Radius arm mount & weak bodypanel
 

I've got an issue thats come up since switching to all poly racing spec bushings last year - the peg that the radius arm large bushing sits on has begun to work itself loose from the body floor panel, I can't believe how thin the metal is so not suprised but has anyone else run into issues here? I feel like its more metal fatigue from the rock solid polybush having zero give, though now there is rust around where the circular plate is welded (poorly) to the underbody.

How much downtime am I looking at to remedy these weak floor panels? Ive never done or paid for bodywork before.

These are well known corrosion areas on the XJS, not really related to the bushings.
It is fairly straightforward but make sure that the rust hasn't spread in to the sills.
Bits are still available.

https://www.britishparts.co.uk/class...-mounting-p928

There is a guy on UK ebay that sells repair panels for the part of the car that Steve's part "the cup" (linked above) is welded to. This is likely to be the real source of the trouble. Essentially, have a GOOD poke at the metal around the edges of the "cup" and see if it is weakened or perforated. If it is, grind out the weak parts first and see what is left. If the corrosion is localised to the area around the cup, it might well be sufficient to weld a repair plate onto the cleaned up panel, and then weld a new cup to it.

FWIW, I feel a poly at the small end only of the radius arm is probably the best plan. I think this is the guy who makes the repair panels:
https://www.ebay.co.uk/itm/Jaguar-XJ...QAAOSw6n5XrPRv

Thanks these are super helpful. The metal around the cup is weak as in you can push it slightly, when applying alot of force, no visible nasty bubbling rust though the whole car underbody was sprayed in that Waxoyl stuff. I just find it ridiculous how thin the metal appears to be at a point where youd think you need more strength.

The cups are riveted on and 3 spot welds applied. I strengthened the area under the cup by welding 2mm plates under the cups.

Poly in the small radius arm bush and rubber in the large one was Norman Lutz's advice to me.

https://cimg9.ibsrv.net/gimg/www.jag...53625c8802.jpg

https://cimg0.ibsrv.net/gimg/www.jag...bc214ff52c.jpg

https://cimg2.ibsrv.net/gimg/www.jag...761d633715.jpg

i did what warrjon did ,, welded a thick plate over that whole area , then rewelded the cup to that plate, did both sides,(helps to have a rotessorie roll jig so you can do better work).
https://cimg5.ibsrv.net/gimg/www.jag...9ef182f3f6.png
https://cimg0.ibsrv.net/gimg/www.jag...fc274fd854.png
https://cimg4.ibsrv.net/gimg/www.jag...56140da6cc.png

Very nice, after feeling how thin it is I will be suggesting my mechanic find a 1/2" plate of top tier steel and warm up the weldinh rig. Now I'm not super familiar with the structural layout of the car but is there a way to 'see' the other side of this thin floor panel and asses if rust is the factor?

I blasted the area with a solvent to remove my underbody coating I see two of 3 welds failed, some very light rust not more then the size of a quarter, and two rivets third hole is empty, also a small drill hole in the box beam segmemt, but this was behind a 1/4" thick layer of hard wax-oil spray and no rust at that side.

The other side of that plate is under the rear seat metal former and it no accessible.
1/4 inch steel will be plenty! I would use 3mm!

1/2" would be insanely overkill. the 5th wheel on a semi is held on by 3/8, 1/8th would still be overkill but much more reasonable. plating it after removing any rust and then re installing the cup would be the proper repair though.

I used 2mm (0.080") plate is more than enough to stabilise the area as long as all the other steel is in good condition.