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therealpinto Reader
2/2/21 9:41 a.m.

Cool to see, if Miata donors were cheaper in Sweden that would be tempting...

I recognize the cutting from when I fitted the IRS (Sierra) into my Capri. What struck me afterwards that in some ways it would have been quicker and easier to cut a big hole right away, rather that trying to keep as much as possible of the original floor sections. But now it's also kind of nice to have some Capri left...


Shavarsh Reader
2/2/21 11:59 a.m.

In reply to Nitroracer (Forum Supporter) :

Thanks Nitroracer, I appreciate the comment.

Shavarsh Reader
2/2/21 12:00 p.m.

In reply to therealpinto :

I have similar feelings about the route I took... but its only possible to see in hindsight :)

Shavarsh Reader
2/2/21 12:39 p.m.

One of the goals during the IRS swap is to keep the rear seat functional. I believe this will work out even though the people in the back seat will loose about 4" of leg room.

So the plan for the frame started to come together at this point. Here is the first draft:

With this in mind space was made for the main frame rails:

Shavarsh Reader
2/3/21 2:39 p.m.

Next the cross bar was fitted:


The model was finalized and frame construction began in earnest:

This process involved lots of test fitting:

Shavarsh Reader
2/5/21 11:16 a.m.

Another frame rail was built for the other side and attached at the crossbar. This process includes running around with the level, T-square, and a pack of wood shims to try to achieve close to square on an uneven surface.

Shavarsh Reader
2/5/21 12:52 p.m.

More test fitting, at this point it was time to add the rear legs to hold the subframe.

Shavarsh Reader
2/5/21 5:08 p.m.

Of course more test fitting comes next, as well as the first time seeing the rear subframe supported by the frame (and ratchet straps) in the car.


After searching for months for an appropriately priced set of seats, I got these, originally from a Neon Srt. The guy who sold them to me had a sweet eagle talon project and some other cool projects and when he heard what the seats were going into he cut me a deal. Thanks!

They can't be placed properly due to the support bars currently welded in the way. I'm hoping that I can get them low enough in the car with the frame rail taking up space underneath them.

Shavarsh Reader
2/7/21 12:13 a.m.

So next up was mounting the differential and putting the rear subframe back together. I found that I cut more out of the subframe than I needed to so I found the scrap piece and welded it back in.

I decided to tackle the rear mount first:


Then started on the front. I like to build mounts on the item they mount to then just hold them in place with the mount and connect the dots to attach the mounts to the frame.

Shavarsh Reader
2/7/21 12:26 a.m.

I'm sure some of you who have done the 8.8 miata swap before will recognize this site:


If the author is on this board I really appreciate the documentation you provided. I did in fact use the same bushings (rears cut down to fit) as shown.

More subframe progress:


This is not the lightest solution. It will work for me, but if I had more of the subframe intact I would probably not have built such a heavy front mount.

Shavarsh Reader
2/8/21 11:19 a.m.

So after finishing the subframe modifications it was time for another test fit.


Then the surfaces that were about to become hidden got a proper coat of etch primer and paint.


More test fitting:


Thankfully wearymicrobe dropped by and knocked me out of my analysis paralysis for the subframe placement and later that night we ended up here:

Shavarsh Reader
2/10/21 12:33 p.m.

After looking at prices for miata coilover kits and running down the rabbit hole of generic vs vehicle specific options I ended up ordering a set of coilovers from Viking. 


Thanks to Keith and NoHome for answering my messages looking for confirmation of sanity.

Here are the napkin drawings for how I decided on shock length and spring length. Don't use these values if you are ordering shocks the values are all estimated based on scaled advertisement pictures and are likely not accurate, just showing the thought process. 


After a while the shocks came in and I started to modify the control arms to accept the new shocks and ensure plenty of axle clearance.

Obviously the premade tabs from the metal store needed to be modified.


Shavarsh Reader
2/10/21 5:06 p.m.

After both control arms were finished we needed a place for the top of the shock.

Cardboard version:

And in metal:

Shavarsh Reader
2/12/21 11:02 a.m.

Then there were 2:


A bit of test fitting:


I made the passenger side tower a little too tight and it touched the top of the spring when mounted. After some love from a big adjustable wrench we had clearance.

Welded in:


759NRNG (Forum Partidario)
759NRNG (Forum Partidario) UberDork
2/12/21 11:10 a.m.

Bet you sure don't miss these conditions.......what's the temp today?



