The Unior HG-IG Sprocket Wear Indicator can be used to check 12D21 tooth sprockets. The chain is put on the sprocket and the tool placed with its round end between two teeth on the upper sprocket side. The last chain link is moved away from the sprocket and a force of 100 N or 10 kg is applied to the tool in the direction of sprocket rotation. Now the last chain link is moved towards the sprocket. If it skips easily to the cog the sprocket is in a good condition and will function flawlessly even with a new chain. However, if the last chain link gets stuck on the tip of the neighbouring tooth or if we have to force it to skip on the cog, the sprocket is worn out and should be replaced. In case the chain falls off the clog when applying force, the sprocket is heavily worn out and a replacement is overdue.