I have had good luck with Marine-Tex on external cracks if I prepared the cracked area properly. First sand or media blast to grey metal, warm along crack to extract any moisture, make a shallow "V" in crack with a Dremel tool, rough up the surface with a 24 or 36 disc on an angle grinder to an inch or so on each side of the crack, apply the Marine-Tex no more than 1/8" thick over entire prepared area. Picture is a 40 Pontiac block done in 2010 that is still holding fine. The crack has been warmed to drive off moisture.

[img]https://s1.postimg.org/1k8ao9btln/1940_Pontiac_Cpe_004.jpg[/img]


Evan