How to Change the Coolant in a 2018 KTM 125SX

Shop parts for a 2018 KTM 125SX Dirt Bike.

Changing the coolant in your 2018 KTM 125SX is a straightforward maintenance task that helps protect your two-stroke engine from overheating and corrosion. KTM's 125SX shares its cooling system design with other 125/150 SX models, making this process nearly identical across recent years. Below, you'll find a step-by-step guide, as well as important model-specific notes every owner should know.

What You'll Need

  • Coolant compatible with aluminum radiators and ready-mixed for dirt bikes
  • New radiator cap seal (recommended but optional)
  • 8mm socket or wrench
  • Drain pan
  • Funnel
  • Shop towels

Preparation

  • Always begin with the engine and cooling system completely cool to avoid the risk of burns or scalds from hot coolant.
  • Keep the bike upright and ensure the area is well-ventilated.
  • Wear gloves and eye protection, as coolant can irritate skin and eyes. If coolant contacts skin or eyes, promptly rinse with water and seek medical help if needed.

Step-by-Step Coolant Change

  1. Remove the Radiator Cap: With the engine cool and bike upright, remove the radiator cap. This allows air in, helping coolant drain faster.
  2. Locate the Coolant Drain Bolt: On your 2018 KTM 125SX, the drain bolt for the cooling system is positioned at the water pump. It's typically an 8mm bolt on the lower right side of the engine case. Place a drain pan underneath to catch all old coolant.
    Pro tip: Loosening the radiator cap prior to removing the drain bolt speeds up draining–for all recent 125SX models.
  3. Drain the Old Coolant: Use your socket or wrench to carefully remove the drain bolt. Allow all coolant to fully drain from the system. Inspect the drained coolant for dirt or oil–any contamination may indicate a more serious engine problem.
  4. Replace and Tighten the Drain Bolt: Once drained, install the drain bolt (use a new seal ring if needed) and tighten to a torque of 15 Nm (11.1 lbf ft) if you have a torque wrench. Wipe away any spilled coolant.
  5. Fill with New Coolant: Using a funnel, slowly fill the radiator until coolant reaches the bottom of the filler neck. The KTM 125SX requires approximately 1.2 liters of coolant.
  6. Remove Air Pockets: With the radiator cap still off, lean the bike slightly side to side and gently tap radiator hoses to help trapped air escape. Add more coolant if necessary.
  7. Reinstall the Radiator Cap: Secure the cap. For extra reliability, use a new cap seal if you have one.
  8. Run and Recheck: Start the engine and let it idle until warm (do not exceed operating temperature); this circulates the new coolant. Shut off the engine, allow to cool, and recheck coolant level. Top off as needed.

Key Notes for 2018 KTM 125SX Owners

  • Use only aluminum-safe coolant (avoid those with silicates, which may damage your cooling system).
  • Inspect radiator fins during any cooling system service; bent fins can be carefully straightened and should be kept clean for proper airflow.
  • If riding in sand or dusty environments, consider an air filter dust cover and periodic radiator cleaning for best performance.

Troubleshooting and Cautions

  • Never open a hot radiator cap. Serious burns can result from pressurized, hot coolant spraying out if the system is not fully cool.
  • If the old coolant appears contaminated (milky, rusty, or oily), seek professional diagnosis before running the engine again.
  • Dispose of old coolant responsibly at an approved recycling center–don't pour it down drains or onto the ground.

Following this guide will keep your 2018 KTM 125SX running cool and reliable, whether you're hitting the track or trail. For routine maintenance or rebuilds, Shop parts for a 2018 KTM 125SX Dirt Bike.

Related Shopping Categories

Shop Radiators for a 2018 KTM 125SX Dirt Bike.

Shop Coolant for a 2018 KTM 125SX Dirt Bike.

Shop Temperature Sensors for a 2018 KTM 125SX Dirt Bike.

Shop Tools for a 2018 KTM 125SX Dirt Bike.

Shop Radiator Hoses for a 2018 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.