Carbon fiber seatposts and handlebars have low torque limits, and the smooth surface finish on carbon and alloy components doesn't always generate enough friction to stay put at those specs. That's where assembly compound comes in. Park Tool's SAC-2 Supergrip Assembly Compound uses silicon dioxide particles suspended in a paste carrier to increase friction between clamping surfaces, so your seatpost stops creeping and your bars stay where you aimed them without cranking past the torque values stamped on the component.

The SAC-2 is not grease and should not be confused with it. Grease reduces friction; this does the opposite. You apply a thin layer to the contact surfaces between your seatpost and seat tube, your handlebar and stem clamp, or your stem and steerer tube. The silica particles bite into both surfaces and resist rotation or sliding, which means you can torque to the manufacturer's recommended spec and actually have the part stay put. That distinction matters on carbon parts especially, where overtightening to compensate for slippage can crack the component. A lot of frame and component manufacturers now explicitly require assembly compound during installation, so using it isn't just a shop preference, it's often a warranty condition.

The compound works across material combinations: carbon-to-carbon, carbon-to-aluminum, aluminum-to-aluminum, aluminum-to-steel, and titanium interfaces. One note worth knowing is that the silica particles can leave minor cosmetic marks on finished surfaces where the parts contact each other. Those marks are hidden under the clamp area and don't affect structural integrity, but they're there. Don't use SAC-2 on threaded parts, bearing surfaces, or in a grease gun. Threads and bearings need lubrication, not friction, and using assembly compound there would work against you. A thin film is all you need per application, so the 4 oz tube lasts through dozens of installations.