No set mileage, just depends on how worn they are. Not sure how to check them, but I'll look around and see. Hmmm, that clunk could be a bad rack, could also be a bad tire rod end, which, done by a pro, should be about $120-130. $800 is a reasonable price for a rack, but I would want more assurance that it is the rack. I would see about getting it checked somewhere else besides that place.