Number Plate Screws and Corrosion

This is close to a worst-case fastener. Two dissimilar metals in permanent contact, at the lowest and wettest point on the car, salted every winter, held under tension. Galvanic corrosion does the rest, and the screw effectively welds itself into the bumper.

The whole of the fix is preventive and takes ten minutes: stainless screws, anti-seize on the threads, and taking them out again every couple of years. Do that and the plate comes off in two minutes forever.

A note on this guide: this rests on how galvanic corrosion and these fixings behave, not on plates removed here. See how we test.

The problem, as it appears

The plate needs to come off — a bumper repair, a new plate, a car being sold — and the screws will not turn.

Then it gets worse in one of two directions. On an older car with a metal thread, the screw is seized and the head cams out, leaving a rounded recess in something that is now immovable. On a newer car with a plastic insert, the screw turns freely and forever, because the insert has stripped or is spinning in the bumper.

Both end up as careful, awkward work on a painted panel.

Why replacing the screws afterwards disappoints

Because by the time you are replacing them, the damage is already done — and because "stainless" on its own does not fix the mechanism.

Stainless resists rusting, which is the visible problem, and it does not prevent seizing, which is the expensive one. Stainless against steel or aluminium is still a galvanic pair, and stainless has its own failure mode: galling, where threads under friction cold-weld to each other. A stainless screw can seize as firmly as a steel one and look immaculate doing it.

What actually breaks the mechanism is a barrier between the two metals. That is what anti-seize is — and it is the part people leave out, because the screws come in a bag and go straight in.

What actually matters

Find out what the screw threads into

Before applying any force, because the two cases fail differently and want different handling.

A metal thread or a captive nut, common on older cars: the failure is seizing, and the answer is penetration and patience.

A plastic well nut or moulded insert, common on modern bumpers: the failure is the insert stripping or spinning, and force makes it worse rather than better. Once it spins there is nothing to react against, and the fix involves holding the insert from behind — which usually means getting to the back of the bumper.

A quick look behind the plate, or at how the previous plate was mounted, tells you which you have.

Soak it properly, which means overnight

Five minutes is not soaking. The corrosion filling the thread needs time for the oil to travel into it, and that is measured in hours.

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Heat is the usual next suggestion and it is a poor idea here — everything within reach is painted plastic.

Most of these are lost to a cammed head, not to the corrosion

The technique that saves the screw.

Use a driver that fills the head completely, and push hard into it while turning. Cam-out happens when a slightly undersized bit rides up out of the recess under load, and once the head is rounded the screw is a far harder problem than it was.

A driver that is nearly the right size is the thing to avoid.

If the head has already rounded, extractors work where there is material left. On a plastic insert, cutting a slot with a rotary tool and using a flat blade is frequently easier — and both are careful jobs, because everything around the screw is painted.

Where a thread in metal has gone entirely, that is a different repair — stripped thread repair and threadlocker covers it.

Stainless plus anti-seize, not stainless alone

The actual specification.

A thin film, on the threads only. This is a barrier between two dissimilar metals, not a lubricant to be applied generously.

Caps are worth using, with one caveat

The plastic caps supplied with plate kits keep water out of the screw head, which genuinely slows the corrosion.

The caveat: they also make a cammed-out head harder to get a grip on later, and they encourage sealing and forgetting. Use them, and take them off when you do the periodic check.

Do not overtighten

Tempting, because a plate that rattles is annoying, and counterproductive.

On a plastic insert, overtightening strips it immediately — which produces the spinning failure with no corrosion involved at all. On a metal thread it increases the contact pressure that drives the galvanic reaction.

Snug, and no more. If the plate still moves, the problem is the mounting or a missing pad rather than the torque.

The periodic check is the entire maintenance

Once every couple of years: out, clean the threads, fresh compound, back in.

Ten minutes, and it is the difference between a two-minute job and a bumper repair on the day the plate actually has to come off. It is the same argument as locking wheel nuts and the missing key — a fastener that has not moved in five years is the one that fails.

Why it is worth caring about at all

Because the cost is asymmetric and lands at the wrong moment.

A seized plate screw is trivial until the day the bumper needs painting, or the plate needs replacing, or the car is being handed back — and then it is a careful extraction on a painted panel with somebody waiting. On a leased car especially, damage created while removing a plate is chargeable, which is the scenario in keeping a leased car unmarked.

The order to work in

1. Establish what the screws thread into — metal or plastic insert.
2. Soak overnight with penetrating oil.
3. Use a driver that fills the head, and push hard while turning.
4. Stop if a plastic insert starts to spin — force makes that worse.
5. Clean the threads and the recess once out.
6. Fit stainless with anti-seize on the threads.
7. Snug, not tight, especially into plastic.
8. Fit the caps.
9. Diarise it for two years.

The mistake to avoid

Fitting stainless screws dry and considering the problem solved. It is the obvious upgrade, it looks like the fix, and it addresses rusting rather than seizing — which is the failure that actually costs you. Stainless can gall into a thread and hold just as firmly as a rusted steel screw, while looking perfect from outside. The compound is a few pence per car and it is the part that does the work.

The second mistake is applying more force to a screw that has started turning without loosening. On a modern bumper that means the plastic insert is spinning, and every further turn destroys more of the housing it sits in — turning a fastener problem into a bumper problem. The moment the resistance goes soft rather than releasing, the answer is to stop and get behind it.