I am in the middle of a V8 swap. I have the subframes modded and the 302 and T5 in the car. My problem is that my frame rails are crushed in several places. No doubt from years of bottoming out and poorly placed jacks.
Do I....
1) bang and pull the rails into some kind of shape and then build reinforcements around the bent metal.
2) put 90* angle iron over the dammaged rails. Two 90* rods on both sides making a U to cover the damaged rails.
3) cut the rails off and replace them with 1 inch square tube. Two tubes side by side where the old rails were.

I know there are a bunch of ways to go at this problem. I wuld like to know how you would over come this.

 

I would use a hammer and pound them sort of back into shape. Then weld some steel around those areas to support the damaged metal.

I recommend using a weld through galvanizing compound, and drill some holes in the bottom plates.

 

I saw where somebody removed their interior, cut the floor pan above the rails, beat the bent rails back into shape from the inside, & filled the rails with some 2" (I think) square 1/4" wall tube that was welded along the area. This is how I would go about repairing it if mine were badly damaged. It would also provide a much more stable mounting location for the transmission crossmember. You would also never have to worry about the rails bending again, & you would have a great location to jack the car up with.

 

That's a really really good thought. I like it. This is basically how subframe connectors on Mustangs and F-bodys are done, is it not? Sounds similar to my Thirdgen. Basically cut the floor and weld in a connector along the length of the car. It just so happens you are placing yours within the frame rails.

This would change the crash characteristics of the car quite a bit, though, no? Something to keep in mind.

 

First up you should know those rails add little for support in the car. The real structure is in the rockers which is why Mazda just used tinfoil on these so called frame rails. It's not a huge deal if you even fix them at all except for appearance.

Nick a great thread on how to rebuild them from the inside is HERE and probably the best idea I have ever seen to fix and reinforce them.

I really didn't care much about the dents but needed a good foundation for the trans mount and wanted to tie the front frame to the back frame. So I just put a piece of real box along side the existing rail. It was easy, fast and cheap and you could use smaller box or rectangle material to be lower. Mine are 2x2, 16 ga and strong as a tank.

Details HERE

From there I tied the car corner to corner through the trans mount,



and from front to back through the added rails.

More details HERE.

It's a lot of crap under the car but it is a really rigid chassis.

 

Those "frame rails" are essentially floor stiffeners. The real frame rails are made up of a 3-piece weldment at the rockers. Somewhere there is a pic of a cross section showing how it's done. That's why rocker rust is a big deal in a Miata/

That being said, most v8 swaps involve a trans crossmember between the rails you speak of, which aren't really substantial enough for that application. So, the bolt-on stiffeners like V8R's and FM's come into play, to create a good place to hang the crossmember and add some additional chassis stiffness. Since these add-on pieces cover up the existing rails, you can beat on them as required to get the new rails on. I find that a big drill press vice works pretty well to squeeze the sides in.

Most sheetmetal shops can bend up a "hat section" channel to your specs which can then be bolted on. Or welded if you're a glutton for punishment.

Mike

 

I think i found it! Looks like a great solution
My frame rail solution - Miata Turbo Forum - Turbo Kitten is watching you test compression.

 

I like the idea of tying the front and rear frames together. But I would not want to weld them. Maybe bolt on frame connecters. I also like the idea of running a large square tube along the rail. It would be nice to put them on the inside of the rails so that they would be well attached to the tranny crossmember. The problem is those pesky fuel and brake lines on the passenger side.

 

If i do not cut into the rails and re enforcing them from inside then I think I will make my own impression of the V8 stiffiners. Good design just too pricey for me.

 

I put them on the outside of the frame rails so my X bracing would extend as far out as possible to the corners of the car. My goal was chassis stiffness, front to back, side to side, corner to corner and twisting. Also you can go a little farther forward with them on the outside and yeah the fuel lines. But the best reason to go outside is it's super easy to get at with the floor jack.

So why would you not want to weld them?

 

For future servicing. If i needed to drop the front subframe for a new oil pan. Replacing rear end parts from drifts gone wrong. I anticipate some bottoming out and bolt on parts will make easier replacement of damaged parts. Also i do not own a lift so liquid hot metal inches from my face does not sound like fun. I am too accident prone for that.

 

The frame rails do not connect to the engine cradle just support what it is bolted to. It will drop right out just as it did stock. Ditto with the diff and axles.

Though I did plenty of upside down work under the car where I welded the 2x2 rails was welded from inside the car on/ through the floor not underneath.

 

I'm liking the frame rail work that guy did. Might have to implement it. I was planning on doing caps like most kits, but his option is better, and while I have the welder out, can't hurt. I'd preferable also cut and weld mounting tabs for the transmission bracing and weld those to the frame rails as well. Rust prevention is going to be a big issue, I'll have to figure out how to get in there for paint.

 

Jim, you'll love this!
Keith@FM
"Nice!

On my MG, I welded a second frame rail in beside the original. I needed the new ones (which extended into the engine bay to provide mounts for the new suspension) to be a bit further apart, so I laid the new square tube along beside the old and welded them together. I like the idea of the slot in the floor, that would have worked well."

You're in good company.

Here's a related search I did on painting inside Rails/subframes, I will be implementing this when I move on to the chassis. Exciting stuff.
Need to paint inside rockers, frame rails, subframe??? - NastyZ28.com
Underseal Spray Gun and Probe - Undercoating Spray Gun - RustSeal - KBS Coatings


http://www.miataturbo.net/attachment...ine=1361215001
I'm adding this photo from that previous thread. Good visual. I wonder how much if any the seat mount in the middle helps for the chassis. In the end, everything adds its few bits to make the whole. Always a fun subject.

 

LOL thanks Nick!

I'm not following why paint would be a problem. The inside of the tubes are sealed off from the elements and don't need paint and the outside is easy to get at. I painted as far as the can would shoot and thought about capping them but they are a super handy place to run battery cables and what not through. Besides by the time that 16ga rusts out the car will crashed, parted to build something else or I'll have been dead 25 years...

 

When I started working on my LS powered car, the "frame rail" on the driver side was crushed to the point where the floor was pushed up about an inch. The rail itself was too smooshed to easily be bent back, so I split it down the middle and straightened it out as best as possible including banging the floor back down with a sledge hammer. After that I cut the very bottom of the rails off so that I had a "U" channel pointing down from the bottom of the floor. I then took a piece of 2"x2" .125" wall tube and cut it just long enough that it could be slid inside the heavier steel that is at the front and back of the rails. I put just a little taper at each end of the 2x2s so that it would slide easier into the pockets. I then welded the front and back of the 2x2 to the heavier steel at the ends of the original frame rails, welded the remaining portions of the original rails to the sides of the 2x2s and spot-welded the top of the 2x2 to the floor from the inside of the car.

Once this was done, I have a much stronger structure, that looks almost stock.

 

Don't suppose you happened to take some photos along the way SofaKing? ^

 

I don't have photos, but it turned out so well, we did it on the other LS car that didn't have any frame rail damage. I will try to get a photo tonight when I get to the shop.

 

Here are a couple of photos of the way we beefed up our frame rails. Sorry for the quality of the pics, I took them without jacking up the car.

 

Second attempt at attaching photos!

 

SofaKing,

Does the 2x2 sit any lower than the factory rail?

Nice work, thinking I may go this route...

 

It is pretty close to factory. If it is lower, it is no more than 1/8" lower!