This guy used a piece of glass as a cylinder head so you can see the combustion happening. It's a really cool idea that we would love to see on a four stroke engine. It would have to be a flat head though, we think oil would block the view in an overhead cam.
![his-glass-cylinder-head-is-awesome](https://altdriver.com/wp-content/uploads/sites/2/2016/09/this-glass-cylinder-head-is-awesome.jpg?w=1056)