How to Repair a Radiator With J-B WELD

by Kris Gaines
cpu radiator image by saied shahinkiya from

Car radiators leak, and the easiest way to tell that your radiator has a leak is a consistently low coolant level or simply a green radioactive looking puddle under your vehicle. Though it may seem difficult, repairing this problem is quite easy.

Step 1

Drain your radiator. To do this, make sure your engine has cooled for several hours, your battery is disconnected and the engine is covered with an approved blanket or tarp to prevent splashing from reaching it. Unscrew your radiator cap to provide air flow and place your container below the radiator plug, which is usually located at one of the bottom corners of the radiator. Remove the plug and let the fluid drain until it stops dripping.

Step 2

Clean the area where the leak is located thoroughly. If need be, scrape rust and other debris away, and make sure to remove any grease, grime or dirt with a rag and a solvent if necessary.

Step 3

Mix the J-B WELD. To do this, mix equal parts of the black and red tubes on your disposable plastic surface with your wooden tool. Make sure to work quickly, as the J-B WELD sets very fast.

Step 4

Apply your mixed J-B WELD all over the leak area liberally with the wooden tool. The mixture will set in 4 to 6 hours and be completely cured in 15 to 24 hours.

Step 5

Replace the radiator plug on the bottom of the radiator and refill the fluid to the appropriate level. Replace the radiator cap and remove the engine cover that you placed to keep fluids from reaching your engine. Reconnect your battery and start your vehicle.

Check for remaining leaks after the car has been started and repeat as necessary.


  • To create an even better seal, attach the radiator overflow line over to the intake manifold and remove the distributor cap. Start the engine up a few times. The suction will bring some of the J-B WELD inside, creating a better seal. Do this before the J-B WELD has completely set.


  • Radiator fluid is extremely toxic. When working with radiator fluid, always park the vehicle on level ground to avoid run off into gutters and streams.

Items you will need

  • Bucket or other container
  • Blanket or tarp (approved engine cover)
  • Disposable piece of plastic (such as a jar lid)
  • Wooden tongue depressor or match stick
  • Rag or shop towel

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