DIY Workshop: Cooling System Flush

2 weeks ago | Words: Ian Hancock | Photos: Ian Hancock

Riding a dirt bike during the hot summer months means more fatigue for both rider and machine. And while there’s not a lot you can do to keep your body temperature down when the mercury soars, there are precautionary measures you ought to be taking to avoid the hefty mechanical bills often associated with an overheating engine. So let’s take a look, step by step, at how to properly prepare your machine with a cooling system flush.
This content was first published in the May 2013 (Issue #31) of Transmoto Magazine.

How to disassemble, flush, inspect and rebuild your bike’s cooling system in less than an hour.

Your cooling system has the gruelling job of dispensing with the huge amount of heat put out by your bike’s high-performance engine. Modern coolant has a higher boiling point (and lasts substantially longer) than the coolant of old, but grit and grime can still build up inside your engine and radiator over time, greatly reducing its efficiency. Losing a race or getting stuck or held up on the trail because of an under-performing cooling system is easily avoidable. And blowing-up an engine from overheating is, frankly, embarrassing. So it’s well worth spending an hour in the shed to refresh things.
Here, Chris Hope shows us how it’s done.


Accessing your radiator hoses with the tank on is more difficult than checking that pimple on your arse without a mirror, so whip the seat and tank off. Also, remove the bashplate if it hinders access to the water-pump cover (where the lower radiator hose connects into the engine). With a clear view of the engine and radiator, give your bike a good wash to remove any dirt. By doing this now, you’ll save any loose dirt getting into the engine.

2. Take a Leak

Drain the coolant. Most bikes have a coolant drain bolt on the water-pump cover. Identify it by finding the bolt on the cover with a brass washer behind it. Slowly remove the bolt and, with a large bucket at hand, moderate the flow of coolant draining from the bike (do this by turning the radiator cap back and forward to breaks and form a vacuum). Ensure all coolant is drained by holding the bike on an angle with the drain at the lowest point.

3. Give it a Tug, Love

Remove the hoses. Undo the hose clamp using a socket (save rounding those Phillips heads for the bush) and remove the ends of the rubber hoses that fit onto the radiator. Before pulling hard on the hoses, give them a twist to gently crack their compressioned seal, then pull them off. Gently use vice grips if the hoses are in hard-to-reach spots.

4. Flush with Success

Flush the engine and radiator. Flush them separately (to avoid washing grit from the engine into the radiator) and in the opposite direction to normal flow of the coolant, catching the waste water in a bucket.
Do this by inserting a hose into the radiator hose and form a seal with your hand. If your bike has two lower hoses, you’ll need to block the other one with your thumb. Flush till the water runs out constantly clear.

5. Getting Dressed Again

Reassemble. Check that the hoses are free from grit and reassemble them, then put the tank and seat back on. Before putting the drain bolt back in the bike, closely inspect the copper washer for damage, as this can cause a coolant leak. If there are dents or scratches on the mating surfaces of the washer, replace it. Don’t forget to reinstall the bashplate.

6. Cool Down & Carry On

Fill, ride, re-fill. Check that all old water is removed from the system by pouring just enough fresh coolant into the radiator and checking that it comes out the drain bolt hole. Now fill the radiator and overflow bottle (if you have one, to the maximum line) with coolant. Start the engine and run till hot, then wait for the bike to cool down. The system should now have bleed itself, so top the coolant up, make sure the radiator cap is tight. And, voila, the job is done.

Coolant Flow

Your cooling system works on the principle that cold water sinks. So the hoses at the bottom of the radiator feed cooler water from the radiator, through the water-pump and into the engine. Hot water exits the engine’s head and is fed to the top of the radiator. When flushing the engine and radiator, ensure the water flows in the reverse direction to better dislodge any bits of grit, and push them out of the system.

Expert Tips – With Chris Hope

  • Use Vaseline to get the hoses back on if they’re stiff and difficult.
  • After loosening the hose clamps, don’t take them off. Instead, push them down the radiator hose to make life easier when re-assembling.
  • Most dirt bikes don’t have thermostats. If your bike does, you’ll need to remove it to flush the engine, so get a spare water-pump cover gasket.
  • If your bike doesn’t have a thermostat, you’ll need to fill the system with coolant while the engine is running.

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