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Picture of How to make a custom seat bracket for your car
There are many reasons for replacing the seats in your car, ranging from spills and tears in the current seats, to full-blown race car conversions.  Whatever the reason, aftermarket seats can be pricey, and the pre-made brackets that manufacturers sell with them are pricey as well, and that's IF they even make a bracket for your car.

So when I set out to fit some aftermarket seats to my 1989 BMW 325i, I decided to make my own brackets.  The level of complexity can vary depending on whether or not you want to add adjustable slider brackets, etc..

My seats didn't have sliders, so I decided to add some aftermarket slider brackets, which upped the complexity, but I'll just go over the basic idea here.

Necessary parts/equipment:

-1/4" x 1-1/4" mild steel bar stock (~5-10' depending on your setup)
-measuring tools (you could get by with just a measuring tape, but something like a caliper comes in handy as well)
-metal working tools (bandsaw, chop saw, angle grinder, drill press, etc)
-welder

I built my brackets at the TechShop, in Menlo park, where they have all the tools and equipment I needed, but if all you have is a hacksaw and a cordless drill you could probably do everything but the welding.  If you want more info about TechShop, check out their website, www.TechShop.ws
 
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Step 1: Remove your current seat

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The first step is to remove your current seats.  Typically this is as simple as taking out four bolts and removing the seat.  Usually you will not have access to all four bolts without sliding the seat forward and back, and if you have power seats or heated seats, there will be electrical connections underneath that need to be disconnected before the seat can be removed.

Step 2: Measure

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Once you have your seat removed, you can take some measurements to establish the 2 bolt patterns that you are trying to adapt.  The car will have (most likely) a simple 4 bolt pattern, and the seat will have (most likely) a different 4 bolt pattern.  The goal is to make a bracket that contains both bolt patterns.

Once you have the measurements, it helps to overlay them on top of each other to visualize how the two will fit together.  I used Autodesk's AutoCAD Mechanical 2012 for this step, as it is readily available on computers at the TechShop.

Step 3: Design

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With the measurements taken, you can start designing the bracket, which in most cases is as simple as a strip on either side, each with two holes to match the cars bolt pattern, and two more strips perpendicular to the first two that adapt to the seat's bolt pattern.

Keep in mind when designing your brackets that you'll need to be able to assemble it, which means there needs to be clearance for bolt heads, nuts, wrenches, sockets, etc.  I designed my brackets using Autodesk's AutoCAD at the TechShop, which made things a lot easier.

The two bolt patterns for my bracket were so close to each other that I had to notch the opposing sets of strips to make room for the bolts.  I did this with an angle grinder and some patience.

Step 4: Fabricate

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Once you have the design all laid out, it's time to cut the necessary pieces and drill the holes there they need to be.  It's a good idea to make the holes just a BIT larger than necessary to allow for a little wiggle room upon fitment.  I used the horizontal bandsaw at TechShop, but there are a number of tools you can use to cut your pieces, such as a metal chopsaw, a miter saw with a metal cutting disc, or even a hacksaw.  Any drill or drill press should work as long as you mark the holes and make sure your drill bit doesn't wander.

After testing the fitment, you're ready to weld.  I usually put a few tack welds on there and then go back to recheck fitment before fully welding.  I used the TIG welder at TechShop, but if you have a MIG welder at home that would work too.

Step 5: Finish/Install

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I chose to powdercoat my brackets because TechShop has a powdercoating setup with a curing oven.  If you're not a TechShop member, you can always just use some spray paint.

Now install your finished bracket and enjoy your new seat!