A bicycle chain’s apparent stretch comes from wear within its joints. As wear increases, the chain meshes less well with sprockets and can accelerate their deterioration. Cleaning and lubrication matter, but neither replaces a wear measurement.
First check that the gauge suits your particular chain. Some chains use different roller dimensions. For example, the CC-4.2 uses three contact points and accommodates SRAM Flattop and T-Type chains; another tool’s compatibility should not be assumed from a similar appearance.
General replacement guidelines are 0.5% wear for 11–13-speed chains, 0.75% for 5–10-speed chains and 1% for a single-speed drivetrain. The specific chain or drivetrain manufacturer’s limit takes precedence, however, and may differ.
Follow your tool’s own measurement sequence. With the CC-4.2, tension must be maintained at the specified point while taking the reading. Incorrect placement can mislead; a sound measurement supports a replacement decision before pronounced shifting problems develop.