I work in a heavy truck shop for a utility. When small cracks appear in cast iron blocks, or thermo housings on our trucks or equipment, or whatever, we use a product called JB Weld. It's a two part epoxy that you mix together and apply to the crack. Cures in 24 hours and it works great! Seals that bad boy right up. You can even file of grind it afterwards. I get it at a local Napa store here in Jersey. This is a good question for the engine "guru" on this site Dick L. Hey Dick are ya out there?