Do yourself and your cage builder a favor by gutting your own doors. It saves you money and saves the fabricator time.

Without a doubt, the biggest expense in building a racecar from a production car is having the roll cage installed. That step in the process involves prepping the interior, removing the dash and gutting the doors to accommodate the “Petty bars” or the pyramid X designs.

Some steps are best left to the fabricator, but gutting the doors is a process you can do in your garage at home, which saves you money. It also lets your fabricator focus on the important task of building the cage.

The tools you will need, if you don’t already have them, likely won’t cost as much as the labor the fabricator would charge. You need an angle grinder, and several cutting wheels. It’s probably $50 worth of stuff at Harbor Freight. You probably already have most of the rest of the tools you need: Sharpie, straight edge, ball peen hammer, decent file and a shop vacuum.

You’re going to need to buy the “value pack” of cutting wheels. I think I went through seven or eight of them. You’ll also need safety glasses, hearing protection and an N95 mask at minimum. You should be used to wearing those by now, anyway, right?

You’ve got to remove the window and regulator first. I didn’t document that process because, well, I forgot. My bad. But on a Miata, and probably most cars, just remove every nut and bolt you can find and eventually it comes apart. Some of the fasteners that hold the glass to the mechanism might only be accessible when the window is in a specific position within its travel. If you have power windows, keep the battery and the switch connected so you can move things around.

Once you get the regulator and glass out, you can take the Sharpie and the straight edge to draw the lines you’ll be cutting. Yes, a straight edge. You want to build a nice racecar, don’t you? Where the door bends toward the outer panel, you can hold the Sharpie and your thumb and forefinger in such a way that you can “trace” the contours of the door with your middle finger. That photo below didn’t turn out so well, so I hope the description works. Half the reason I wanted to cut the door up is so I could get rid of a lot of the sealer used to hold the plastic film that keeps out road grime. It’s messy and it gets on your clothes almost every time you open the door.

As you cut through the steel, it’s easier to let the rotation of the cutting wheel pull grinder along, like a chainsaw. If the sparks fly into the interior or hit the windshield, cover them with a canvas tarp or something that won’t melt or catch fire. Those hot little sparks can speckle a windshield and make it look like you drove through a sandstorm. They can do similar damage to a vinyl dashboard. These photos walk you through the process.

Using a Sharpie and a straight edge, draw lines where you plan to cut. It’s better to have a plan in place before you begin cutting.
Where the door bends toward the outer panel, you can hold the Sharpie and your thumb and forefinger in such a way that you can “trace” the contours of the door using your middle finger as a guide.
It’s much easier to cut hard corners with a cutting wheel than it is to cut rounded corners, so do that where you can. They clean up nicely in the end.
Sometimes you have to cut a rounded corner, and you just have to move the wheel along the line slowly until it cuts through the panel. If you rush, you get sloppy results.
You can get a grinder like this from Harbor Freight and a value pack of cutting wheels for less than $50.
Not having the guard in place on the grinder lets you position however you want. It’s not OSHA-approved, but it works. I also like to let the spinning wheel pull the grinder along, like a chainsaw, rather than pushing against the rotation.
The lower portion of a door on a Miata is a single panel of steel, so the wheel cuts through it pretty easily. Look at the pretty sparks! Cutting steel is fun!

This video takes you quickly through the process of cutting out the interior door panel to facilitate the installation of a roll cage.

In about 15 minutes, the inner door panel comes out, usually in one piece depending on how you draw your lines.
Once the inner panel is cut out, file the edges smooth so they don’t catch on clothing or an expensive race suit. This view shows how dirty the inside of the door can get after 91,000 miles. That’s going to need a good cleaning. Also be sure to vacuum the bottom of the door of all the grinder dust and steel shavings. Obsessive compulsion is not a disorder when building a racecar.
If you really want to add a nice finishing touch, you can wrap the edges of the door where you cut with Trim Lok.
This particular car came with power mirrors, which is a great thing to have in a racecar. I trimmed off all the wires for the power windows, the speaker and the power locks, and wrapped what remained in plastic conduit and zip-tied it to the door bar so everything stays put. Still need to do a little cleaning in here. OCD is a wonderful thing.
With all the road grime cleaned up, we’re ready to go do the other side.

 

This video shows a time-lapse capture of gutting a door. Because of the rotation of the grinding wheel, sparks were flying toward the dash panel and the windshield, which we covered with a canvas tarp. Those hot little sparks can speckle a windshield and make it look like you drove through a sandstorm. They can do similar damage to a vinyl dashboard. A welder’s jacket is a good idea, too, if you have one.

Image courtesy of Brett Becker

4 COMMENTS

  1. Great article. Two additional recommendations… I removed the doors to make them easier to work on (at this point it’s just 3 more bolts), and a jig saw in the deeper sections made gutting the doors go a lot faster.

  2. You only need a “value pack” of cutting disks if you are buying from Harbor Freight. If you use high quality cutting disks you will easily be able to do the doors with 2 disks max of you are being rough on them, or 1 disk if you use them as intended.

    For example, with 1 disk I gutted 2 doors and had enough meat left to cut a hood for louvers. Now…miata has aluminum hood so it doesn’t require much of a cutting disk.

    I like the thin red ones from Home Depot…everything cuts like butter with that disk.

  4. When I gut a Spec Miata door I leave enough metal to mount the upper part of the inner Door panel held in by two or three sheet metal screws. Also I glue “edge trim” to the cut parts after filing sharp edges. Also, I leave enough steel on the lower edge to retain strength. No sharp corners. I welded a bracket to the brace bar to attach the opener to using the stock wire rod re bent to fit. And yes, take the door off to do this job.

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