what water ways would be narrow enough to block with k-seal? none that i can think of. its a good product imo, ive used it to 'fix' a heater matrix on a frontera, it worked pretty well to be fair. ive used it in a ****ty mgzr with a head gasket leak, ran fine for several thousand miles, used it in my subaru forester to seal a pin sized hole in the radiator, its been fine for the last 6 months. a lot of people put this stuff in their coolant as a preventative measure, to seal cracks as they happen, and ive never heard of it doing any harm to any engine. obviously its better to fix the problem properly, but these are old cars, and not worth much, so why bother doing a heater matrix when u can use a £10 bottle of k-seal. just my opinion