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