How to Fill RC Shocks with Oil: Complete Shock Filling Guide

It’s imperative to fill your RC shocks with oil properly as they control suspension movement by forcing oil through small piston holes. Improperly filled ones produce air bubbles to disrupt this process and create inconsistent damping, causing blown seals, inconsistent handling, and hydro-locking.

Your goal: create a clean oil environment with minimal air at the correct level for your shock type.

After thousands of shock rebuilds with our racing team, we've perfected techniques for all three shock types: emulsion (RTR cars), bladder (performance cars), and competition setups. And we’re more than willing to share them with you in this guide!

Universal Preparation for All Shock Types

RC shock filling requires specific tools and supplies before starting. You'll need a small funnel or syringe, paper towels, a disposable cup, and appropriate shock oil for your car type.

Remove the shock from your car and take off the spring. This makes handling easier and prevents accidental damage. Clean under any rubber boots on the shock shaft - dirt here often gets pushed into the shock during operation.

Oil Removal Process: Place a paper towel on the bottom of a disposable cup to absorb the old oil. Remove the shock cap and pour out the old oil. Move the shock shaft up and down several times to push out oil trapped under the piston.

Oil Quality Check: Clean oil appears clear and flows smoothly. Dirty oil looks dark, thick, or jelly-like and contains particles that damage internal parts. If the oil is dirty, clean the shock cylinder with a rolled-up paper towel and inspect the seals for wear.

Place the shock upside down on a paper towel for a few minutes to remove the last traces of old oil.

Initial Filling: Pull the shock shaft to its fully extended position before adding new oil. Fill the shock body about halfway with quality shock oil. Controlled filling prevents bubble formation that disrupts damper performance.

Air Bubble Removal: Move the piston slowly up and down in the oil 4-6 times. This releases air bubbles trapped under the piston. Wait 30 seconds between movements to let bubbles rise to the surface.

Final Oil Level: Oil fill levels vary by manufacturer. Fill to 1-2mm from the shock body top (XRAY recommends 1mm, Traxxas suggests 2mm). Keep the shock perfectly upright during this process. Let the filled shock sit vertically for 2-3 minutes so remaining air bubbles can rise and pop.

Emulsion Shock Filling (RTR/Standard Cars)

Emulsion shocks, designed to allow air mixing with oil, are found on most ready-to-run cars including Traxxas, Arrma, and HPI models. RC shock caps come in two designs: bleeder caps with a small screw and standard caps without bleeders.

For Bleeder Screw Caps: Remove the small screw completely. Fill the shock body to create a slight dome of oil above the rim. Install the cap finger-tight only, leaving the bleeder hole open.

Slowly push the shock shaft all the way down. Oil and air will spray out of the bleeder hole, which is normal. Keep pushing until only clean oil exits without bubbles. Hold the shaft down and reinstall the bleeder screw. Tighten the cap securely.

For Standard Caps: Add a few drops of oil inside the cap before installation. Fill the shock body to the top. Install the cap loosely and carefully push the shock shaft down. Excess oil will flow out through the cap threads.

Proper Rebound: The shaft, when properly filled, should stay compressed when you release it: no rebound means proper emulsion shock filling.

Bladder Shock Filling (Performance Cars)

Bladder shocks, engineered with rubber membranes, create more consistent damping than emulsion designs. They're common on touring cars and high-end kits from manufacturers like XRAY and Yokomo.

Bladder Cap Setup: Fill the cap to 1-2mm from the top, as the bladder needs space. Position the shock shaft to set your desired rebound before installing the cap. A good starting point is leaving about 1/3 of the shaft visible above the shock body.

Assembly: Install the bladder carefully into the oil-filled cap without trapping air underneath. Hold the shock shaft in your desired position and install the cap. The shaft will try to extend as you tighten the cap, so maintain pressure.

Shock Matching: Test both shocks at the same axle to ensure they behave identically. Push each shock down and release; they should extend the same amount at the same speed.

Pro Tips for Competition Shock Rebuilds

Competition drivers often use vacuum pumps for faster, more complete air bubble removal than manual methods. Connect the pump to the shock body and create a partial vacuum while adding oil.

RC maintenance for racing applications requires precision tools to match shock pairs exactly for IFMAR racing standards. Small differences in oil level or air content create noticeable handling imbalances during competition.

Suspension Tuning Through Oil Selection

Shock oil selection directly affects your car's handling and performance characteristics. Thicker oils provide more damping for high-traction surfaces, and thinner oils allow faster suspension action for mechanical grip on loose surfaces.

Basic Oil Recommendations by Car Type:

Key Rules for RC Maintenance: Always use the same oil brand throughout your shock rebuild process. Different manufacturers use varying formulations that don't mix consistently. Quality shock oils maintain consistent thickness across temperature ranges.

Damper tuning requires understanding temperature effects on viscosity. Hot weather makes oil thinner and reduces damping; cold weather thickens oil and increases damping. Adjust oil weights by 50-100 cSt for major temperature changes during your maintenance schedule.

Common Problems and Quick Fixes

Persistent Air Bubbles: Stubborn bubbles require an extended bleeding time. Let the shock sit vertically for 10-15 minutes before final assembly. Vacuum pumps speed this process for complete bubble removal.

Inconsistent Rebound Between Shocks: Oil level differences as small as 1-2mm cause different behavior between shocks. Add or remove small amounts until both shocks at the same axle match perfectly.

Oil Leaking During Assembly: Check cap tightness first. Under-tightened caps leak immediately, over-tightened caps can crack plastic threads. Damaged seals require complete shock rebuilds with new parts.

Wrong Oil Weight Selection: Test drive before changing oil during your maintenance routine. Small viscosity changes (50-100 cSt) make noticeable handling differences in suspension tuning.

RC Shock Oil FAQs

How do I stop my RC shocks from leaking?

Stop RC shocks from leaking by replacing worn O-rings, using high-quality shock oil, and tightening shock caps properly. Check for cracks in the shock body and ensure the shafts are clean and smooth. Regular maintenance prevents pressure buildup and oil seepage.

Why do shocks need oil?

Shocks need oil to absorb impact, control suspension movement, and dissipate heat. The oil flows through internal valves, creating resistance that slows down the shock’s compression and rebound, providing a smoother and more stable ride.

How do oil filled shocks work?

Oil-filled shocks work by forcing oil through internal valves as the piston moves. This creates hydraulic resistance that slows motion, absorbing bumps and stabilizing the suspension. The viscosity of the oil and valve design determine the damping performance.

Can you refill oil in shock absorber?

You can refill oil in a shock absorber by disassembling it, draining old oil, and refilling with the correct type. Ensure all seals are intact and bleed air bubbles before reassembly to maintain consistent damping and performance.

Start Your Next Shock Service Today

You now have everything needed to fill your RC shocks properly, maintain peak performance, and deliver consistent results that improve handling and extend shock life.

The key to success is taking your time during the filling process and matching oil weights to your driving conditions. Start with the basic recommendations in our guide, then fine-tune based on how your car responds on your favorite tracks.

Grab your shock oils and maintenance tools from EuroRC's tested selection and get started. Our racing team uses these same products and techniques to keep their shocks performing at a championship level.

Questions about your specific setup? Our technical support team understands RC shock systems inside and out. We're here to help you get the most from your investment.