Rebuilding your shocks

Shocks are the most neglected part of most cars and the one that repays attention most. Oil breaks down, seals wear, and air works its way in. The car does not fail dramatically — it just gradually stops doing what you ask, and because it happens slowly you adapt to it without noticing.

Do all four in one sitting. Matched shocks matter more than perfect ones.

When

Every few race meetings, or straight away if you see oil on the shock body, feel one shock behave differently from its pair, or the car has started to feel unsettled over bumps.

Stripping

Work over a tray — the small parts are easy to lose and awkward to source at a meeting.

1. Remove the shocks and take off the springs, noting the preload spacers on each so you can put them back the same.
2. Unscrew the cap and empty the old oil out. It will be darker than it went in.
3. Push the shaft out and inspect it. Any pitting, scoring or bend means a new shaft — a damaged shaft destroys new seals in one run.
4. Pop the seals out of the lower cartridge. Replace them. They are cheap, they are the thing that actually wears, and reusing them wastes the whole job.
5. Clean everything with motor spray or isopropyl alcohol and let it dry fully.

Reassembly

Put a drop of green slime or shock oil on the new seals as you fit them, then slide the shaft in from below so the seal lips are not forced backwards. Fit the piston and secure it — check the pistons are the same across the set, since it is easy to mix hole counts between cars.

Filling and bleeding

This is where the job is won or lost.

1. Stand the shock upright with the shaft fully extended and fill with oil to just below the top.
2. Cycle the piston slowly up and down a few times, staying under the oil surface. You will see bubbles come up. Slowly — a fast stroke whips air in rather than out.
3. Let it stand for a few minutes. Bubbles clinging to the piston will find their way up.
4. Top up, then set the shaft position your kit calls for and fit the cap. With a bladder, fit it so trapped air can escape past the edge as you screw the cap down. With an emulsion build, a deliberate small air gap is the point — keep it identical on every shock.
5. Wipe the excess and compress the shock fully. It should rebound smoothly. If it springs back hard, there is too much air; if it will not fully compress, there is too much oil.

Matching the set

Hold each pair side by side, compress both by the same amount and release. They should move at the same rate. If one is quicker, it has more air in it — bleed it again. Two minutes of comparison here saves an afternoon of chasing a handling problem that is not a setup problem at all.

Log the rebuild against the car with the oil weights and piston you used. Next time you will know exactly what was in there rather than guessing.