Soma Hobson-Zingo Chainring Nut Wrench

It fits into the notches on the nut on the negative side of the chainring and keeps it from spinning when you use a 5mm allen wrench to tighten the bolt on the other side. You can do without it if you grease the bolt threads and awkwardly use a tiny flat-bladed screwdriver to hold the nut in place, but this little tool is made just for the task, and you’d be nuts not to get it. 

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Intentionally
made without plastic or vinyl handles for the environment’s sake. We
recommend you wrap them with Newbaum’s Cotton Tape and finish the ends
off with twine. The tool factory Rivendell contracted with took about a
month to wrap their head around the idea.