This is a measure twice, cut once job. Leave yourself plenty of time to get it done. Be prepared to be without your car for a couple of days.
All holes are drilled 1/64" over the specified bolt diameter. My assembly shows no lock washers since I'm going to take it back apart for painting at the end.
Aluminum race seats require a seat back brace to be safe. I'm not covering that here. Install the brace per manufacturer directions. If you don't have a roll cage or roll bar, I'd recommenced a composite seat.
Materials and tools:
3" wide 1/4" Steel Flat Bar. 32" long minimum. (you could use structural 2024 aluminum if you need it light)
1.5" angle flat bar
Misc. bolts, nuts, washers, and lock washers (called out below)
Loctite Blue (Optional, I'm running a very stiff vibration heavy setup)
Cardboard for layout templates
Drill Press (you could use a hand drill if your a masochist)
Hand drill
Center punch
Painters tape
Angle grinder for cutoff and edge cleanup
Standard hand tools
Welder (totally optional for mounting spacers)
Beer
Preparation:
Remove the stock seat and unbolt the seat sliders.
Clean the seat sliders. Your going to be handling these a lot and they will be disgusting. I wiped them thoroughly with shop towels and vacuumed them.
Mount layout:
Cut and debur two 15.5" lengths of 3"flat bar.
Use the flat bar pieces to make two matching cardboard templates.
Bolt the seat sliders back into the car hand tight.
Fit up your cardboard template. You want the inside edge flush with the inside edge of the slider. Forward aft position is a matter of preference. I set mine with a little more than an inch of bar behind the rear rail bolt. Bolt your template to the sliders using 5/16 x 1" bolts. Move the sliders through the full range of motion and verify clearance. Test your seat on the mock-up. I have about 1/16" gap between the front of my seat and the front of the flat bar. This positioning puts the back angled portion of the seat very close to your rear 5/16" bolt. Mark a notch location for your slider lever.
Remove your sliders and the cardboard. Transfer your holes and and notch mark on the inboard sliders to your flat bar. Center punch and drill the holes. Cut out the notch with the angle grinder. Bolt the flat bars back into the car. Set your sliders to the same position left and right.
Spacers:
Your going to need to get the flat bar up a bit off the rails for clearance. Two 5/16" flat washers are perfect for this. There's a long span between these. I wasn't comfortable with that so I welded in a center blocking piece. You could plug weld a two washer stack if you wanted. You could also use washers and JB weld them in. If your using aluminum, the washers and epoxy are your only choice unless you TIG weld. These pictures show the spacer washers, gap, and added spacer block. The spacer block is only attached to the flat bar, not to the slider.
Seat Layout:
A friend helps with the next step. If you don't have any, I'm sorry. It can be done solo. Put your seat on top of the sliders. We're going to set the fore/aft position of the seat on the slider. Center it up left to right and then move it backward until it's up against the bolts sticking up through the bar. Wiggle it until the position is just where you want it front to back. As I said, I set the front lip of the seat about 1/16" ahead of the front edge of the bar. Make sure the seat is square on the rails. Using a Sharpie, mark the front and rear edges of the flat portion of the seat pan. Take the seat out of the car.
Now, more cardboard. We're going to make a template of the flat portion of the seat pan. Make the template so that it sits in the pan like a seat cushion. it should be just small enough that is doesn't touch the welds.
Move your template into the car and position it using the marks you made using the seat. Put your washers on it as pictured. They should be fully on the cardboard (which will clear the welds on the seat) and mark the holes on the cardboard. Make sure your marked holes are well clear of the slider below the plate.
remove the cardboard and punch out the marked holes. Next, attach your cardbourd template to the bottom of the seat and transfer the holes to the seat. Drill the holes for 3/8" bolts using a hand drill.
Discard the template and put the seat back in the car. Line it up as discussed earlier and mark the holes on the slider flat bar. Again, use a friend to help if at all possible.
Mounting the Seat:
Remove the sliders from the car and unbolt the flat bars. Drill the flat bars for 3/8" bolts.
Bolt the seat to the flat bars. Use 3/8" x 1" button head cap screws. Leave the sliders off for now.
Reinforce the Seat Mount:
Four bolts through the seat pan are not adequate to support an aluminum seat. Were going to add an angle brace at the front to make the connection solid. Were going to cut 1.5" structural angle aluminum down to 1" x 1.5" and make two 2" lengths.
It should look like this. Mount it in the car and make sure it fits. If you used aluminum flat bar, you're done! Drink some of that beer! If it's steel, take it all back apart and paint it before final install. You can drink the beer while the paint dries.
On an NA you'll need to come up with a mount for your seat belt.
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