Removing and installing the front and rear diffusers is more trouble than it should be because the cap screws are secured with Nylock nuts and washers. The front diffuser is especially tricky, even with two people so I installed floating nut plates. M5 for the rear diffuser and M6 for the front:
RivNuts (aka Nutserts) would have been simpler to install, but these are floating so they give a little wiggle room and they use Jet nuts which have a locking property. I got these from Pegasus (not cheap, but good). It took a little while to rivet these in place, but now such a time saver every time I need to remove and re-install a diffuser.
I have not seen an RS, but yes I have heard that it is different than the RSX an XX models. There are two M6 bolts for each side of the rear diffuser, and four M5 in the rear that mount onto a bar.
When I take my diffuser off, that bar remains fastened to the chassis, not the diffuser. It looks the bar (on an RS) stays attached the diffuser?
Those bits are the same - if you were to remove the long bolts Charley references that bar would come off and stay attached the diffuser. Leaving the bar on the chassis and dropping it with nutserts is much easier having done it both ways.
