When using the Wolf Tooth GC 42 (50-3898) or GC 40 (50-3899) replacement cogs with a Shimano rear derailleur, a longer B-screw will be needed for proper adjustment. High quality 18-8 stainless steel screw is compatible with most Shimano mountain bike derailleurs. Replaces stock B-limit screw on Shimano MTB derailleurs Use if you are using Wolf Tooth GC 42 or GC 40 large cogs High quality stainless steel