The Bean

Fair enough. I wouldn't say most Miata guys have to drop subframes often at all really. I was just curious. 

Thanks for posting so much detail!    This thread is awesome.

In reply to gumby (Forum Supporter) :

Absolutely love your truck. Thanks for the comments

In reply to AWSX1686 (Forum Supporter) :

Its a good question, helped me to crystalize my thoughts about it to write it down.

In reply to V6Buicks :

Thank you for commenting, I'm enjoying keeping up with your camaro as well.

I saw that the miata started with a 7.5 diff. 

What happened to the axles from it? A 7.5 would be plenty in my v6 miata. 

In reply to Dusterbd13-michael (Forum Supporter) :

I got the miata from someone planning an LFX swap. The 7.5 was in his shed and he had decided he wasn't going to use it so I got it as part of the deal. Unfortunately it didn't come with any swap parts. I'd actually prefer the 7.5 I think at my power levels just due to packaging and ease of moving it around. However the 8.8 already has a trac lok so it's likely what I'll use. One nice thing is that they both mount up the same and can use the same axles (in this application) so I can swap between them if desired in the future.

Edit:

See a few posts down for clarification of "same axles" 

After removing the trunk floor it became apparent that the corners of the rear panel were actually seam sealer and not metal. Not a particularly complex fix but I am happy with how it turned out.

While cleaner welds and an all metal finish would be preferable, you can see the solder running out of the rest of the panel as I weld the patches in. These will receive some high build and maybe a lick of filler and will be just fine.

In reply to Shavarsh :

Thanks! Didn't know the axles would be the same between 8.8 and 7.5

What did you wind up doing for axles? Cant find the other thread 

In reply to Dusterbd13-michael (Forum Supporter) :

Near the bottom of page 4 on this thread I talked about it a little. I went with the Monster Miata axle kit. 

Also I want to be clear the axles are not the same between the 8.8 and 7.5. The 7.5 has smaller inner plunge joints, outer cv, and smaller diameter center bar (and a different length center bar) However the inner stub on the plunge joint is the same (as are the splines that engage the hub on the outer cv). This allows you to use either axle on either differential. The 7.5 axle is preferable due to the shock clearance gained with the smaller outer cv and the ability to use a small cv boot. As I spoke about in the axle thread the parts store axles will likely be 8.8 spec axles or redesigned 7.5 axles. Neither of these are compatible with the monster kit so the junkyard is the current source for compatible axles.

Another tube went in to support the back of the frame and shock towers:

Then the areas of the bars intruding into the front foot wells were removed:

After doing a few more simplified simulations it became clear I needed to include a bar connecting the two main frame rails. After a series of iterations including different size/gauge tube, chamfers, notches for the driveshaft, and diagonal additions I decided to use a 2x4x.100 section under the rear seat area and running below the driveshaft. Disclaimer: these cases are quick comparisons and not very accurate (just pretty pictures to show).

Some test fitting with the prototype (wooden 2x4 not shown) to make sure the driveshaft can still be removed and there is room for an exhaust:

A lucky trip to the metal store netted a piece of 2x4x.120 that had fallen back behind the remnants rack, which was exactly what I was looking for. During some of the previous gusseting the frame rails splayed apart. So with some help from the ratchet strap the cross bar welded in nicely.

Final mock up before marrying the body to the frame:

Old vs New:

Next up the frame went in for the final time. Here is the method used to get the wheels pointing in the right direction and centered side to side. It took me awhile to find features on the body that were straight/centered enough to use for reference points.

Then the beginnings of the connections between frame and body to hold things in place:

After getting the frame secured to the rockers (above) the clamps were freed up to work on the trunk. The trunk was framed out in .055 2x2. This gives me good structure for the rather flexible rear of the body and a good place to mount the gas tank/exhaust/etc. I tried out the tape method discussed by a few others on their threads for measuring for rectangular tube and had success. I will be using this method again for sure.

The trunk support was welded in at the original trunk floor height. This means the main frame sits about a half inch higher at the junction. I can remedy this with a piece of plywood for the trunk floor in the future if it presents a problem. Mounting at original floor height prevents any further space loss in the cabin, and gives better access to the surfaces that need to be welded.

Moving right along, the top side of the front subframe/floors were welded to the new frame rails. Whoever brazed these floors in used thick sheet metal and it welded quite nicely.

Each side also received a light gauge 2x2 support to the frame rail just behind the factory floor support structure:

Next up I needed to see some visible progress to keep the motivation up. So the next project was closing in the passenger side shock tower. 

First up remove more wheel well to gain shock clearance:

To transfer lines like these to the other side of the work piece (where there is better grinder access) I like to drill small holes in each corner then redraw the lines on the other side.

After some cardboard work has been done its time for the metal version:

After some bending, fitting and welding we end up here:

Repeat the process a few times and we have one shock tower closed in:

So a question directed at those of you who have done this type of work before. In the past I have always welded both sides of a seam like the ones shown in the post above (.040 sheet butt welds), however this time around I am considering seam sealing the top side rather than welding it again. What are your thoughts?

The welds on the bottom have plenty of penetration. If the area was going to be on display it might be nice to have a clean bead running around all the junctions however these will be hidden on the inside by sound deadening and carpet. There is also a good chance I cut this up again to modify it or weld new brackets to the floor,etc. With this in mind dousing the seam in self etching primer and paint/welding both sides may be a better way to go. I'm sure there is plenty of experience on this board with each of the methods.

I learned today that bean is used by kids these days as a synonym for bitch/bastard. From my experience with my 73 pinto wagon, I'd say it's an apt name. 

In reply to Shavarsh :

Single sided welds for me every time if it is less than .125 thick.

This is coming along so well.  I'm even more impressed to see all this come out of a little two car garage!

I'll second that there is no need to weld the inside in this situation. You already have more weld than the OE type unibody seams.

If there are future mods planned, I would definitely skip the seam sealer as well.

Thanks V6 and Damarble for your comments, I really appreciate the engagement.

Turner and gumby, thanks for your input. I am not concerned about adding strength with the top side weld, rather I like to use it as metal "filler" to ensure no moisture finds its way into the seam from the top (there is a small channel between panels). It won't take me long and it wont add appreciable weight. Do you see a downside to adding the weld? If there is I would like to learn. I am planning to skip the seam sealer on these parts at this point and only apply to the areas I don't foresee revisiting.

In reply to Shavarsh :

The down side I can forsee, but not garrauntee happening is extra chances to build in stress from the extra heat cycle of the weld. I do use seam sealer liberally.

I skipped over finishing the main connections between the frame and rockers so I'll toss some pictures showing the process here. I ended up with 2 .120 plates welded to the floors then a cap made from 2x4x.120 cut to form 2 angles.

First, I finished closing in both wheel wells:

Then after removing the temporary supports I tossed the rear seat in to see how everything is looking. This is a very exciting mock up for me. It shows that I will be able to achieve my goal of keeping the rear seat functional in this car despite the addition of the frame/IRS setup.