RC... here's 25 year old'ish OEM fan housing. Magnesium. Same sort of cracking at blade as yours.
More views of same housing.
How long will a cracking housing last? Could be indefinitely... Could come totally unglued tomorrow. Fact that housing is wrapped by retainer strap probably has a great deal to do with ability of housing to crack and remain operable for any length of time.
Sure time to change housing is when housing warps out of round enough so fan scraps housing. When this contact happens... makes distinct bell ringing sound when engine is at idle.
Is good idea to immediately change out housing when bell ringing starts. Doing so saves fan. (Fan will burn up from friction.) OEM fan is $315. (2012 price.)
Inside fan hub... Supposed gas pocket blow out from frictional heat.
Am sure there are different opinions on why fan housing's crack. I'm inclined to believe the press/pull fit of alternators into these housings has something to do with it. Rather than pull alternator into this new housing and stress it, housing was put on lathe and seat for alt was ground out so alt slips in. Cost to do so is a few dollars--well worth it I suggest. Not only is it good for housing, makes it much easier to change alt in future. Touch of anti-seize inside seat keeps parts from "welding over time."
