My car popped a head gasket water leak last thursday night, I also have an o-ringed block with aluminum heads and copper head gaskets. The Yamabond4 sealant has held up great all summer til last thursday. I ran around the corner to Summit Racing and bought a bottle of Moroso ceramic sealer, shook it up good(this is critical), took some water out of the radiator to make room for the ceramic stuff, ran the engine up to 150 degrees, poured the ceramic sealer in and let the motor run for at least 15 minutes. The leak stopped after only 5 minutes.

I shut it off and let it cool, pulled the lower radiator hose off and let out as much as possible, put hose back on and filled the radiator with fresh water. I loaded it on the trailer and it was good to go.