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.
Shocks are the easiest thing on the car to ignore and the biggest thing you can feel when you fix them. Fresh oil, good seals, matched rebound, and the car just calms down. It stops pogo-ing off jumps and tracks straighter through the rough. 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 often than I probably need to, because I chased a mystery handling problem for a whole afternoon once and it turned out to be a torn o-ring in one rear shock.
Shock oil doesn't wear out exactly, but it gets contaminated and it leaks. Every time the shaft strokes, a tiny film of oil rides out past the seal. Over a few packs 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 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 depends on how you run. Bashing in the yard, once or twice a season is plenty. Racing every weekend on abrasive outdoor clay, 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 and dump the old oil into a rag, not on the kitchen table. Pull the shaft out the bottom. The shaft guide, o-rings and spacers live down in that lower cartridge, and 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 rather than digging at them with a screwdriver, or you'll nick the bore.
Look at the shaft under a light. Any dull spot, bend, or rash of tiny scratches means replace it, because a scored shaft will shred a new seal in one run. Straight shafts are a couple of bucks and this isn't where you save money.
This is where everybody gets confused, and it's the manufacturers' fault. RC oil is sold two ways. American brands like Associated, Losi and ProLine print a weight number, 30wt, 40wt, 50wt. European brands and a lot of the pro stuff print centistokes, which is the actual scientific viscosity. Weight isn't standardized, so one company's 40wt is not another company's 40wt. It's a marketing number.
cSt is the real one. If you're comparing across brands, find the cSt figure, usually in small print on the bottle, 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 mean something. I run one brand front and rear for exactly that reason.
Thicker oil slows the shaft down, so the car feels planted and soaks up big hits but gets lazy in tight stuff. Thinner oil moves quicker and is better over choppy bumps, but it'll blow through its travel and bottom out on big landings. Most 1/10 buggy setups live in the 30-45wt neighborhood, and you go thicker for heavy 1/8 and 1/5 rigs.
There are two ways to seal a shock and you should know which your car uses.
Bladder shocks, which is most modern race buggies, use a little rubber cup under the cap that flexes to absorb the volume the shaft displaces. To bleed one, 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 full rebound, meaning the shaft pushes 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, so back it off.
Emulsion shocks, common on trucks and older kits, have no bladder and 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 as precise as a good bladder setup. Build whatever your kit was designed for.
The exact oil volume in any one shock matters much 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 except 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 has less oil or trapped air, so pop the cap and even them up. Do this every rebuild. It takes thirty seconds and it's the difference between a car that tracks and one that fights you.
An x-ring seal kit for four shocks runs a few dollars, and new oil in a nicked seal is a wasted evening since it'll leak back down within a pack or two. I replace the lower seals every second or third rebuild, and always if I see weeping around the shaft. A dab of green slime on the seals and shaft before reassembly makes them slide smoother and last longer.
When the car isn't 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 that you can't feel what you did.
If the car packs down and bottoms on jumps, go thicker before you go to a stiffer spring. If it feels harsh and skittish over small chop, go thinner. If the rear steps out on power, try a touch thicker in the rear first. Make one change, run it, feel it, and write it down. Log it on a setup sheet so you can 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.
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. 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.