AT27-GL pistons are single piece aluminum castings. They are a four ring design, with the top two compression rings being housed within a Ni-resist steel insert. The third ring is an intermediate ring while the fourth is an oil control ring. The AT25 uses five rings, two compression, one scraper, and two oil control rings. Previous AT27 pistons used a black graphite coating for break-in purposes while current AT27s do not.