Tight fitting, hard metal and labeled for which direction to tighten. Avenir is a great brand. Not high end, but fits right and is plenty good enough for a fixed gear buildup.

• Avenir Fixed Gear Cog 17 Tooth (1/2 X (1/8)
• Avenir Track Cog - 1/8", Fixed, RH 24 x 1.370, Black, 16T
• Park Tool Head-Gear Lockring Wrench
• B003BC8IW6
• Avenir Fixed Gear Cog 18 Tooth (1/2 X 3/32)