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I've heard of three C3's breaking anodized lowering bolts, bought from accessory houses, on the rear. I had a stock one break on my old C3 as well, which was kinda understandable as it was 28 yo at the time...
In a recent thread, someone reported a bolt breaking.
In discussion, it appears as though the nut had been turned downward
on OEM bolts, placing it at or below the hole for the cotter key. The
hole removes a large amount out of the cross-section of the bolt and
consequently out of the tensile strength of the bolt at/past this
location.
Bolts for rear spring retention are 14mm or 9/16 dia. I believe the
ones intended for lowering are not drilled for a cotter pin and are
typically supplied with Nylock-style nuts.