Here’s the left piston from the mid-60’s Yamaha 100 Twin Jet I’ve just started restoring. This is one of the most damaged pistons I’ve ever encountered in over 45 years of working on Japanese motorcycles. I don’t know what actually happened, however, the cylinders/pistons are one-size-over stock. I suspect the cylinder ports were not camfered for the rings when the cylinders were bored and consequently the rings snagged the ports.
The piston is literally split up, you can set the crack across the top of the piston in the second photo.
