Steel thickness of chassis floor (Mk 2.5)?

Technical Area Body, Interior & Styling
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  1. My model of MX-5 is: Mk2.5

I have an '05 NB and there’s a hole in the under side at the front of one of the chassis rails. I’m gonna weld in a fix but I’m not sure what gauge of steel I should be using. Can anyone help with this info?

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Is that the piece that wraps under from the back of the front wheel arch (Part of the ‘torque box’)?

If so, it is quite thick - I used 1.5mm (16 Gauge) for a repair to that panel.

If it turns out to be worse than you think, you can get the full panel from MX5 Restorer

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Thanks for that link. I’ve had a more of a look and it seems that most of the floor / sill panels need to be replaced. I’m a bit confused about some of the panels in that site though. Do you know if I can find a diagram anywhere showing where each of those would be fitted, e.g. full sill panels or inner sills uprights?

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There are very general parts diagrams, but I’m not sure that they’ll help much ( Link )

The full sill is the part that you can touch under the door, but it dives inside body panels at both front and rear (behind the bolt on wings at the front and the welded on lower quarter panel at the rear). The sill upright aka reinforcement aka membrane spans the top and bottom of the sill panel for most of its length. There’s a diagram on the MX-5 Restorer site:

At the front and rear, it gets more complicated, though, and I’m pretty sure that the panel they sell is just a repair section, rather than the full thing.

This is what it looks like inside the front bottom corner with the outer panels cut away (the central part has already been repaired):

This is with the inner and outer sills repair patches tacked on:

An this is with the ‘apron’ (aka torque box) panel repaired:

(All of the repair panels above were fabricated.)

I found a few youtube videos helpful to figure out the construction:

And also this series (5 videos - although it’s a MK1 rather than a MK2, it’s worth sticking with it IMHO):

I’ve got some previous experience of welding and fabrication, and I think this is quite a difficult repair to do properly.

FWIW: Give the rest of the car a thorough assesment for rust issues before starting to repair this corner - there may be more.

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Thanks m8, that’s a brilliant reply and a lot of helpful information. I’d been watching videos from the first guy there but the second one was new to me. I really appreciate the time you took to share this with me.

I’ve had a good look all over underneath and there’s a lot more than just that corner needing done. So I’ve got a project for my brother and I for the next few months :grin:

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