An external crack in the water jacket can be brazed. I've done a bunch of them. Silver solder? I've never tried that. Be sure to find the ends of the crack and drill a hole to prevent it from extending itself as the heat is applied to fuse the brass to the cast iron, and be sure to grind the block smooth for about 1/2" on each side of the crack so the brass can make a good bond. Otherwise, it will leak coolant by following the rough casting surface. Once you've got a good bead of brass built up and fused well, the surface can be dressed flat with a coarse-grit sanding disc and the patch can be painted over. If you want to hide the repair completely, spread a thin layer of Bondo over it and press a piece of 40 grit sandpaper to the patch after it's about halfway cured. The texture of the sandpaper will transfer to the Bondo and hide the repair completely once the block is painted.
Jerry
