How to Change the Coolant in a 1991 Suzuki RM125
Shop parts for a 1991 Suzuki RM125 Dirt Bike.
Keeping your 1991 Suzuki RM125 running strong means regular maintenance, and refreshing the coolant is one job that should not be overlooked. Old or contaminated coolant can cause overheating, corrosion, and internal engine damage. The 1991 RM125, with its lightweight frame and powerful two-stroke engine, depends on optimal cooling–especially if you put it through its paces at the track or on the trail. Here is a step-by-step proprietary MotoSport guide to changing the coolant on your RM125.
Tools & Supplies You Will Need
- Fresh coolant recommended for aluminum engines (check your owner's manual for specifications)
- Distilled water (never use tap water due to minerals that can harm the radiator and water pump)
- Clean catch pan
- 8mm socket or wrench (for the water pump bolt)
- Flathead screwdriver (for hose clamps if needed)
- Compressed air tool (optional but helpful for flushing)
- Protective gloves & rags
Step-by-Step Coolant Change Guide
- Let the Bike Cool Completely
Cooling systems are under pressure when hot. Make sure the engine and radiator are cool to the touch before starting. - Remove Skid Plate (If Equipped)
If your RM125 has a skid plate, remove it to access the water pump drain bolt. - Open the Radiator Cap
Twist the radiator cap off slowly to relieve any residual pressure. - Drain the Old Coolant
Place your catch pan under the water pump (right side of the engine case). Remove the 8mm drain bolt at the bottom of the water pump. Allow the coolant to drain entirely. Tilt the bike slightly if needed to get every last drop. - Flush the Cooling System
For a thorough job, replace the bolt temporarily and pour distilled water into the radiator. Remove the drain bolt once more and let the water flow out, carrying away any residual coolant or contaminants. You can use gentle bursts of compressed air at the radiator neck to clear stubborn pockets of old coolant, but do not use high pressure–delicate fins and seals can be damaged. - Inspect Hoses & Clamps
While the system is empty, inspect radiator hoses and clamps for cracks or wear. The 1991 RM125, being a legacy model, can develop leaks from aging rubber and corroded factory hose clamps. Replace any worn parts for reliable operation. - Reinstall the Drain Bolt
Tighten the drain bolt snugly by hand, being careful not to overtighten–stripping the case is costly. - Add Fresh Coolant
Pour new coolant into the radiator up to the filler neck. Use a coolant suitable for high-revving two-strokes and compatible with aluminum radiators. Avoid automotive coolants with silicates–they can eat away at motorcycle cooling systems over time. - Bleed Air from the System
Gently squeeze the coolant hoses to release trapped air. Some RM125s develop air pockets in the system, leading to overheating under load. If necessary, "burp" the system: fill, run the engine briefly with the cap off, top off, and reinstall the cap. - Check for Leaks & Test
Wipe down and inspect for leaks around the drain bolt and hoses. Start the engine and let it idle. Check for proper circulation–the radiator should feel warm (but not boiling hot) after a few minutes.
1991 RM125 Model-Specific Notes
- The 1991 RM125 radiator and hose connections can become brittle over time, especially if the bike has seen extensive off-road use. Inspect plastic and metal joints carefully for signs of fatigue.
- Look out for powdery white residue around the radiator cap or tank–it could indicate a failing seal or past use of the wrong type of coolant.
- The factory water pump seal is known to wear by this age. If you spot coolant leakage at the pump housing, consider a water pump rebuild kit.
- This model's narrow frame makes access straightforward, but always double-check that tools and hardware don't drop into tight spaces.
Coolant Change Interval
For best results and to protect your vintage RM125 engine, change the coolant every 12 months or 30 hours of riding, whichever comes first–especially if you ride in mud or in hot climates.
Proper Coolant Disposal
Never pour used coolant down the drain or on the ground, as it is toxic to people and animals. Take it to an approved recycling center or hazardous waste facility in your area.
Final Check
Give your RM125 a gentle test ride and recheck coolant level after it's fully cooled. Top off as necessary. Perform a final look for drips or leaks and you're ready for another season of riding.
Related Shopping Categories
Shop Radiators for a 1991 Suzuki RM125 Dirt Bike.
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Shop Temperature Sensors for a 1991 Suzuki RM125 Dirt Bike.
Shop Tools for a 1991 Suzuki RM125 Dirt Bike.
Shop Radiator Hoses for a 1991 Suzuki RM125 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.