Motorcyclist reader Patrick Hunt emailed recently asking about the Coates rotary-valve engine. Back in the 1990s, an interested firm asked for my opinion about the Coates valve system. Readers can easily learn how the Coates system works on their own, so I won’t go into great detail here. My research found that Coates rotary valves, which are roughly the same diameter as the cylinder bore, necessitate a combustion chamber shape very similar to what used to be called a “hemispherical” chamber with conventional valves splayed about 30 degrees from the cylinder axis. Modern valve angles are typically 15 degrees or less, and piston crowns are flat or very close to it. The Coates arrangement, by comparison, has a domed top, and its valves leave short “port remnants” when closed, creating isolated areas in the chamber where combustion is likely to be incomplete—bad news for both emissions and efficiency. My conclusion was that the Coates system couldn’t compete effectively with more modern, conventional valve technology.