R107 Rear License Plate Riveting Nut Replacement A Step-by-Step Tutorial by GlueckAuf Woo-hoo!I've crossed another of my SL's few remaining annoyances off the to-do list. On my long-departed 1984 W123 as well as on my 1987 560 SL, Mercedes installed four blind aluminum riveting nuts as the mounts for the rear license plate.And on both of these cars, most of these soft, weak rivets either stripped their threads, or broke loose and became free-spinning, a real annoyance when trying to change the license plate. Figure 1.Four M5 aluminum threaded inserts, called riveting nuts, secure the rear license plate to the rear panel of the 560 SL (and presumably its other R107 cousins.)Forcing improper screws into them can damage these nuts, necessitating replacement of one or more.This tutorial will show you how. Part of the problem with these riveting nuts is that they appear to be threaded for an M6 screw, but they're actually M5.What misleads the license-plate installer (whos usually on his knees, in the rain, in the DMV parking lot) is that the threads are recessed considerably from the opening of the insert, but the visible opening itself is plenty big to accommodate an M6 screw.So when some previous owners of both of these cars probably first muscled-in M6 screws or bolts into those holes (or worse, SAE-threaded screws), the damage was done. Installing new riveting nuts requires a means to expand them, and my research led me to believe there must be a special Mercedes-Benz tool designed to expand the nuts into the hexagonal holes in the R107's rear panel.Generic tools exist for installing similar, but round, rivet nuts on aircraft.But at $20 to $80, they are mighty pricey, particularly for a one-trick pony of a tool one might use just once in a lifetime.But I could see no reason why a simple screw and nut could not serve to expand these Mercedes-style hexagonal riveting nuts.So armed with less than a dollar's worth of hardware for the home-brewed tool and four new riveting nuts from the local Mercedes dealer ($2.90 each, part number 201 990 05 58), I put the hypothesis to the test. Figure 2.Four of these $2.90 each riveting nuts will set you back just $11.60 from your local Mercedes dealer.But the highly-specialized tools advertised to install these types of fasteners range from $20 to $80.Fortunately I found that an M5 screw, nut, and a couple of washers perform just as well. Removing the Old Riveting Nuts But first, how to get the old, damaged riveting nuts out?I found that a 1/4" drill bit is just about the perfect size to drill out enough of the collar--without drilling into the rear panel sheet metal's hexagonal holesso that the inside portion of the riveting nut can then be wiggled and broken free from the weakened collar.The only difficulty is accessing the rest of the old fastener that hides inside the rear panel beneath the carpeting. Now I suppose one could simply drill out most of the old riveting nuts collars and, using a slim punch, drive the remainder of the old riveting nuts into the rear panel to stay, and not bother with removing the carpeting at all.But I didnt like the idea of those four old fasteners rattling around inside the rear body panel where they might end up being both noisy and irrecoverableso off came the carpeting. Removing the rear panel carpeting requires removing one or both of the two finishing strips that hold the top of the carpeting to the trunk lid opening.Four small Phillips-head sheet metal screws hold each strip on, and the carpeting itself is loosely glued to the top of the trunk's rear panel.After removing all eight screws and both metal strips, I used a plastic scraper to loosen the carpeting glue as I gently pulled the carpet free. Figure 3.As should be apparent, a 1/4" drill bit is pretty close to the perfectly-sized tool to thin the collar of the old riveting nut enough to remove it without contacting the rear panels painted sheet metal within which it resides.Work slowly and carefully with low pressure and slow speed to drill out the fastener.Once the carpet is free, the right-side (passenger) riveting nuts are easily visible and accessible to be grabbed and held with a pair of needle nose vice-grips so they won't fall deep into the trunk rear panel crevice when drilled out.