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Mild steel for frame rail repair?

Motorsport Fabrication Fundamentals

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Hi all, I am restoring an '85 RX7 and need to do some rust repairs on the frame rails (chassis legs), so I am wondering what steel should I use? I will be going the same thickness as the steel I'm replacing, but what grade/spec of steel should I be using? I have some cold rolled steel (EN10130) that has yield strength of 140-280MPa and tensile strength of 270-410MPa, would this be good enough to use for the repair?

Cheers, James.

Hi James,

In practice, any steel should do fine as far as strength goes when doing simple patching. I would be more concerned by potential weldment issues with alloy mismatching. In this case, the frame rails are most likely a very similar alloy to the plate you plan on using, so this is another non-issue. You can simply choose a mild steel filler like ER70s2 or ER70s6 and complete the weld.

Hi John,

Thanks for your reply.

I managed to get the factory body shop manual for the car, and it mentions that the structural parts, including the frame rails, are made of high strength steel. With this in mind, would it still be acceptable to use a non high strength steel for the repair?

I am hoping to get away with just patch panels, see attached image of frame rail to see what I'm dealing with!

Attached Files

Hi James,

That is some fairly major rust damage to a structural component of the vehicle (its probably also part of the designed crash structure) and will need more that just plating over. Is it possible to purchase a undamaged set of these rails (donor car at a wreckers or similar) so that you can cut the damaged sections off and completely replace them with non rusted metal? Or otherwise fabricate a replacement set of rails from a compatible steel?

I just think that you are going to be doing a lot of work to repair those components by patching, and that you'll get a better result by replacing them.

Hi Stephen,

Getting replacement frame rails is one of the options I'm looking into, but it's not going too good. So far every site I have found that has the parts listed tells me they are discontinued, but I'll keep looking!! I didn't consider getting frame rails off a donor car simply because here in Ireland they are most likely rusted to nothing (That's the lovely Irish weather for ya!).

On the off chance, would you know of anywhere that might have new replacement parts or maybe donor parts?

If not, I think the only alternative is fabricating the frame rails from scratch!! 😲

Hi James,

There are possibly places local to me (Australia) that could supply these parts, but by the time you get the parts shipped to you it would be more cost effective to have fabricated a whole new front end onto the vehicle.

Hi Stephen,

That's a fair point, with shipping and customs it'd work out pretty expensive. All the same, if you had any websites or contacts I could try, that'd be great.

In the meantime, I'll start looking into the possibility of fabricating the frame rails from scratch.

The joys of vintage cars!!!!

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