KMC Cassette, 10-speed, 11–42T, steel

KMC’s wide-range cassette is a ratio change rather than a replacement, and the derailleur decides whether it is possible.

What the manufacturer publishes

A 10-speed cassette sprocket set from 11 to 42 teeth, in steel.

The 42-tooth question

This is the whole decision. A 42-tooth largest sprocket is a dramatically easier climbing gear than the 32 of the cassettes below it in this hub. It is also beyond what many standard 10-speed rear derailleurs are rated for.

The maximum sprocket size is published by the derailleur manufacturer. If yours states 36, a 42 will not clear the pulley. Some riders solve this with an extender hanger; that is a further purchase and its own compatibility question.

The second consequence

Chain length. A larger sprocket needs a longer chain, and the derailleur’s total capacity figure has to accommodate the range. Both figures come from the derailleur maker.

The gear steps

Ten sprockets across 11 to 42 means larger jumps than ten sprockets across 11 to 32. On a climb that does not matter; on a flat road at a steady cadence it does. That is the honest trade.