Shavarsh Reader
2/12/21 1:47 p.m.

In reply to 759NRNG (Forum Partidario) :

I sure don't, the only thing I miss is driving in the snow. I'm afraid to admit it but its mid 60's here and considered chilly!

Shavarsh Reader
2/12/21 4:51 p.m.

So one of the expensive parts of this build and one of the reasons I was putting off getting started was finding suitable axles for a reasonable cost. Some of my exploration was discussed here: Link

After lots of deliberation reliability became the number one concern. This is a street car and I have no desire to get stranded somewhere due to a broken sleeved axle. I took a trip to the junkyard and visited a couple of these:


To grab a couple of these:

I took a trip to see Martin at Monster Miata and picked up his axle kit and broached hubs. After disassembly and ready for reassembly:

These tripod bearings were suuuper tight on the splines and took quite a bit of persuasion to install.

At the end of it all we ended up with these:


Rocambolesque Reader
2/12/21 7:47 p.m.

Very cool! But you welded the Miata subframe onto that tube frame? Once you weld the tube frame on the car, you won't be able to remove the rear subframe?

Shavarsh Reader
2/14/21 5:01 p.m.

In reply to Rocambolesque :

That's right, I don't see a need to remove the subframe in the future with how things will be set up.

Shavarsh Reader
2/16/21 12:38 p.m.

After the shock towers were final welded the frame went back in for some more test fitting.


Its about this point that I start to question my initial assumptions and decisions in a project. So to avoid stagnation I focused on small projects like capping the open box sections and grinding back weak looking welds in favor of better ones.

Shavarsh Reader
2/16/21 5:53 p.m.

At this point the main goal is to get the frame into the car. In preparation for that the floors got welded to the edges of the under-floor supports (front frame rails). This welded seam will be welded directly to the 2x3 frame rail.

I don't have finished pictures but you get the idea.

Shavarsh Reader
2/16/21 6:01 p.m.

I also needed an attachment point to hold the rear of the car to the new frame. I have never liked the trunk floor or any of the underfloor supports. There have been at least 3, possibly 4 new trunk floors put in at various times, some welded, some brazed, some held in with seam sealer. My solution is to start fresh. 


First remove the bumper and back panel:

Then start removing:

Until you end up here:


Evidence of further hackery, for now it stays. Have to stop the scope creep somewhere.

Weight reduction!


That brings us up to date with the project. 

AWSX1686 (Forum Supporter)
AWSX1686 (Forum Supporter) UltraDork
2/17/21 11:03 a.m.

Looking good! I love the whole idea. 

One question, what made you decide to weld the Miata subframe to the rest of the frame instead of having it bolt to the frame like it would have on a Miata? Just curious here. 

Shavarsh Reader
2/17/21 1:05 p.m.

In reply to AWSX1686 (Forum Supporter) :

It's a good question. Here are some of my thoughts:

1. When I removed the front part of the subframe (to maintain room for the rear seat) I lost a bolt hole and some nice stiff curves of metal. While it was an option to drill a new hole, the leverage and forces on that bolt would be different and it would be less effective than the original. Fully welded I have more "supported area" on the flange hopefully meaning higher strength and better stability. 

2. The stock setup in the miata no doubt has some locating elements so that the subframe bolts into the same position each time (or very close). While I could devise some simple locating features to aid a bolt in setup, welding it in takes that concern out of the picture.

3. There is added strength to the frame and subframe to have them joined. This is a bit of a stretch, but the subframe can act as a psuedo shear panel on the frame rather than being connected at 6 points.

4. It is far easier and faster to weld in and locate once rather than weld in nuts and studs in the proper location. I don't see a need to remove the subframe in the future so this really could be the only bullet point because it is 80% of why I went this direction.

gumby (Forum Supporter)
gumby (Forum Supporter) Dork
2/18/21 8:25 a.m.
Shavarsh said:.

I don't see a need to remove the subframe in the future so this really could be the only bullet point because it is 80% of why I went this direction.

Boom. That's money. I am not intimately familiar with the Miata platform or service methods, but how often are Miata guys dropping subframes?

When I was doing a bunch of work on sn95 IRS cars the only time the subframe came out was to mod the cradle itself or unibody above it. All parts bolted to it were serviceable with the subframe in the chassis.
I also integrated the mn12 IRS cradle with the frame in the back of my truck. There is no need for it to be removable in that application and, similar to your situation, all other considerations are improved by reducing potential flex/failure points.

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