Shock Oil and Pistons: Why wt Does Not Compare Between Brands
Springs decide how far, shocks decide how fast. What oil weight actually is and why cSt is the only number that compares across brands, how pistons change the character of damping rather than the amount, a full rebuild with proper bleeding, and a symptom table.
Updated Jul 22, 2026 · RC Crash Crew
Springs decide how far your suspension moves. Shocks decide how fast. Get that distinction and everything else on this page follows.
A car with the right springs and wrong damping feels wrong in a way no amount of spring changing will fix — it will either pogo and bounce, or pack down and refuse to extend. This covers what oil weight actually is, why the number on the bottle is not comparable between brands, how pistons change the same thing from a different direction, and how to rebuild a shock so both sides match.
What a shock actually does
Inside the body is oil and a piston on the end of the shaft. The piston has holes in it. When the suspension moves, the piston is forced through the oil, and the oil has to squeeze through those holes.
That squeezing is the damping. It converts the suspension's movement into heat, which is how the energy of a bump gets absorbed instead of bouncing the car around.
Two things control how hard that squeeze is:
- How thick the oil is — thicker oil resists flowing through the holes
- How much hole area there is — fewer or smaller holes restrict flow more
Which means thicker oil and a more restrictive piston do the same job, and you can arrive at similar damping from either direction. That is the single most useful thing to understand here, because it explains why two people can recommend completely different oil weights for the same car and both be right — they are running different pistons.
The oil number, and why brands do not match
Two scales are in use:
Weight (wt) — the traditional RC number. It is not standardized. One brand's 35wt can flow like another brand's 30wt or 40wt. There is no agreed test behind it, so comparing weights across brands is guesswork.
Centistokes (cSt) — a real, measured unit of kinematic viscosity. This is the number that compares. Most quality shock oils print both.
⚠️ If you are switching brands, match the cSt figure, not the weight. People who "went up two weights" and got a car that behaved completely differently usually changed brand at the same time and did not notice.
One more thing nobody mentions: oil thins as it heats up. A shock a few minutes into a hard run is damping less than it was on the first lap. That is normal, and it is part of why long runs feel different from short ones.
Which direction to go
Thicker oil (more damping):
- Slows the suspension down
- More controlled on big jumps and hard landings — less bottoming, less rebound kick
- More stable on smooth, high-grip surfaces
- Too thick: the suspension cannot recover between bumps. It "packs down" — each bump compresses it further before it has extended from the last, so it rides progressively lower and lower and eventually has no travel left. The car feels harsh and skittish over rough sections.
Thinner oil (less damping):
- Suspension reacts faster, follows rough ground, keeps tires in contact
- More grip on bumpy, loose or low-traction surfaces
- Too thin: the car pogos and oscillates after bumps and landings, feels floaty, and rolls further in corners because nothing is slowing the transfer.
The practical read: if the car bounces and keeps moving after a bump, go thicker. If it feels harsh and rides lower and lower through a rough section, go thinner.
Front versus rear
They are almost never the same, and the difference is a tuning tool.
- Thicker front slows front weight transfer → less initial steering, more stability on entry
- Thinner front loads the front tires faster → more turn-in
- Thicker rear reduces rear squat under power → more stable, less rotation on exit
- Thinner rear lets the rear squat and find drive on loose surfaces
Change one end at a time. Changing both tells you the car is different but not why.
Pistons: the other half of the equation
A piston is described by how many holes and how big they are — "2×1.6" means two holes of 1.6mm.
More total hole area = less damping (oil flows through easily). Less area = more damping.
But hole count and hole size are not interchangeable, and this is the part worth understanding:
- Many small holes produce damping that builds progressively with shaft speed. Gentle over small, fast bumps; firm on big, slow compressions. Good for rough surfaces.
- Few large holes produce a more linear feel — more consistent across speeds, and generally preferred on smooth, high-grip tracks.
That difference is why racers change pistons rather than just chasing oil weight: a piston change alters the *character* of the damping, while oil weight mostly alters the amount.
Practical rule: if the car is right in slow, big movements but wrong over fast chop (or vice versa), that is a piston question. If it is uniformly too fast or too slow, that is an oil question.
Rebuilding a shock properly
This is where most damping problems actually come from. A perfectly chosen oil in a badly built shock is worse than an average oil in a good one.
1. Strip and clean. Everything, including the shaft. Inspect the shaft for bends and scratches — a scratched shaft destroys seals, and a bent one binds.
2. Check the seals. O-rings are consumables. If a shock has been leaking, new oil alone will not fix it.
3. Fill slowly, and let it stand for a few minutes so trapped air rises out.
4. Bleed it. Slowly work the shaft up and down through its full stroke a few times with the cap off, letting bubbles escape. Rushing this is the number one cause of inconsistent shocks.
5. Set rebound. With the cap off and the shaft fully extended, top up, then compress the shaft slowly while fitting the cap so excess oil escapes. How much you leave out sets the rebound:
- No air (full) — the shock resists compression at the end of its stroke and will not stay compressed
- A little air / emulsion — the shaft can be pushed in and stays put, and the shock returns more gently
6. Test both shocks side by side. Hold them together, compress both at the same rate with your hands. They should feel identical and rebound at the same speed. If they do not, rebuild both again. Two shocks on the same axle with different damping is exactly the fault that makes a car behave differently turning left than right — and people chase that through the whole setup sheet.
Always rebuild in pairs. If one needs doing, do its partner with the same oil, the same technique and the same session.
The diagnostic table
| What the car does | Likely cause | Try |
|---|---|---|
| Bounces / pogos after landings | Too thin, or too much air in the shock | Thicker oil, re-bleed |
| Rides lower and lower through rough sections | Packing — too thick, or too little rebound | Thinner oil, more open piston |
| Harsh over small bumps, fine on big hits | Piston too restrictive for fast movement | More holes, smaller diameter |
| Bottoms out hard on landings | Too thin, or wrong spring | Thicker oil first, then springs |
| Behaves differently left vs right | Not an oil problem — a shock is aerated, leaking, or bent | Rebuild both, check the shaft |
| Feels vague and slow to react | Too thick, or binding shaft | Thinner oil, check for a bent shaft |
The order to work in
- Get the car mechanically right first. Bent shafts, aerated shocks and leaks will make any oil choice look wrong.
- Start from the manufacturer's setup sheet for your chassis. It is the only source with numbers that are genuinely right for your car.
- Change one end, one step, at a time. Test the same section between changes.
- Write it down. Oil brand, cSt, piston, and what the car did. Memory of how a car felt three packs ago is not reliable.
What we could not confirm
We are not publishing recommended oil weights or piston specifications, and this is deliberate rather than lazy. The correct values depend on chassis, spring rate, piston, vehicle weight, surface, ambient temperature and the specific brand of oil — and because "weight" is not a standardized unit, any figure we printed would be wrong for most readers even if it were right for one car.
Your chassis manufacturer's own setup sheet is the authority for starting values, and the club or track you run at is the authority for what works on that surface. Everything on this page is the direction to move and how to tell which way — that part does transfer.