Had one of my Exige diffuser rivnuts let go this past weekend. Ever since I started working on this car I noticed this one bolt was always trouble. Rivnut finally cross threaded, locked up, and came loose under torque.
Having just added a bunch of rivnuts during the install of my GRP side skirts, I felt confident I could tackle this job.
What you will need:
Rivnut tool kit, preferably one that includes a variety of adapters and rivnuts. You will need to set it up to install an 8 mm rivnut.
8 mm rivnuts
Dremel + grinding stone 8193
Small standard (flat) screwdriver
It's worth it to have a Rivnut tool kit. Buy the best one you can. Rivnuts come in very handy. Need a threaded hole where you can't tap? Rivnut...done.
Make sure your rivnuts are coated and not exposed stainless steel or they could interact with and corrode the galvanized frame when moisture gets to them. If you don't want to buy a kit then youtube search "diy rivnut tool" and find some examples of how to do it that way. You can start this whole thing by threading the rivnut tool into the old oem rivnut and see if you can re-crimp it. But once they come loose they tend to be done so that may not work. Worth a shot though.
Remove the diffuser.
Cover the frame area around the problem rivnut with a couple layers of masking tape to protect the frame. I was glad I did, as you will see in the pics. My hands are steady but my arms do get tired in the odd positions we must work in under the Lotus. You don't want to graze the frame without any protection over it.
If you are in the situation where the bolt is stuck in the loose rivnut then search this forum or google for "removing a stuck bolt from a rivnut". There are tons of how-to's out there. One way is by using force to pull on the bolt while unthreading and hope that the downward motion causes the old rivnut to seize and stop spinning long enough to remove the bolt. Another method is to get the rivnut to seize in place using a tool so you can remove the bolt. On our cars you can access the backside of the rivnut through the open side of the frame rail but you will need long needle nose pliers or a long serrated blade as one LT user did
. OR you shave off the head of the bolt as close as you can to the rivnut surface without risking the frame, then just include the remaining bolt in the work below.
Once everything is protected by masking tape, get your Dremel to work and carefully, slowly, start shaving down the rivnut.
If the bolt is in there, get that flat too and just work it all as one surface. Avoid the frame. What you are trying to do is shave down the exposed lip of the rivnut enough to make it easy to work with. You can take it all the way down if you are brave, to where there is almost nothing left but I wasn't that brave. Take your time. Go slow. Use a medium-high speed.
Once there is nothing but a thin layer of rivnut remaining, take a small flat screwdriver and gently pry up the thin remaining bits of wall. Try not to pry against the frame and more just lift the remaining bits of lip of the rivnut.
Then take your needle-nose pliers and grab one of the little bits of rivnut and pull it towards the center of the hole. Repeat everywhere you can, basically just crushing the rest of the lip inwards. If you can, grab two sides of the rivnut with the pliers and gently crush it.
Again, you could have just taken the sanding all the way down but I found it was only an extra minute or two to take the time to do the last part by hand rather than risk grinding on the frame accidentally.
Now you can just use your needle-nose pliers and wiggle/pop the rivnut loose, up into the frame rail. Be gentle! You want it to come loose but also to stay put to make the next step easier.
Now slide a magnet into the frame rail access and you've recovered your old rivnut!
Use a new 8mm rivnut, test the rivnut by screwing in the diffuser bolt. Check for snags, damaged threads, and debris and adjust or repair accordingly. Once satisfied with the condition of the rivnut, thread it onto the 8mm rivnut tool adapter, and give the rivnut a good shove to fit snugly into the frame rail. Notice the angle of the frame rail. You need to get the rivnut tool at this same angle for a flush, straight attachment.
It can help to use some kind of leverage extension on your rivnut tool to get a better crimp. I like to separate the handle of my floor jack into two pieces and slide that onto the rivnut tool handle.
Crimp the rivnut, remove the tool, check the new rivnut for a firm fit then try threading just the diffuser bolt once more, and go slow feeling for a loose rivnut or cross-thread issues.
And you're done! Reassemble the car and go for a drive. Check for vibrations and rattles. Be careful when torquing a rivnut in the future! Most rivnut applications are body panels, covers, etc that only need 10-12 ft lbs which is pretty much "hand tight then another half turn" and you should be good.
If the rivnut does come loose again, first try threading the rivnut tool into the rivnut and get it a good crimp again and it may sort itself out and lock down again. But really, once crimped then dislodged a rivnut is never the same again and should be replaced.
Sorry almost all of the pics are upside down! They are correct locally. So I can only blame the forum software. But you get the idea.