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The nuts of the rs crossmember are not good anymore. It's impossible to tighten the long bolts. Has anybody ever replaced these nuts?

Not sure which part you referring to ..have you got pics of it ?

I take it that you are referring to the lower control arm bolts and nuts.
If that is the case, and there is any doubt about the condition of the threads, you would be advised to obtain a complete new set of high tensile bolts and nuts. My preference is to buy the bolts a bit longer to enable "double nutting" in preference to nyloc nuts because the locking function is compromised if you re-use them say after maintenance on the lower control arm bearing or ball joints.

sorry, it's the rear suspension. There are 6 bolts. The nuts for the shorts bolts are easily accessible. But the nuts for the 4 long bolts are in between the two metal floor sheets.
The garage put in helicoils. But then the top part (without thread) of the bolt is too thick. So the garage cut the top part off. This was not a good idea,as the top part is used to guide the bolt into the bolt.

Their have been loads of cars I have worked on when they nuts in the panel are stripped . What I have been forced to do is drill throught the nut to make it bigger then when you want to tighten up crossmember you need two people as now you will put a loose nut with your finger tips into the recess pannel and you can still put a spanner on it when some one tightens from underneath . It's pain . If you feel like making a hole in pannel get a hole saw . It will make life easier . Forget helicoil those ..waste of time
Last alternative see this pic

Hi KevinR, that seems a better solution then helicoils.