Proprietary grain and bond system.Premium performance for severe edge grinding.Type 27., The highest quality zirconium grain.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?âòÀÞ?â Hub combines both a mechanical locking system and chemical bond providing one of the safest, vibration-free designs in the industry.