Folks occasionally ask us, "How do you install sidemount brackets?" Well, there is a good bit of flexibility in how to do the installation of seats with side mount brackets. I just finised putting a FX1 Pro and side mounts into our race car (see above). I can assure you that it takes several careful iterations of guessing, checking, and trying before you get it just the way you want it. How you do it really depends on how much room you're dealing with too. As you can guess, in a Miata it's pretty tight.
If the floor has a flat floor pan everything is too easy. We'll start by making this assumption and will then make some suggestions for addressing situations where it is not.
First, bolt the side pieces to the seat. Don't worry too much about the angle at this point. Set the seat in the car and mark where they touch the floorpan.
Next, lay the floor rails on the floor at approximately the same location where the side rails touched the floor. It doesn't really matter if you face both rails in, both rails out, or one in one direction and one in the other. Likewise, it doesn't matter if you bolt the side rails to the floor rails both on the inside, both on the outside, or a combination. At this point the goal is to try and figure out about where the side rails will bolt to the floor rails. Make some marks with a Sharpie and remove everything from the car.
Now, completely assemble the brackets to the seat. Adjust the position until it you sit comfortably on the floor. Don't get too wrapped up about which holes you use--as long as the seat is firmly attached with grade 8 hardware at all 4 corners, you should be set. Don't try to tighten it too much until you get it where you think it is right. You will probably spend several iterations getting it just right.
When you have it where you think it is going to work, put the seat back in the car. Does it fit? Does it sit firmly or does it rock? If it rocks, is it a lot or something where you can slip a washer underneath one of the floor rails and fix it? Based on what you find here you may need to take the seat out of the car and make some changes. If you do not have a flat floor pan, this might get pretty tricky. We were able to simplify our problem by welding some plate steel into the floor pan to smooth things out a bit. That's not something that most folks would want to do but it's worth considering if you have access or if the task seems like it will be too difficult without it.
When the seat sits comfortably without rocking you can mark where you want to drill. Again, you have multiple options at this point, but as long as you use good hardware in at least 4 locations you should be good. Use a good, thick steel washer underneath the head of the bolt and on the other end as well. Keep in mind that this bracket is what holds you in place in the event of a serious accident. If you are unsure then get some professional assistance before driving the car.
Here is a picture of our installed seat bracket. Click to see a bigger version. To orient you, this is looking back under the seat from the area of the pedals. The blue at the top is the lower edge of the front of the seat.
As you see, the left-hand side and the right-hand side are not even, yet the seat itself is level. You do not have to get the two long front-to-rear pieces level. It's not even mandatory that they be parallel. You have many vertical mounting holes to choose from and you can likely find one that fits even if they aren't level and parallel. This is a really tough installation. Most cars won't be this bad. If you look closely, you'll see that we even had to put a slight s-bend in the vertical brackets on the left side of the picture. If you want to see more pictures of our race car, visit EPMiata.Com. (Not now, after you check out!)
I think you'll have no problems once you get started, but it will probably be more interations of "install, check, mark, remove, change, repeat" thn you might expect. Let us know how this works out for you.