If you have an aluminium frame which has thick end plates, you need these chain tensioners, and not the NJS type. The NJS type only fit on steel frames. On really wide ended frames, you may need to file out these tensioners an extra mm or so...but most popular track frames will accept these as they are. Very high quality, made in Japan by Mikashima. ***Sold as a single tensioner***, and the adjusting hex wrench is included.