Rust looks a lot worse than expected

  1. My model of MX-5 is: NBFL 2003
  2. I’m based near: South London
  3. I’m looking for technical help or recommendations on: Can it be repaired to safe enough standard?

Since it had rusted through the inside of the sill, I decided to cut open the repair on my recently purchased '03 NBFL MX-5. It’s a horror show inside. What more is the inside sill/floor also looks bad.
I’m okay with welding but now I’m getting worried about how far to cut and what is still safe to do?
Hopefully a Video link give a good idea of what I found:
Rust inside sill

Have you looked at the front chassis legs underneath yet?

that does indeed look nasty

Yep, one has been welded before but looks sound. The second (intake side) seems okay but I realise it’s very tough to tell from the top. I’ll move the ramps to the front wheels and spend some more time under poking around.

Here’s my biggest concern, the seatbelt mount. Does that look like surface rust I should just treat and seal or are we into serious problems?

In isolation not really terminal.
The deeper you dig…etc.
Swelling & splitting.
As they do!
It’d need refabricated.
(Been there, plus VAT & Delivery!)
Anything is fixable with sufficient cash, man maths,…and a 1st Hons Degree of madness.


MOT regs state that a fail is where structural rust is within 30cm of a seatbelt anchorage. That looks less than 3cm !

This is exactly my worry. I could easily patch over the outer sill and you’d never know, I bet there are a ton of cars out there like that.
I’m thinking I might build a mini roll cage from angle iron. Weld it way down the side, across the seat belt anchor and up the pillar towards some good metal.
I’m not sure if there is some crush structure I’d be messing with if I do that, I guess there must be since that inner sill usually has so many bends and holes.

Designed to crumple…in bits…after a fashion.

You have come this far with it so may as well finish it off properly, especially if you can weld yourself. I don’t know quite where these welding guys are dreaming up their prices from sometimes. I appreciate it’s a bit of a skill but some are wanting more than a consultant brain surgeon!! Not of course that I would be asking a brain surgeon loose with a welder near my car.

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if you can weld just cut out and replace.take seat out lift carpet the rusty bit is the inner sill which is spot welded to the floorpan joint is hidden under seam sealer at floor level.the floor pan is probably fine so you wont see anything inside but the seam sealer will ignite if you weld there without removing it.
just dealt with this on my mk1.the mx5 restorer sells a replacement channel for this if you need to do more than a small patch

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As others have said it will require dealing with properly. There’s no point slapping a plate over it and not worrying about it as you’ll be back there in another year sorting it again.

I’d clean it up with a proper wire wheel to see the true extent however I suspect you will be chopping a section out and welding in a piece.
If you can get a decent repair panel then all the better

Strip out the interior on that side to get proper access to it. Measure it up or scribe around your repair panel and cut out as much as you need to get back to clean metal. The key is then to get the metal you’re joining as fastidiously clean as you can to get a nice weld.

If you’ve welded before I know why you’re reluctant to tackle this but take your time and do it one step at a time. It’s not rocket science and even nasty looking welds can be tidied with a flap disk.

That rust isn’t too bad but it needs addressing before it becomes a real issue

Thanks all - just the confidence I think I needed.

I got all four wheels up on ramps this weekend and spent some more time under the car. To my relief the front rails on both sides do look to be in good condition. I think I’ll investigate some sort of spray on/in wax to keep them that way.

I found a 2 inch rust patch under the passenger seat. Very specific spot which seems strange. Anyway I’ll knock that through and plate over that too.

I need to buy a bigger can of seam sealant.

As I wrote earlier, been there.
Might be of assistance to see what these so called crumple zone/deformation chassis legs look like inside.
These were mine. Note the moisture trapping double skin.

Now note the OEM “factory replacement” from Mazda UK.
Single skin. Go figure!

Nice though.

Good as new still. Better actually.

You may wish to consider doing a wee bit of drilling on the chassis rails and pressure flood with aggressive stuffs like BiltHamber make as far as possible into that air gap. You can use rubber bungs to well…bung 'em.
The slightest evidence of chassis rail swelling or distortion heralds rot, and don’t trust any undocumented or unphoto’d repairs…ever if you find any.

Then we started on my Green one.
Good fun innit?

What would we all do without a bit of dirty rotty-botty now and then. How boring it would all be.