Motorcycle Shock Piston Retaining Locknut

Your rear shock absorber has a complex thick metal disc with machined passages flowing between both flat sides. To make sure the suspension fluid can only flow through the holes, o-rings & a bush (or soft PTFE band) seal the outer diameter. Stacked on both sides of this piston are valving shims, a collection of thin steel washers that flex when pushed by oil pressure. They are arranged in a specific order to create the right damping characteristics, with compression valve stack on one side & rebound on the other.

The piston and it's shim stacks are then attached to the end of the shock shaft using one of these locknuts. You can't use a normal nut due to the constant vibration, and you can't use loctite or nylocs either, due to the heat & oil. You need a special fujilok lock nut (or OEM equivalent) with a locking stainless steel spring insert. Read our nuts & bolts tech article to become a fasteners expert.

When the motorbike's coilover compresses, some of the oil flexes the comp shims open and pushes through the piston holes to the other side, creating friction & 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 & reb shims to the other side, absorbing more energy.

If your shocks are adjustable with clickers they adjust the initial pressure on the shim stack, which can get it closer to what you need for your terrain & riding style. If you are out of adjustment but still need more, an experienced tuning workshop can revalve both stacks on the damper rod piston. Revalving takes a lot of testing to get right - using a suspension dyno and advanced resources like our shim equivalence calculator - can take your internals way beyond factory spec. For a better idea on what mods work, watch Teknik's suspension guru Nick rebuild and revalve a shock.