Rebuild your RC shocks step by step: why they go mushy, oil weight vs cSt explained, how to fill and bleed, and matching rebound so your car tracks straight.
Your shocks are the single easiest thing on the car to ignore and the single biggest thing you can feel when you fix them. Fresh oil, good seals, matched rebound — the car just calms down. Stops pogo-ing off jumps. Tracks straighter through the rough. And most people are running shocks that got a rebuild sometime around when they bought the kit, which is to say never.
I rebuild mine more than I probably need to, because I've chased a mystery handling problem for a whole afternoon exactly once and it turned out to be a torn o-ring in one rear shock. Never again. Here's how I do it.
Shock oil doesn't magically wear out, but it does get contaminated and it does leak. Every time the shaft strokes, a tiny film of oil rides out past the seal. Over a few packs of running you lose enough that there's air where oil used to be, and air compresses. That's the mushy, vague feeling — the shock isn't damping anymore, it's just bouncing on a hidden air gap. Grit works its way in too, especially on dusty outdoor tracks, and it turns your seals into sandpaper.
How often? If you're bashing in the yard, once or twice a season is plenty. If you're racing every weekend on an abrasive outdoor clay track, I'd pull them apart every few race days. The tell is simple: push down on a corner and let go. A healthy shock rebounds once, smooth and controlled. A dead one either shoots back up fast or feels gritty and notchy on the way. Trust the feel more than a calendar.
The whole job is small parts on a bench, and shock bits love to ping off into carpet forever. Work over a light towel or a shallow tray. I lay the parts out left to right in the order they come off so reassembly is just reading it backwards.
Unthread the cap, dump the old oil into a rag (it's not nice stuff, don't do this on the kitchen table). Pull the shaft out the bottom. The shaft guide, o-rings, and spacers live down in that lower cartridge — most Team Associated and TLR shocks stack them the same way, a spacer, two x-ring seals, another spacer. Push them out with the shaft, don't dig at them with a screwdriver or you'll nick the bore.
Look at the shaft itself under a light. Any dull spot, bend, or a rash of tiny scratches means replace it — a scored shaft shreds a new seal in one run. Straight shafts are a couple bucks. This is not where you save money.
This is where everybody gets confused, and it's the manufacturers' fault. RC oil is sold two ways. American brands (Associated, Losi, ProLine) print a "weight" number — 30wt, 40wt, 50wt. European brands and a lot of the pro stuff print centistokes (cSt), the actual scientific viscosity. The problem is that "weight" isn't standardized, so one company's 40wt is not another company's 40wt. It's a marketing number, not a measurement.
cSt is the real one. If you're comparing across brands, find the cSt figure (it's usually on the bottle in small print) and compare those. Losi and Associated both publish conversion charts, but the simpler fix is to pick one line of oil and stay in it so the numbers actually mean something. I run one brand front and rear for exactly this reason.
What the number actually does: thicker oil (higher number) slows the shaft down. The car feels planted, soaks up big hits, but gets lazy and doesn't react fast in tight stuff. Thinner oil lets the shaft move quicker — livelier, better on choppy small bumps, but it'll blow through its travel and bottom out on big landings. That's the whole trade. Most 1/10 buggy setups live somewhere in the 30–45wt neighborhood; go thicker for big heavy 1/8 and 1/5 rigs. When in doubt, thicker in front than rear is a safe starting bias on a lot of buggies, but check your car's baseline sheet first.
Two ways to seal a shock, and you should know which your car uses.
Bladder shocks (most modern race buggies) use a little rubber cup under the cap that flexes to absorb the volume the shaft displaces. To bleed: fill to the top, stroke to remove bubbles, seat the bladder, then push the shaft in a hair and thread the cap on while a little oil weeps out. You want the shaft to have full rebound — it should push itself all the way back out on its own. If it doesn't, there's too much oil and you've got a hydraulic lock; back it off.
Emulsion shocks (a lot of trucks and older kits, no bladder) deliberately whip a bit of air into the oil so the mix itself is compressible. You fill a little below the top, cap it, and the trapped air is the spring. They're more forgiving to build and consistent enough for bashing, just not quite as precise as a good bladder setup. Neither is wrong — build what your kit's designed for.
Here's the thing nobody tells you when you start: the exact oil volume in any one shock matters way less than having the pair on an axle feel identical. A car with both rears slightly overfilled but matched to each other will drive fine. A car where the left rear rebounds fast and the right rear rebounds slow will pull and hop and drive you nuts, and you'll blame everything but the shocks.
So after you build a pair, hold them side by side, compress both fully, and let go at the same instant. They should return at the same speed and stop at the same length. If one's faster, it's got less oil or trapped air — pop the cap and even them up. Do this every rebuild. It's thirty seconds and it's the difference between a car that tracks and one that fights you.
An x-ring seal kit for a set of four shocks runs a few dollars. New oil in a seal that's already nicked is a waste of your evening — it'll leak back down within a pack or two. I replace the lower seals every second or third rebuild as a matter of course, and always if I see any weeping around the shaft. A dab of green slime (the assembly grease in most kits, or a tube of Associated Green Slime) on the seals and shaft before reassembly makes them last longer and slide smoother. Don't run them dry.
When the car's not handling right, the instinct is to swap springs. Resist it. Oil changes are cheaper and usually the actual fix, and springs change so much at once you can't feel what you did.
Car packs down and bottoms on jumps? Go thicker before you go to a stiffer spring. Feels harsh and skittish over small chop? Thinner. Rear stepping out on power? Try a touch thicker rear first. Make one change, run it, feel it, write it down. Change one thing at a time or you're just guessing, and log it on a setup sheet so you can actually get back to a baseline that worked. If you're not sure what a baseline even looks like for your car, the kit setup templates are the manufacturer's own starting numbers — begin there, then deviate on purpose.
None of this is hard. It's fiddly and it rewards patience. Grab your car's baseline numbers off a setup sheet first so you know your target oil weights, then pull the shocks and get started — and if a rebuild uncovers a bent shaft or shot seals you don't have on hand, the used parts listings are usually the fastest way to get running again.