The two main fixes to repair leaking radiators are to either weld the offending leak or use an epoxy, like JB Weld, to fill the crack or hole. The best solution will be entirely dependent on where the leak is.
The easiest way I’ve found to locate a leak is to submerge the radiator underwater and pressurize it. To do this, all but one of the inlet and outlet holes will need to be sealed off. This can be easily done using tubing and hose clamps to connect the outlets together. Here I’ve used hose from the cooling system and plugged the remaining end with a piece of aluminum.
Once the leak has been pinpointed, carefully clean the area that must be epoxied. After cleaning rough up the surface so that fresh aluminum is exposed. This can be done by using small diamond files or other suitable tools and will help the epoxy stick to the aluminum.
Mix up a small amount of JB Weld or your favorite epoxy and apply a small amount to the cracked area. Once you’ve applied the epoxy use a torch or heat gun to carefully heat the affected area. This will make the epoxy runny and help it fill in the hole. Next, create a vacuum by sucking on the end of the long hose. A MityVac or similar tool is best for doing the sucking. Your lungs can work in a pinch, but the concern here is that any chemical vapors from the epoxy you breathe in won’t be real good for your respiratory system. If necessary, you can omit creating a vacuum and simply rely on gravity to draw the epoxy into the hole.
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