Overhead cam engines all use either a timing belt or a timing chain. They are all overhead valve. Overhead valves may either be driven by an overhead cam (see above) or by a gear-driven cam + push rods + rockers. Side valve (flathead, L-head, etc.) designs have a gear-driven cam + push rods (usually) + the valves themselves. Two cycle engines generally have no valves (except a reed valve to keep fuel/oil/air mix from blowing back through the carburetor). Various other designs have been built, including sleeve valves. Diesel (compression ignition) and Otto cycle (spark ignition) can have any of these valve configurations.