We’re finishing up six weeks of engines in class, and today’s project for me was to reassemble the 1986 (or so) Kawasaki GPz900 engine I had taken apart on Tuesday. While I say “engine,” “powerplant” or “powertrain” might be more accurate — nearly every modern motorcycle engine is in fact a unit-construction engine plus transmission. This is to say that a single case houses both the engine (the part that converts gasoline into mechanical work) and the transmission (the part that directs that work to the driven wheel).

As I inspected the little details of how this engine worked, it struck me that I was also peering through a window into the early 1980’s when it was first imagined by the engineers who created it. Did they think it was clever to turn on the neutral light by inscribing a broken, non-conductive circle into a metal plate turned by the shift drum? Did they love the steam-era look of the soldered-copper pipes running high-pressure oil inside the crankcase? How many minds molded each decision that led this engine into mass production? When they saw their first shift drum, what did they think of it?

I’ve decided that my favorite part inside the powerplant is the shift drum. To me, this smallish cylinder with deep paths cut into the surface looks like an ancient Mayan computer: the paths control three shift forks, each of which has three possible positions that control the sliding gears of the transmission. This may sound oblique, but the shift drum is really a seven-line program for a three-digit, ternary computer, whose output controls the ratio between the engine’s rotational speed and the motorcycle’s linear speed. The seven lines represent the seven gear ratios, including neutral (a ratio of 1:0).

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