"Change one...change them all... "
If you aren't sure what counts as a crack, go to any auto parts supply and buy some dye check, used for inspecting metal parts. Clean the part, apply the dye, wipe off, apply the "developer" and you should be able to see dye in any cracks. Hardest part of it is finding the dye--but any real machine shop or auto repair supply will have it.
All one really needs is a piece of Scotch-Brite and a magnifying glass. The corrosion/rust actually acts like the dye in that it's not removed by the scrubbing and the cracks show up clearly upon close inspection.