ZEV Technologies™ stainless steel recoil spring guide rod for compact frame size Glock® pistols. (Compatible models listed on Tech Specs page)Recommended usage of either an ISMI recoil spring in a corresponding correct weight for your pistol. In most cases 13 lbs fits best.ZEV Guide Rods are NOT Compatible with Glock® Factory Guide Rod Springs.If you plan to use this Guide rod with a Generation 4 model Glock® you must also use a Generation 4 reducer (RDC-4G-SS or RDC-4G-SS-DLC)