As related in a different post, I needed to replace the fuel filter on my '05 XJ8L, but a 1 hour job turned into an overnight PIA when two of 4 studs holding the under-body plastic cover were corroded solid with the nuts/fender washers, all up inside conical plastic stand-offs where they were almost unreachable. The studs were in threaded inserts and the inserts were spinning while the nuts stood tight.

I solved this dilemma by selecting a 7/8 hole saw whose I/D was close to the O/D of the fender washer on the studs, took out the pilot bit, and ran it at medium speed until it drilled through the plastic at the top of the stand-off, and the plastic pan dropped away.

That exposed the rusted studs which I burned away with a cutting wheel in my angle grinder, leaving just the insert to deal with. I cautiously cut a cross in the face of the insert, leaving 4 quadrants I could chisel off, leaving the top of the insert and a bit of the original stud to drive upward out of the hole, into the insulation above the sheet metal.

I have a Shurebonder Quick-Thread kit from the home center, whose largest size, 1/4-20, calls for a 25/64 hole, the exact size remaining in the sheet metal of the under-body. A quick thread insert install, a spritz of grey primer, and the plastic tray was ready to go back up. I needed to use an over-size fender washer and a 1/4-20 machine screw to fit the now oversize holes in those 2 stand-offs, and a bit of anti-seize compound on all the bolt threads under the car, and down off the lift.

 

Nice job and good write up detailing it. Refreshing to see one of the PITA situations end up with a positive outcome.