Waterglass (oldtimers' name for sodium Silicate) only works if the crack is exposed to combustion heat. The heating cures it to, literally, glass, with a melting point of 900+ degrees. When the 503 cubic inch GMC in Ed Pruss' retired fire truck had a seep leak at the head gasket, I flushed the system several times and added sodium silicate, and drove it from middle Tennessee to Kansas City. Ed picked it up there and drove it on to Denver. After about a year, it started leaking again and he had a terrible time getting the glued-down head off the engine. It's a temporary fix at best, but a pretty good one- - - -as long as the leak is internal. The only way to fix an external crack is to weld or braze it, or possibly to use overlapping cast iron threaded tapered plugs- - - -a process called "sewing" a block. The plugs and the tooling to install them is expensive- - - -let a machine shop that's already set up for the process do it, and be glad you don't have to buy a tap, a taper reamer, and a minimum purchase of a hundred plugs.
Jerry
