I swear by this stuff: "Dike" cooling system sealer.
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Sealed up a John Deere 2520 gas farm tractor, leaking head gasket running down the outside of the block over 20 years ago and is still holding.
Also use it as preventive leak maintenance for used cars, hoses, and gaskets that can seep and leak. Won't plug the heater core.
Want to give it a better chance of working? Flush your cooling system a couple of times, to get the white crap and oily film out.
Then put in a new 50/50 mix of Distilled Water and Anti-Freeze and the Dike cooling system sealer.
Get all the air out of the system and make sure it is full. (sometimes have to top it off the next morning after it cools down from first run up to get warm)
Then take it out for a good long drive and let the healing begin.
Put in a new thermostat too at the same time, as you don't want your system running warm or even hot from a wore out used up thermostat causing problems.
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