Broken Captive Nut / Fender Stay Repair

Today a minor miracle happened, and I managed to get enough time in the shop to complete a project from start to finish. It wasn’t a large project, but it did resolve an annoyance that the car has had since the delrin motor mounts went in: a rattle from the driver’s side fender that made it sound like the car was falling apart at idle.

There are 2 bolts that hold the fender to the bottom of the chassis, and many moons ago, 1 of them sheared in the hole thanks to rust. All methods of extrication failed, and in trying to get the bolt out I managed to unseat the captive nut inside the tub. That was, as my kids would say, a Thumbs Down Day. There were a few options to fix it, but it being a blind hole made most of them pretty difficult.

What I went with was opening the hole with a step-bit enough that a 6mm weld-nut would fit through the hole, make a patch panel, and weld that into place.

Here’s the hole, hogged out to fit the new nut:


The patch, made from a piece of a mostly-thrashed Honda Civic fender my neighbor left at the curb on trash day (Honda steel is compatible with Mazda bodies, right? :D)


Tacking it into place:


A little bit of hammering, welding, and then a couple coats of rust inhibitor, and the fender fits on good as new:



Pro-Tip: welding upside down sucks. A lot.