GT4 rear bumper rubber resto

Gt4 rear bumper rubber restoration.


There’s a few threads on here , but seem to miss how to fix ?




Problem is the black rubber ( EU spec ) is bolted on to the elegant chrome bit by 5 bolts from behind .

Theses bolts attach to a metal lattice embedded in the rubber which have the nut part welded on in the 5 places .

Water ( cars 40 this year ) inevitably finds it way through , not just to the nut part which makes sense tracking down the bolt ?

It’s finds it’s way along the lattice .

Subsequent corrosion of the base metal lattice , as it “ blows “ it expands and flakes pushing the rubber away from the bumper .

Usually at the corners first , there’s more metal to expand .


Replacements cost between £450 to €850 from the usual suppliers .

There’s also a horrible fibreglass copy bumper which dispenses with the nice chrome and OEM black rubber .


Removal is a 10 min job two 14 mm bolts in alignment tubes , disconnect the reversing lights wire plug in so a doddle .

Tap out with a mallet and another socket the bumper from its hole mounts .


Once off remove the reversing lights - easy .


Now this is the tricky bit undoing those corroded 5 retaining bolts of the rubber .Three just were spinning as rusted away and fractured off the lattice inside.

Try and push down your hand between the chrome and rubber to get a bite , foiling that - basically just rip out the retaining / spinning nut ( s)


Then use a vice to hold the ceased nut ( still through the chrome ) with a socket on the bolt .


Next bit cutting out the lattice .

Stanley knife - no joy and risky tbo .

What I found best was an angle grinder ,it melts the rubber ,just enough to cut and sparks on the metal lattice , once you find the tubular lattice , just cut above it and again brute force - rip it out .


Then clean up use a tool to scape out rust flakes etc .


Now what ?


Re fixation hmm .


The lattice is about 5 mm only under the surface .

I did think about using those car underbody fixers for a bolt , but worried about realignment of the rubber .


The holes in the chrome allow for some adjustments they are say 10 mm in dia .


Maybe fixing some hard wood where the nuts where and using self tappers inside the chrome to refit - hmm - what would happen if the screw was overly tight and the wood attempted to turn inside the rubber .


In the end I opened up those “ fixation “ things and got some M6 bolts .

Idea is to embed the bolts in the rubber .

There’s sufficient space for this inside the chrome bumper for the studs to come through without interfering with the body work .


I I cut out some more rubber to sink them deeper - carefully so,s not to perforate through .Its plenty thick enough .


The corrosion had distorted the under side a bit so I cut the lips off so it’s flush .

Hence the wavey edges where some of the lattice came out .


The corrosion has left nasty orange ferrous oxide stains and flakes in the groves .I spent some time scraping theses tiny bits out , prepping .


In the end I painted the grove with kurust to neutralise any surface oxidation that had some how impregnated into the rubber .



Next stage is to think about the future if needs to come off later .

So I used some 3M grab adhesive,” super strong “ to embed the hex bolts and bracket into the rubber in such away to hopefully prevent them spinning at a later date ? Long set time and quite sticky “ grab “ means you can offer the rubber to the chrome to check the final alignment .Once happy everything is centred - walk away and wait for the set .

Could have just placed them and covered with black rubber backfilling .

Problem with that idea is they may spin later , and how do you turn it over to check the final alignment with runny rubber mix in ??


I bought some wooden dowl same size as the lattice with the intention of refitting to put back some rigidity potentially removed by the lattice .

How ever the rubber is still very rigid in the vertical plane as it is .

Also the dowl will take up too much space , so less room for the flexible back filling .So I went off that idea .

So I figured the lattice was just to support the mounting nuts , put in the mould as they poured the liquid rubber during manufacturing,it’s does not appear to be crucial to the rigidity.


Anyhow some pics paint a 1000 words

TBC
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Corrosion the blowing metal forcing the rubber out

 

Sorry not very IT literate with these pics - grrr

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I thought with the metal lattice removed it would sag in the vertical plane too and need reinforcements.
However pleased to report it does not .The 5 studs are evenly spaced as well .

I did get some same size wooden dowl ,but it’s a tight fit and the inside where the lattice was removed had left a rough surface in the worst corrosion areas .
This means when the dowl was stuffed back dry it’s actually kinda distorted the edges .Plus you leave little room for the back fill .

Back fill

The cavity internal shape is all over the place ,so what ever goes back need to flow ,have a decent working time and be mouldable .
Could be any colour but black seems the obvious choice .
Thought about various “ liquid rubbers “
How runny ? If too much it’s gonna run to the lowest points ?
Two pack stuff — hmmm how do you control the setting - to get it right ?
Also wanted a adhesive, not a mild one a really strong one .
Reason partly to reinforce the studs , although they are very firm with the instant grab white stuff .
But mostly to try and recontour one edge that warped .

I used a wooden jig to push the edge in see pick .
Did this indoors so room temp for setting and flexibility of the rubber in the jig
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Alignment of the studs rechecked as per previous post before back filling
The white stuff “ instant grab “ set solid , just enough hopefully to prevent the stud heads spinning in future .
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Warped edge - temp jig pushing it in while hopefully the strong Sika sets ?

 

This is the warped bit at the bottom edge .You would not notice it on the chrome on the car .
But while it’s off - ......... may as well attemp to unwrap it .

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The actual visual parts are excellent cosmetically nice unmarked front and edges , so seemed a shame just to bin it and buy another .
Also it’s I guess got the right 40 y old pattena , would hate to have the two bumper rubbers different shades .

 

Bolted it back together everything lined up
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You can see the mouting tubes from the inside , it’s secured with a 13 bolt at each end .
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Happy with the result , removed the corroded parts the lattice , refitted some retaining bolts , back filled , refitted Also dismantled the back up lights and cleaned the grime out .

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Dabbed a bit of waxoil on the studs,and the inside lip of the chrome bumper ,esp the lower corners that seem to act as water traps , the chrome here although out of sight was starting to corrode .