Halt Chain Checker
Nice easy to use chain wear indicator in a go - no go design.
Guage determines less than 0.75% wear - which means the cahin does not need replacing.
If the 0.75% indicator drops in, but the 1.0% side does not - then the chain is between 0.75 and 1.0% worn and needs to be replaced whilst a new chain will still be compatible with the chainrings and cogs.
If the 1.0% side drops in - the chain is more than 1.0% worn and cannot be replaced with out replacing the chainrings and cogs.