Shimano's Hollowtech II external bearing bottom brackets use a specific spline interface that doesn't match older square taper or ISIS tooling. The Park Tool BBT-79.3 fits that 16-notch pattern found on Shimano road and mountain cranks, plus several other brands that license or copy the interface. If you're installing or removing a Hollowtech II bottom bracket, this is the tool that engages those splines without slipping or rounding them off.

The 44mm outer diameter matches the plastic preload cap and lockring dimensions Shimano uses across their current lineup. That includes Dura-Ace, Ultegra, 105, GRX, Deore, SLX, XT, and XTR bottom brackets—basically any Shimano crankset with external bearings made in the last fifteen years. The tool also works with FSA MegaExo, Race Face X-Type, and other systems that adopted the same spline count and diameter.

Park Tool machines this from steel rather than the softer aluminum you'll find on some budget options. That matters because bottom bracket cups can seize from corrosion or threadlocker, and a soft tool will strip out before the cup moves. The 3/8-inch drive interface accepts standard socket wrenches, ratchets, or a breaker bar when you need serious leverage on a stubborn cup. A 20mm hex fitting on the opposite end gives you a second engagement option if your drive tools are out of reach.... Read More