Tough zirconium grain.High performance grinding for the toughest applications.Type 27., Highest quality zirconium grain assures consistent high stock removal for tough applications.All sizes are made with uniform grain throughout, no filler grain used.All wheels are internally reinforced with the highest quality fiberglass for maximum safety.Our unique Super-Lock??????????Hub combines both a mechanical locking system and chemical bond providing one of the safest, vibration-free designs in the industry.