Chassis leg rust repair

[JamesDF]

Hi All,
So as the name suggests the chassis legs (Frame rails) on my FB could be in better condition, see below:

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About a year ago I got stuck into it and started cutting out the rusty metal and welding in some nice new metal. I was making good progress until, due to different things I ended up not working on the car for months. Of course when I got back to it my lovely new metal had a layer of surface rust on it. Frustrated but not beaten I decided to start over!

I got my hands on the factory RX7 bodyshop manual and have procedures to follow for chassis repairs and rust prevention. BUT, there is a page in this manual the states that "high strength steel" is used in the structural parts of the chassis, see below:
(I realise that the table below doesn't have a column for Rx7, but the manual is shared across a number of models including the Rx7 so I'm going to assume this applies to it also).

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The issue here is, the steel I have been working with is made to EN10130 DC01 (BS CR4) which has a tensile strength of 270-410MPa and the high strength steel stated in the table above is 343-441MPa (35-45Kg/mm^2). So, for me the steel I have is not suitable for this repair.

I have found different standards of steel that give me the correct tensile strength, i.e. EN10139 DC01 C340 (340-490MPa), but I haven't found anywhere that will sell in small quantities yet. I did some research into welding higher tensile strength steels and apparently they are very sensitive to heat, so I would need to be very cautious with my MIG. Until I can source a steel supplier this option is on the back burner.

Another option is to get replacement chassis legs. I believe that the part numbers are FA54-54-310C and FA54-53-310B, and I found several sites with these parts listed, however none of them actually stocked them! This is looking like a losing battle. I don't think getting a chassis leg from a donor car is an option, as I'd imagine these are just as hard to come by. But, I love to be proven wrong!!

The last option I have is to get the chassis legs fabricated. In the coming weeks I hope to do a CAD drawing of the chassis legs and send them to a fabricator for a quotation.


So that's my story so far.
I am open to other idea's or suggestions on getting this repaired!

Thanks for reading, James.

[gt_james]

I reckon you're overthinking it, the tensile strength ranges overlap significantly, and the material is likely within the original manufacturers range unless its right at the bottom of the range for the material supplied, but usually materials tend to be under rated. Good luck getting old stock frame rails, I think your only option will be to fabricate the new sections.

[JamesDF]

Hi gt_james,

You may be right, but my fear is that the tensile strength of the steel I have may be on the lower end of the range. I know it would be miles better than the way it is at the moment, but for a structural part like this I'd rather over engineer it than under engineer it!

Agreed, it's looking like newly fabricated chassis legs is the only way forward!