Drivetrain cleaning separates into two problems: the chain itself and the cogs it runs through. Degreasers and chain cleaning tools handle the chain, but cassette cogs and chainring teeth need something that can get between those tight spaces where grime builds up. The Park Tool GSC-1 combines two brush types on a single tool—stiff nylon bristles on one end for scrubbing cog surfaces and chainring teeth, and a curved plastic scraper on the other end for digging packed crud out from between cassette cogs.

The bristle end works like you'd expect: dip it in degreaser or soapy water and scrub across the cassette face or along chainring teeth. The nylon is stiff enough to dislodge dried chain lube and road grit without scratching aluminum or steel cog surfaces. Where the GSC-1 earns its spot on the workbench is the opposite end—that curved scraper fits into the gaps between cogs where brushes can't reach. Spin the cassette by hand while holding the scraper between two cogs, and the accumulated black paste that builds up in those valleys comes out in satisfying strips.

The handle shape matters more than it might seem. Park Tool designed it with enough curve that your knuckles stay clear of the chainrings when you're working on the crankset, and the grip section is wide enough that the tool doesn't rotate in your hand when you're applying pressure. It's the kind of detail that only shows up after you've used cheaper brushes that spin when you push down.... Read More