Use the Bars Leak Gold which is a small tube of ground ginger root powder and other fine stuff. Only use one tube. (Less than $2.00 at WalMart). There's no metal in it. It will not clog heater cores and was OEM specified on many early GM Northstars due to block porosity. I've used it as a last resort when I absolutely couldn't find very small leaks on several different motors, and it always worked and never clogged anything. Also, it will flush right out during coolant changes.