(Remember, these riveting nuts are aluminum, and wont be attracted to your magnetized screwdriver if you have to fish them outone more reason to securely hold them with a pair of vise grips before drilling.) Figure 4.Within the rear panel, behind the carpeting, reside the inner halves of the four riveting nuts.While those on the right side (right photo) are readily visible, access to those on the left side (left photo) is complicated by the presence of the trunk lock vacuum actuator for the central locking system.I held this end of the fastener with small vise-grip pliers to prevent the inner half from falling into the panel when drilled free from its collar.The left (driver's) side, though is little more complicated, because the central locking mechanism for the trunk lock is in the way.There is a snap-on access panel that can be removed to get at the top riveting nut, but to grab hold of the lower one, the two screws that hold the top of the vacuum actuator mechanism bracket to the rear panel should be removed and the bracket pulled slightly away make the nut accessible to the vise grips.(A rod from the vacuum actuator to the lock mechanism remains connected, but need not be removed.) Once Id locked the small, needle-nosed vise grips onto each old riveting nut, I started the drilling, using a very light touch and slow speed.A sharp bit will cut through these soft aluminum rivets like the proverbial hot knife through butter, so I used low forward pressure and very slow speed.Because the drill bit is just a hair smaller than the riveting nut collars outer radius, it must be wallowed a bit to cut it away near-completely.But easy does it, to avoid nicking sheet metal.Installing the New Riveting Nuts To expand the new riveting nuts, I simply threaded an M5 nut onto a 30mm M5 screw, slipped on two M5 washers (to help protect the paint from the 8mm open-end tightening wrench) and threaded the screw almost fully into the riveting nut.Then, firmly holding the collar of the new riveting nut flush against the rear panel by way of my thumb pressing on the screw head, while holding the screw head fast with an 8mm box wrench, I carefully tightened the nut against the washers and riveting nut.This gradually expands the portion of the riveting nut between the collar and the threads tightly into the hexagonal hole. After turning the M5 nut several partial turns at a constant amount of resistance, it became suddenly more difficult to turn. I prudently took this as the sign that expansion was complete.(Further tightening may strip the riveting nuts threads or deform the fastener!)I removed the tools, removed the screw, nut and washers, and checked to ensure that the new riveting nut was secure.I repeated the process for the remaining three riveting nuts the same way.Finally, to protect the threads of those aluminum riveting nuts from galvanic corrosion with the steel M5 license plate screws, I coated the threads generously with anti-seize compound. Figure 5.(LEFT) Why buy an $80 aircraft-quality threaded insert installation tool when a simple nut, bolt, and two washers will do the job just as well? (RIGHT) The outer 8mm wrench holds the screw stationary while the inner wrench is turned clockwise to tighten the 8mm nut.This pulls the riveting nuts threaded portion toward the collar, expanding the riveting nut to fit tightly within its hexagonal hole.The Finish To complete the job, reattach the trunk lock vacuum actuator bracket and the snap-in access panel, if removed.Check the trunk lock mechanisms vacuum-powered locking and unlocking to ensure it is still working properly. Reposition the loosened, rear panel carpet and press it into position firmly.(The glue on mine was still a little tacky, just enough to hold the carpet, so no re-gluing was necessary.)Replace the metal strips that hold the top edge of the carpet to the top of the trunks rear panel with the four-per-side sheet metal screws. Now, after struggling with putting a new rear license plate on my SL every few years for over a decade (Tennessee; US Army, Europe; Texas; Michigan; Texas again), the rear license plate is finally securely Rear Panel fastened to the rear panel, thanks to four, new, tightly-installed riveting nuts with healthy threads and four properly-sized, M5 screws. Figure 6.The finished product--four new riveting nuts that, with the properly-sized, M5 screws, hold the license plate securely, unlike the stripped and/or free-turning nuts they replaced.No more DMV-parking-lot-on-a-rainy-day-on-my-knees aggravation for me, by God! Of course, having gone to these extraordinary lengths to remedy a small and infrequent but annoying issue, I have thus guaranteed one thing. I will never have occasion to change my license plate again. Good road,________________ GlueckAuf Sterling Heights, MI 1987 Mercedes-Benz 560SL Benzedrine 2002 Volvo S80 T6 2004 Litespeed Vortex/Campagnolo Record