If you are a tall person like me and drive a first generation Mazda MX-5 or Miata NA you will be familiar with the problem of finding enough space for your knees in the car. Using an aftermarket wheel with a spacer to bring it slightly closer to your torso and farther away from your knees helps, but might not get you all the way to total comfort yet. I use a 320 mm aftermarket wheel with a 15 mm spacer and while both my knees clear the wheel just fine, my left one occasionally hits the stock armrest and door pull when letting out the clutch.

My car has the bulky armrests that came with the early models and I quite like the way these armrests look, so throwing them out is out of the question. I decided to try and modify the driver side armrest to make it slightly thinner, but preserve the look and functionality. I will describe the process of shaving 11 mm (almost half an inch) off the stock armrest while mostly preserving the original look in this post. You could probably shave up to 15 mm off without running into serious problems. The thickness at the highest mounting point is the limiting factor here.

The tools I used for this are a side cutter, a small dremel tool with a cut-off wheel and a grinding attachment, a file, a knife, some high quality instant adhesive specifically made for plastics and a vernier gauge. You can easily substitute the vernier gauge for a ruler, but I reckon the dremel tool can only be omitted if you use a very small saw and awl and a lot of patience instead.

Measuring and marking 11 mm to cut off. The scratched line marks the cut. The marking goes all the way around.

I started off by ordering a used part because I am a chicken and decided to take 11 mm off because I think this is the maximum amount you can take off and still fit your fingers into the lower handle groove after you are done. To take your existing armrest off you need to loosen three Philips screws. One is at the frontmost highest point behind a small cover that can be pulled off with a flathead and the two lower ones can be accessed freely.

I marked the cutting line by scratching the surface with the sliding calipers and used the knife to cut away the foam behind that line. There is a plastic frame further inside that we can’t cut through with the knife so we cut around it for now and remove any superfluous foam outside our 11 mm border. It’s good to be careful with the knife and not make any blemishes into the material that will be visible later, but small faults can be disguised with a black felt pen.

The cutting begins. Cut all the way around. After cleaning up all the superfluous foam we can clearly see the plastic frame. We don’t need that anymore.

I busted out the dremel tool with the cut-off wheel and took off the protruding part of the frame except for three points. These are the two lower mounting points where the bolts go and the reinforced area between them.

I cut off the plastic piece at the upper mounting point horizontally (looking from above, orientation like in the picture below) because I saw no way to reconnect it to the frame without a lot of filing anyway. I ended up filing this piece into an almost rectangular shape so it could later be fitted 11 mm deeper into the remaining part of the frame and glued back together in a shape similar to two cuddling “L” letters with the maximum amount of surface for the adhesive to stick to. Unfortunately I don’t have a photo of that, so I made a little drawing below. You also need to shorten the cap that goes over this mounting point later. I cut it off at the desired length.

These are the four spots we need to cut and glue. The shape for the top mounting point piece.

Next I cut off the other three areas horizontally just below their surface and trimmed them down with the file and the grinding attachment for the dremel tool to fit back into their old sockets, but 11 mm further inwards. Of course I had to cut out more foam to do that. When trimming always remember that surface area is the name of the game for adhesives, so make sure to fit the pieces as well as possible into their intended places. I ended up trimming, test fitting and trimming again a lot of times for each piece.

All mounting points gone. And ready to be glued back in!

It’s also important to have the mounting points end up at the right depth (11 mm inwards in my case) and I also test fitted their placement on the car and compared it to my unmolested armrest to figure out the correct depth and orientation. The lower mounting points seems to be pointing slightly upwards on the stock part. In my opinion, going a little bit too deep isn’t a big problem because the door cards have a bit of give and you don’t have to tighten the screws with Herculean strength later anyway.

Once I was satisfied with the fitment of all the pieces, I cleaned up the glueing areas with a rag and benzine and put pieces of paper towel into the bolt holes to prevent glue from soiling the nice side of the armrest. Then I trickled instant adhesive onto the areas, making sure to cover the whole bonding surface. You could get fancy in your choice of adhesive here, but if you glue on clean surfaces and use an instant adhesive that is of high quality and made for use on plastic specifically, your armrest probably won’t fall off later. I used Loctite 406 because I had about 15 grams of that sitting around, but its high temperature resistance is completely unnecessary for this application.

After letting the adhesive become solid over night I touched up some small scratches around the area where I did the cutting with a black felt pen and mounted the armrest in the car. It looks and feels just like stock, but I no longer hit it with my left knee. Success!