which threads in an engine block, if damaged, make it "toast"?

[jtoomasson at yahoo.com (Jeff Toomasson)]

I'm pretty much afraid to admit why I'm asking, partly because I don't actually know what the problem is yet, and partly I don't want to make myself look like any more of a moron than I already have.  :-/

Could anyone tell which threads on the outside of the engine block virtually ruin the block when damaged?

thanks, Jeff