .00195, or less than 2 thousands of a cubic inch!"