Flying Horse 2-Stroke/4-Stroke Motorized Bicycle Engine Spark Plug Cap &ndash; Gas Bike Spark Plug Boot Replacement The Spark Plug Cap attaches to the wire of the CDI on any 48cc or 66/80cc 2-stroke bicycle engine kit To install, remove the stock spark plug cap from the spark plug. Twist counter-clockwise and take the stock plug cap off. Turn the new spark plug cap on to the CDI wire, install back on to the spark plug, and you're ready to ride