During a shock absorber piston, the piston forces a portion of the fluid through the pressure chamber. It is important for the fluid to remain in constant pressure on both sides of the piston. This can be achieved through the use of a piston shim stack. These shims can increase the damping rate on one side of the piston while decreasing the damping rate on the other. This is done by modifying the shape of the shim holes.

The shim stack can be assembled externally around the piston rod, or internally inside the cylinder body. In either case, the spring is mounted on the inside of the cylinder. The piston assembly is then assembled into the pressure tube. It is not a good idea to have the shim stack inserted in the pressure tube without the presence of a reduced diameter flange. This is because there would be little or no clearance between the piston and the tube. This would result in a spongy action.

The outer cylinder acts as a reservoir for the low-pressure gas. The nitrogen gas keeps the fluid pressure constant and helps to reduce the aeration. Unlike the twin-tube design, the nitrogen gas does not mix with the shock oil. Consequently, the low-pressure gas does not degass as quickly. This can help to avoid the problem of boiling. The low-pressure gas is also useful in absorbing vibrations.

The inner cylinder contains the oil. The piston compresses the elastomeric pellets. When the elastomeric pellets absorb energy, they reposition the piston for the next stroke. The energy is then converted into heat energy, which lowers the car's vibrations. The piston then retracts.

The piston has three valving stages. The first stage is a knife-edge orifice that is very short compared to the thickness of the inner cylinder wall. This type of orifice produces a non-laminar flow that is easily controlled. The other two valving stages are staggered. The staggered orifices allow a smaller amount of fluid to pass through. This results in a more uniform distribution of the fluid and increases the resistance to downward motion.

The plastic portion 50 is molded even with the annular surface 86. This provides a non-metallic hearing surface, a slidable engagement with the pressure tube, and superior bearing characteristics. The plastic portion may be made from nylon, or another plastic material. The flange may be plastic or metal. It can be drilled with a blind hole. The plastic flange is a substantial length compared to the total length of the plastic layer. The flange has an outside diameter that is approximately one-quarter to one-fifth of the axial length of the piston.

The lower end of the piston has an annular groove. It is connected to a piston valve, which measures the oil flow rate. The piston rod then connects to the piston assembly. The piston is then retracted back to its initial position by the spring. This is done to prevent the piston from bottoming out. This can cause damage to the shock absorber.