evo.com | Wolf Tooth Components Rear Derailluers > When you outfit your mountain bike's cassette with the Wolf Tooth GC 40 or 42T cog, you might need a longer b-screw so your derailleur clears the cog. The Wolf Tooth Components 25mm B-Screw is just that; made from high quality 18-8 stainless steel for trusted shifts and reliable drivetrain performance. High Quality 18-8 Stainless Steel When using the GC 40 with a Shimano rear derailleur, this longer B-Screw may be needed. The longer B-Screw is almost always needed when using the GC 42 with a Shimano rear derailleur. You will NOT need this longer B-Screw with the Wolf Tooth Goatlink. M4 x 25mm | Wolf Tooth Components 25mm B-Screw in Silver