C5 Install of ST3 Seat
 

Below is a write up from a recent C5 customer installation:

Track day (HPDE) seats for C5 Z06
I have spent a long time trying to find the best seat for my C5 given my strict requirements.
I needed seats that:
1. Could be used for HPDE track use with a 5 or 6 point harness
2. Could work for 2 different height and size drivers (needed to be adjustable)
3. Would retain the stock seat belts so that it would be street legal
4. Would fit a 240lbs, 5’11” driver in a C5 with stock interior

After a long search, which included test fitting in other cars, I settled on the Marrad Industries ST3 seat which met all of the above requirements. http://www.marradind.com/proddetail.asp?prod=ST3

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I will try and describe my install process below in hopes that it may help others.
I am not a professional mechanic and none of the instructions below are certified safe so proceed solely at your own risk. I am not responsible for any issues or accidents to you or your car as a result of this writeup.

I ordered the revised ST3 in August of 2018. I also ordered the Ultra Low Seat Mounting Bracket (for more head room), The double locking Sliders (to adjust to multiple drivers)and the side seat brackets. I did not order the Harness Mounting from Marrad initially because I bought the 6 point harness and harness bar from a fellow C6 owner upgrading to a C7. (This harness bar turned out to limit seat travel rearward. I had to order the Marrad seat mounting bracket with integrated harness mounting to allow the seat to be adjusted for longer legs)
http://www.marradind.com/proddetail.asp?prod=C5UL-RHM

My Install process experience
Removing the stock seats.

Remove the 2 plastic covers over the front 2 floor studs
Remove the nuts on the front 2 studs
Slide the seat forward in order to gain access to the rear 2 nuts
Disconnect the car battery at this point (to prevent any electrical shorts or accidental airbag deployment)
Remove the 2 nuts from the rear studs
Disconnect the 2 electrical connectors under the seat
After lifting the seat off the studs I removed the seat by angling the seat back towards the center of the car and moving the seat bottom out of the door first.

Bolting the ST3 seat to the seat side rails, mounting base and sliders
My seat base mount, slider and side rails came already assembled
Remove the stock seat belt receiver from the stock seat bracket and attach to the ST3 side rail with a 3/8” x ¾” grade 8 bolt (purchased from local hardware store) The stock nut can be reused.

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If you will be using a 5 or 6 point harness, now is the time to attach the sub belt and lap belts bolted to the sub belt bar included with the seat mounting base.
(this took several iterations of testing to determine the length the belts needed to be to accommodate both drivers)
I guessed at the length, then bolted the seat to the side rails, sat in the seat, tested the fit of the belts and readjusted as needed. (installation should be per your harness manufactures instructions)

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Because the ST3 seat bolts to the side rails on the side of the bracket it is easier to test fit the driver to the seat prior to installing in the car to adjust seat angle to personal preference.
Once the seat is bolted into the car the tunnel side bolts are not accessible.

Dealing with the 2 rear seat mounting studs that are too short (couple options for this step)
Option #1 Marrad Industries sells stud extenders since the rear stock studs will not be long enough for the seat mount plus the harness mount plus the upper harness bar brace.( basically you will have 2 or 3 pieces of steel sandwiched together to be bolted to the floor.)
And the stock stud closest to the door will not be long enough to get the nut on) http://www.marradind.com/proddetail.asp?prod=M18SEb.

Option #2 Remove the stock 2 rear studs and replace with longer bolts

I chose option #2. (I used 7/16” x 2” grade 8 bolts to replace the studs)
The studs are spot welded to the bottom of the car so I chose to grind the head of the stud off under the car. They can alternately be removed by supporting the area under the car around the stud and using a hammer to beat the stud through the floor.
The area around the stud under the car must be adequately supported in order not to damage the floor of the car while beating on the stud.


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This is how the stud heads look under the car.


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this is how it looks with the stud heads ground off

Now that the studs are out, I got a piece of 1/8” x 2” x 20” steel (from local hardware store) to us as a reinforcement bar under the car for the back 2 seat stud replacement bolts.
Mark the 2 hole locations on the reinforcement bar by holding the steel bar under the car and having a helper mark the hole locations from inside the car.
Now 2 holes have to be drilled accurately in the steel without the bit walking. The easiest way I found to do this with only a handheld drill is to drill a hole of the same size in a scrap piece of steel and then clamp the scrap piece of steel to the steel to be accurately drilled and then place the two in a vise and drill away

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here in the picture is the scrap piece of steel used as a drill bit guide clamped to my reinforcement bar so that I could drill an accurate hole.

It is also best to drill one hole in the steel support then place it under the car and check the marking for the second hole prior to drilling. (this I learned from experience)
Then repeat the process to drill the second hole.
The steel support you just made needs to be painted to prevent rust.

Installing the ST3 seat in the car

The ST3 installs easiest by angling the seat back into the car first then placing the mounting bracket over the front 2 studs.
You will need a helper to hold the reinforcement steel plate, that you made, under the car with the 2 new bolts (locally sourced) are inserted through the accurately drilled holes.

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On the inside of the car the 2 bolts should be visible. On the tunnel side of the car a lock washer and nut (locally sourced) can be used to secure the mounting bracket and harness bar (if harness and mounting base are not all one piece) On the door side the bolt from underneath the car will secure the mounting base, Harness bar (if not integrated) and upper harness bar brace. (the stock studs would not have been long enough to attach all 2 or 3 of these items).


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this is the view from the back of the seat looking down to the floor


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seat installed with harness

Installing the shoulder harness belts

I installed the shoulder belts to the desired lengths and attached to the upper harness bar. (installation should be per your harness manufactures instructions)

Reconnect the battery

Went for a test ride

Observations from the install:

1. Had I known from the start that I would have limited adjustment of the seat rearward with the other manufactures harness bar I would have ordered the Marrad combination bracket/harness bar initially.
This combination bracket/bar with stud extenders would have eliminated the need to replace the stock studs and made the install easier.
https://cimg9.ibsrv.net/gimg/www.cor...137d723e85.jpg
this is a comparison of the integrated seat mount and harness bar (foreground) and
the seat mount and other vendors harness bar. (background) notice that the seat will hit the harness bar about 2 inches earlier than with the integrated mount in the foreground.
also with the integrated mount in the foreground there is only one piece of steel to bolt to the floor vs 2 pieces of steel stacked on top of each other in the background.

2. The ST3 Race seat with the ultra low mounting bracket allows me to sit lower in the car giving me extra head clearance with my helmet on.

3. I have not been to the track yet with this new seat install but from all the spirited driving around town the new seat is very stable, fits tight in the car but still allows complete seat adjustment.

4. In my C5 Z06 I have no warning lights on the dash from this install.

5. The seat install was very straight forward after I had all the right parts. So I hope that this write up will help with the planning and install process for others.

You can see the rear harness mount from another company in the top of the photo. Notice that it reaches its full height in front of the rear floor studs. Ours is on the bottom (in this case a welded one piece unit with the ultra low rear bracket) and you can see we have it designed to allow for at least 2 more inches of legroom, since any seat sitting low enough would hit the unit in the upper photo.

The M18 stud extenders were designed for the aluminum rail kits that sit above the carpet. However, they can be modified to solve the problem of the short rear studs in the C5 floor. Normally they install head down, but below I cut the end off of an extender so that it is now threaded all the way thru. If you drill out the holes in the mounting bracket and the foot from the harness bar strut to M18 you can thread this the opposite way and the head will become the head of a bolt tightening everything down.

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