Waterglass isn't up to the job of sealing a block- - - - -possibly a seep leak in a radiator core, or something like that. If you clean the crack thoroughly and maybe grind it smooth for about 1/2" on each side, a product called "Marine-Tex" is about the best product I've found for crack repair, better than J-B Weld by far. If there's much pressure in the cooling system, even Marine Tex will seep after a short time. Welding or brazing is the only effective crack repair, or a process called "sewing" where overlapping threaded plugs are installed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!