There are many aftermarket seats available for the Miata. Virtually none of them are a straight bolt-in swap with the stock seats, and just about every installation is different. Part of this is due to the limited space in the Miata, and part is due to the different reasons people have for installing aftermarket seats. Some do it to lower their seating position, for example to make it legal for the track. Others do it for comfort. Some do it just because the new seats look cool. All of these reasons are equally valid, but each influences the installation process.
The biggest question afecting the installation is whether or not you want the new seats to be able to slide back and forth. In most cases this means adapting the new seats to the stock Miata sliders. This can be very simple if the seats either don't have fixed mount points, or the mount points miraculously match the mount points of the stock Miata seats. If that's not the case, however, you'll have to fabricate some kind of adapter plates to use the stock sliders. How you do that depends on the model of seat.
If the seats did come with non-Miata sliders, it's unlikley they'll bolt right in because of the bumps in the floor of the car. You'll either have to replace the new sliders with stock Miata sliders, or have the floor of the car flattened. The proper way to flatten the floor is to have the bumps cut out, and new pieces of 16-gauge steel welded or brazed in. This is not a bad idea if you plan to try several different seats, as it gives you almost unlimited mounting options. The downside of flattening the floor is that it's not easily reversible, and it's unlikely your Miata will ever wear stock seats again.
If you really want to get low, and you're willing to give up on the sliders, the installation process isn't much easier. Mounting the seat to the floor requires the same cut-and-weld flattening of the floor pan. Installing side mounts presents a bunch of other difficulties. Ideally, particulary in the case of a less-than-comfortable racing seat, you want an installation that makes a swap with the stock seats a reasonable proposition, while avoiding fabrication of a complete custom mount.
Your own aftermarket seat installation will probably require a fair amount of ingenuity, some amateur engineering, and a little luck. To give you some inspiration, here are a few installation stories from Miata owners who have already faced this task.
by Brandon Yang
I purchased a used Sparco Sprint seat that actually had Miata rails already on it. At first I was overjoyed, thinking this would simplify the installation process by a huge factor. My joy was short lived though.
As I test fit the seat, I realized that it wouldn't fit without modification. One of the guide pins didn't line up with its hole. It was a little too far to the right. Rather than modify how the rails attach to the seat and risk messing up the seat, I decided to modify the mounting points where the rails attach to the car.
It's never a fun thing to drill into your car, but my only other option was to grind the guide pins off the rails. This would result in the seat potentially being less secure in the case of something happening. It could've also meant more difficult work, and tools I didn't have readily available. With this in mind, I took the plunge and decided to start drilling.
I located a drill bit with about the same diameter as the hole in the car for the guide pin. I decided to lengthen the existing hole in the car by drilling off to the right a bit. At first, this seemed to work. I was excited because the guide pin was starting to line up with the new, longer, hole. However, after I decided the hole was long enough, I tried lining it up and bolting that side on, only to find out that the bolt's washer (stock rail bolt) was hitting the wall of the mounting area of the rail.
This mounting area is at the very front end of the rail, and includes the hole where the bolt is supposed to go. There is a flat depression in the rail for the bolt and its washer, with very little room around the depression after the bolt is in. Because I altered the right guide pin hole for the seat to fit, the boltís hole in the rail and the hole in the car didnít match up exactly. This meant that the bolt would have to go in at a slight angle. It would have worked, had it not been for the washer hitting the side of the mounting area. This prevented me from tightening the bolt.
I realized that if the rail was shifted to the left a tiny bit, the right side rail's hole and its corresponding hole in the car would line up much better. With this in mind, I set out to lengthen the left guide pinís hole. I did not want to lengthen the hole too much, and risk making the same mistake as I did on the right side. I lengthened the left side guide pinís hole slightly, enough to shift the rails over a bit. At the same time, I made sure the left side rail's holes would still line up.
If I could redo the drilling and install, I would just drill each sideís guide rail hole just a bit. Start out drilling only a little, and check to see if it will fit. In hindsight, the seat may have fit with only modification to the left guide pinís hole.
Ultrasield Spec Miata
by Nick Jenkins
Overall I'm pretty happy with my Utrashield seat. It's a 16" version, and it fits the Miata nicely. A 17" would be a tough squeeze. Anything wider wouldn't fit at all. Workmanship of the aluminum shell is very good, and the seat cover is thick and durable. At 6'0" and 210 lbs. (on a good day), I can slide into the seat easily with about half an inch to spare.
I installed the seat by cutting two 17-1/2" straps from 1-1/2"x1/8" steel, then drilling and bolting them to the stock mounting
Straps for Ultrashield Mount
holes in the floor of the car, using the stock hardware. This wasn't actually as easy as it sounds. I first drilled holes in the front of the straps and bolted them to the floor. Because the floor bumps are at an angle, the rear of the straps stuck up about 10 inches off the floor. So I marked the top of the bumps on the straps with a Sharpie, then unbolted them and bent them at the marks with a big hammer. After some trial and error, the straps finally lay flat against the rear bumps with the fronts bolted in. I then had to mark the straps where they crossed the top of the rear bumps (see photo), unbolt them, and repeat the bending process at the rear of the staps until the ends fit flush with the rear bumps.
The shell could then be bolted directly to these straps. Total cost for the straps was around $10, but if you want to spend about ten times that much, Goodwin Racing sells the straps already pre-bent and drilled.
Straps bolted in. Note Sharpie marks.
The shell has no mounting holes, so I made my own by drilling through the shell and straps, after first locating the seat exactly where I wanted it, which was all the way back against the roll bar. I had to do some minor grinding on the right front corner of the shell to avoid interference with the transmission tunnel, but I've heard of other people using a hammer for this, although I'm not sure whether they used the hammer on the seat or the tunnel.
Once the shell is bolted in place, the cloth cover and cushions can be snapped in place over the front of the shell. The back of the seat is uncovered, i.e. bare aluminum, and Ideally you should run some kind of brace between the seat back and the trunk wall, or roll bar if you have one.
The best you can say about the seat is that it looks functional. Seating position is more upright, not because I wanted it that way
Seat shell bolted onto straps.
(you could shim the front mounts to lean it back), but because I needed all the leg room I could get, and seat back travel is limited by the roll bar. It's not that the seating position is bad or uncomfortable, just different. I could use another inch or so of leg room.
Getting in and out of the car takes practice. The sides of the seat are very tall and the steering wheel gets in the way. A quick-release wheel would be just the ticket. Without it, you have to slide up and back, open the door and plant your left foot on the ground, then sort of spin yourself out of the car. The good news is, you can unbolt the Ultrashield seat and bolt the stock seat back in place in less than ten minutes.
Addendum: Although the seat is fixed in place on the steel straps, later on I was able to drill a new set up holes in the straps 2-1/2" ahead of the old holes, to position the seat for a shorter driver. Theoretically you could drill holes every inch or so along the straps to accomodate almost any size driver, although adjusting the seat would take a little more effort than sliders.