Chemists and backyard mechanics have probably been coming up with ways to plug radiator leaks for as long as water-cooled engines have existed. These solutions range from the brilliantly complex to the elegantly simple. It's not a question of if they work (which all do to one degree or another), the question primarily comes down to which will hurt the engine, and how long the repair will last.
Pelletized Stop Leak
The most popular type of pelletized stop leak is Bar's, which has been around for decades and comes as stock equipment in most GM cars. Pelletized additives use a number of different compounds (Bar's calls theirs Rhizex), but they all amount to basically the same thing. When the pellets are added to your cooling system, they break down into much smaller pellets, and are forced into the radiator leak by the pressure of the leaking fluid. These are quick and effective, but there are other options.
Copper Stop Leak
Stop leaks containing copper work in much the same way as pelletized additives, but use what amounts to copper dust to fill the leak. These additives are preferred over most other types because the copper material is soft and has excellent metal-on-metal lubricity, which means that · it won't cause undue component wear. However, copper additives are only really suited to sealing hairline cracks, and will flow right through larger holes.
Sodium Silicate
Also known as "liquid glass," sodium silicate has been used for over 100 years to seal radiator leaks. Sodium silicate starts out as a fine powdered glass that readily dissolves into water, which melts at about 200 degrees and sticks to whatever it happens to be touching. Sodium silicate forms what is perhaps the strongest of all bonds, but is better suited to sealing very hot head gasket leaks than those in the relatively cool radiator.
Black Pepper
There is surprisingly sound science behind using pepper to seal leaks. When introduced to hot water, the pepper grains will soften, expand, and get caught in the radiator leak, building up a surprisingly thick patch on the inside. Bear in mind though, that the pepper grains will eventually start to fall apart, so this is a temporary fix. Two to three tablespoons of black pepper will fill a surprisingly large radiator leak, but you'll need to flush your coolant system thoroughly when you get the radiator fixed.
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