Project BMW E30 318iS - Custom Racing Seat Mounts October 11 2013

It is a little known fact that the diminutive BMW E30 318iS can be configured to have seating for 7. I was driving away from a Craigslist purchase with two racing seats riding along in the back seat. They were Italian OMP "Record" seats that were expired per FIA regulation and were no longer of any use to the owner. They had a couple of tears in a few places, but were in otherwise great condition. The seller included the accompanying mounting brackets to sweeten the deal. The seats bounced around the back of the car as I drove back home. With the windows open and the 16 valve 4 cylinder at full song, I couldn't help but be in a good mood. Our project car was going to take one step closer to being a track car.

Racing seat with mounting rails installed. Mounting rails do no line up with factory seat mounting points.

Excited to mount the seats, I got the tools out and got to work. A test fit revealed that it wasn't going to be a bolt-on affair. With the mounting bracket feet positioned outward, the footprint was too wide to pick up the factory seat mount points. With the feet pointed inward, the footprint would be too narrow. The factory mounting points wouldn't match up with the seat bracket mount slots either.

Mockup of aluminum bars under seat mount rails to verify fitment. Both flanges of the seat mount rails are pointing outboard.

With one foot pointing inward and the other foot pointing outward, the spacing between the seat mounts and the factory seat mounting points still wouldn't work. However, with a custom seat bracket, it could. I bought 4 lengths of 1"x2"x18" aluminum bar stock and went to work. It took a bit of experimenting, but after I got the driver's side figured out, duplicating it on the passenger side was easy.

Top view of aluminum bar stock placed over the locations where they will be mounted to the chassis.

The idea was to mount the two pieces of bar stock to the car via the factory mounting points. Counterboring the mounting holes would make the factory fasteners flush, and we would be left with two strips of real estate in which we could add all kinds of tapped holes to mount our seat brackets.

Aluminum bar stock in the chassis with dial calipers and a tape measure resting on it.

We measured the distance between the holes very carefully and then scribed drilling locations onto the bar stock. Since the holes that we would be drilling would be only .020" larger than the fasteners going through them, we would have to be fairly precise for everything to go together without issue.

Pilot drill in a drill press about to drill a hole in the bar stock. Intended hole location is marked with red crosshairs.

With the holes scribed and highlighted with red, we brought the aluminum bars to the drill press. Since it is easier to line up a small drill bit precisely, we lined up the work piece using a small bit as a guide. Touching the bit to the mark and turning the drill head by hand left a small indentation that confirmed we were spot-on. 

Full size drill bit in drill press drilling hole in aluminum bar stock to full diameter.

With the drill head aligned, we swapped in our full-size bit and started boring. With such a large bit, it is important to use a slower drill speed. We pulled the bit back out every few seconds to clear the chips and we squirt oil on the bit every few seconds.

Counterbore bit chucked up in the drill press counterboring the aluminum bar stock.

Next, we fit the 1" counterbore and started making a recess for the fastener heads to sit in. With such a large diameter counterbore, the key is to keep a constant stream of oil on the counterbore while it is cutting.

Completed counterbore in aluminum bar stock with bolt inserted in the hole to verify that the head of the bolt fits within the counterbore and is not proud of the top surface of the aluminum bar stock.

A test fit verifies that the proper counterbore depth has been reached.

Aluminum bar stock with all counterbores complete and bolted to the chassis.

The adapters were then test fitted to the car. Everything bolted up with no issue.

Driver side and passenger side race seats placed in chassis.

With an awesome flat surface to mount to, we put the seat in the car and played around with the positioning. We looked at minimizing how much the seat would block the driver's vision and we looked at moving the seat as inboard as possible to give us more room for the future cage. Sitting in the seat gave us an idea of how much legroom there would be.


Aluminum bar stock with mounting rail hole pattern traced onto it with a fine tip felt marker.

Once we were happy with the positioning, we traced the footprint of the seat mounting brackets onto the mounts. 

Drill press drilling a hole into the aluminum bar stock.

Back to the drill press.

Tapping tool in hole in aluminum bar stock.

With the holes drilled, we tapped the holes. We chose M8 threads. We avoid mixing metric with English fasteners on cars if we can avoid it for ease of maintenance. 

Inboard seat mount rail mounted to inboard aluminum bar stock.

We installed the inboard seat bracket and then the seat.

Outboard seat mount rail holes do not line up with the predicted hole locations that were previously traced.

This is the outboard seat bracket. At first, we were going to use the marks from the test fit to drill up the seat bracket mounting holes. However, better sense prevailed. We decided to bolt in the inboard side and see where the outboard side ended up before drilling. The act of bolting in the inboard side was enough to shift the outboard side slightly. You can see how much it would have been off by.

Stainless steel socket head cap screws attach the seat mount rails to the aluminum bar stock.

We put in some stainless steel cap head screws and bolted the seat in. We chose stainless because our door leaks water in the rain.

Finished project with both driver's and passenger's seats installed.

The seats look great and give the car a race car feel. They are extremely supportive during lateral G maneuvers and are surprisingly comfortable on long trips. The seats sit low in the car, which serves to drop the center of gravity of the drivers and passenger significantly. The seats are also shifted slightly towards the center of the car, increasing room for the roll cage, decreasing the polar moment of inertia, and improving side impact safety. We have also shed an estimated 20 pounds from the car.

Driving the car with racing seats is a completely new experience. Even if the seats don't do anything significant performance-wise, it's like the Batman costume that a kid puts on. The kid knows he's not Batman. His friends know he's not really Batman, but who cares? The kid in the Batman costume feels like he's Batman. Every time I get in this car, it feels like a legitimate track machine, and even loping around the neighborhood feels like I'm puttering around the infield on my way back to the paddock. Long stretches of boring highway become Mulsanne straight replicas.

We can't wait to test these seats out at the track!

Model wearing Maxine E30 in Red.

This is our tribute shirt to our project car. If you like what you see, this shirt and others are available in our store.