Motorcycle Shock Piston & Seals Assembly

Your rear shock absorber has a complex internal thick metal disc with machined passages flowing between both flat sides, and to make sure the suspension fluid can only flow through these holes, a rubber o-ring and a teflon coated bush (or a soft PTFE band) seal around the outer diameter.

These OEM KYB & Showa main piston assemblies include the rubber o-rings & metal bushing rings or outer gapless bands already installed - we sell them individually too. The valving shims are then stacked on both sides, a collection of thin steel washers that flex when pushed by the oil pressure. They come in a huge range of diameters & thicknesses and are arranged in a very specific order with valve stop washers to create the right damping characteristics, with compression stack on one side, and rebound on the other. The piston & stacks are then locked to the end of the shock shaft using a locknut.

When the motorbike's coilover compresses, some of the oil flexes the shims open and pushes through the piston holes to the other side, creating friction and heat. If the damper gets pushed harder, the valving flexes further, allowing more through. After the bump, the coil spring pushes the bike back to it's ride height, and the oil is squeezed back through the piston and rebound shims to the other side, absorbing more energy. If your shocks have rebound adjusters, they usually have a pushrod that presses the reb shims more open or closed, acting like a tap for water and tuning the response - learn more from our DIY damping adjustment guide.

It's all basic science but there are thousands of different variables that change the way it feels - our shim equivalence calculator can help during a rebuild - and that's why experienced tuners test and refine all that data on a suspension dyno.