How to Change the Coolant in a 2025 KTM 125SX: Complete MotoSport Guide

Shop parts for a 2025 KTM 125SX Dirt Bike.

Changing the coolant in your 2025 KTM 125SX is a straightforward but critical maintenance task, especially for a performance-focused dirt bike like this one. The new 2025 engine platform, featuring KTM’s advanced fuel-injected 2-stroke system, demands regular cooling system maintenance to ensure optimum performance and engine longevity. Here’s a step-by-step process tailored specifically for your bike, along with special considerations unique to this model.

Preparation – What You Need

  • Premium motorcycle coolant (check your owner’s manual for KTM-recommended formulas)
  • 10mm wrench or socket
  • Collection pan
  • Clean rags/towels
  • Protective gloves and eyewear

Step 1: Cool Down & Access

  • Ensure the motorcycle is completely cool before starting. Never open the radiator cap when the engine is hot.
  • Stand your bike upright on a level surface for best access to the radiator and drain bolt.

Step 2: Remove the Radiator Cap

  • Locate the radiator cap on the left side of the tank. Press down and twist counterclockwise carefully to remove it.
  • This relieves internal pressure and allows the coolant to drain freely.

Step 3: Locate & Remove the Coolant Drain Bolt

  • The drain bolt is found on the lower part of the water pump cover (right side of the engine).
  • Position your collection pan under the drain area. Use a 10mm wrench or socket to carefully loosen and remove the bolt, letting coolant flow out.
  • Allow the system to drain completely for several minutes.

Step 4: Inspect & Refit the Drain Bolt

  • Inspect the sealing washer (replace if damaged or crushed) before reinstalling the drain bolt.
  • Tighten to factory spec: 15 Nm (11.1 lbf–ft).

Step 5: Fill the Radiator with Fresh Coolant

  • With the bike upright, slowly pour new coolant into the radiator until fluid is visible at the filler neck. The 2025 KTM 125SX holds approximately 1.2 L (1.3 qt) of coolant.
  • Tap the radiator hoses gently or rock the bike side-to-side to dislodge air bubbles.

Step 6: Bleed the Cooling System

  • Start the engine and let it idle for several minutes with the radiator cap off. Watch for air bubbles to escape at the filler neck.
  • Top off as needed, then fit the radiator cap securely.

Step 7: Final Checks

  • Inspect for leaks around the drain bolt, hoses, and the radiator cap area.
  • Test ride your KTM 125SX lightly, then re-check coolant level once the engine has cooled.

Key Notes Specific to the 2025 KTM 125SX

  • KTM's 2025 cooling system integrates a highly efficient radiator layout, ensuring even temperature control for fuel-injected engines. Always use a high-quality, ready-to-use coolant compatible with aluminum and magnesium components to prevent corrosion and maximize heat transfer.
  • Be mindful when working around the radiator: The thin-core design optimizes air flow but is more susceptible to accidental dings or bent fins. Use extreme care not to damage fins when accessing or filling.
  • Coolant changes are crucial if you frequently ride in muddy or dusty conditions. Foreign debris can compromise radiator efficiency and promote overheating—stay on schedule with coolant flushes.

Maintaining your KTM 125SX's cooling system is one of the simplest ways to protect its advanced engine, especially under competitive riding conditions. If you need quality coolants, replacement hoses, or OEM gaskets, shop parts for a 2025 KTM 125SX Dirt Bike.

Related Shopping Categories

Shop Radiators for a 2025 KTM 125SX Dirt Bike.

Shop Coolant for a 2025 KTM 125SX Dirt Bike.

Shop Temperature Sensors for a 2025 KTM 125SX Dirt Bike.

Shop Tools for a 2025 KTM 125SX Dirt Bike.

Shop Radiator Hoses for a 2025 KTM 125SX Dirt Bike.

Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.