The best guide I would suggest is the MSA 'Blue Book' regulation, requiring a grip length on the stud or bolt, of 1.5 times the thread diameter MINIMUM. I've not bothered checking what the Clio bolt dimensions are TBH but let's use a typical fitment of 10mm dia and 1.5mm pitch in the thread, as an example. 10 x 1.5 - 15mm of thread required. Conveniently 10 turns of the bolt @ 1.5mm pitch = that 15mm depth. Check this out on the actual bolt dimensions, or more simply, make sure that the bolts screw in by a depth at least 1.5 times the bolt thread diameter. Hope this helps