Wolf Tooth Components 42T GC Cog - Shimano
If you're converting over to a 1x drivetrain, or just need a bigger bail out gear for some super hilly trails, then you need the Wolf Tooth Components 42T add-on cog. This huge cog will give you an extra wide range that can enable you to get up and over pretty much anything in your path. To install, simple remove your cassette from the wheel, remove either the 15T or 17T cog, install the GC 42, then the rest of the cogs in the correct order. It's super easy, and lets you modify your existing cassette for a wider range. After installation, you may need to replace the B-limit screw on your Shimano derailleur with the Wolf Tooth B-Screw (50-3897) for proper performance.
The Wolf Tooth GC cogs are only compatible with Shimano XT and XTR 10-speed cassettes