Wolf Tooth 25mm B-screwWolf Tooth makes excellent components and solutions for riders who want a little extra out of their bikes. The GC 40 and GC 42 are giant cogs that give riders extra large gearing so they can climb tougher hills. However, when you use these cogs, you might start running into compatibility issues. When using the GC 42 or GC 40 with a Shimano rear derailleur, a longer B-screw may be needed. This is a high quality, 18-8 stainless steel M4 x 25 mm screw.FEATURESDesigned to work with GC 40 and GC 42 cogsWorks with Shimano rear derailleurs so they can work with bigger cogsHigh-quality 18-8 stainless steel M4 x 25mm screw | Wolf Tooth 25 mm B Screw 25 mm B-Screw