Napier Deltic engine triangular cylinder arrangement.
https://s3-us-west-1.amazonaws.com/h...e_triangle.jpg
https://s3-us-west-1.amazonaws.com/h..._triangle1.jpg
Previously:
https://www.homemadetools.net/forum/...631#post159237
https://www.homemadetools.net/forum/...772#post133168