Originally Posted by Tom Pedale
Jimv is absolutely right..I remember seeing a number of these cracks on cranks in the 70's..when you use the round file to remove just enough material to eliminate the crack..you've eliminated what is called in engineering terms a "stress riser". Kinda like..give a crack an inch and it will take a mile. If it's no longer there, it won't progress up the spider any further.
The problem with this is, there may be structural damage that can't be seen that will eventually fail too. And if something has already begun to crack, especially if it's old, chances are it past it's useful life.
I'm a cheap bastard, but sometimes you have to know when to retire a part to the junk bin. Dental work is generally more expensive than a new set of cranks.