In a motorcycle engine, the cylinder head is made of a thick piece of plexiglass allows to observe the explosions produced by the spark plug. The last, present in spark ignition engines (two-stroke or four-stroke), ignites the gas mixture in the combustion chamber. For this purpose, They must be able to create thousands of arcs per minute, while withstanding the heat and pressure generated by the explosion inside the cylinder. The final plexiglass will not withstand the high temperature and pressure, and the spark plug will leave his post.