Deflector Piston

Early two-stroke engines used cross scavenging: the transfer port and exhaust port sat opposite each other. To stop the fresh fuel–air charge from flowing straight out of the exhaust, the piston crown had a tall ridge, the deflector, which turned the charge upward toward the cylinder head and pushed burnt gas out.

The heavy, unevenly heated piston limited revs and power. From the mid-20th century loop scavenging (Schnürle porting), with flat or slightly domed pistons, replaced it, and practically all modern two-strokes use it.

You will meet deflector pistons today mainly when restoring vintage motorcycles, mopeds and small engines; replacement pistons must match the original shape exactly.