I recently purchased a 2008 Miata MX 5 NC. I realized right away when I drove it that the shock absorbers were shot. Nosing about on the internet and FB pages I quickly realized that there is a huge community of enthusiastic Miata fans willing to share their knowledge, and most recommend replacing the stock shocks as the single most important modification. Some recommend standard McPherson struts (coilover without adjustment option), and others recommend adjustable coilovers. I opted for the Tein Advance Z coilovers because they seemed to offer a good blend of performance and cost and promised to lower the car by an inch or two.
I placed my order with Redline 360 and received my new Tein Advance Z coilovers in less than two weeks. Pretty darn good service! I then set about finding YouTube videos to help me figure out how to install them. I found one or two that were close, but none quite exact to my situation. The Tein Advance Z coilovers came without a top and required reusing the OEM top and rubber bushings. The instruction that came with the shocks were a bit vague and not clear to me how to install the OEM caps. It turns out to be amazingly simple!
The instructions include an exploded view of the shock assembly. It notes the existence of a circlip located a couple of inches below the top of the shaft with a warning that this part must be installed or terrible things will happen. Looking at the shocks it was not readily apparent to me where this clip was located. I just saw the shaft, a black washer, metal spacer, top silver colored washer and a nut. After removing the nut, washers, and spacers, I observed that the bottom black washer sits on top of a small circlip which is set into a groove on the shaft of the shock. Without this clip, the bottom washer would not have a stop and would just slide down to the bottom of the shaft. I also noted, and this is important because there is no note of it in the instructions or on any YouTube video, that the black metal washer (6 mm thick) has a stepped ID (shoulder on the middle bore on one side). That shoulder must be facing down and slide over the circlip so it cannot expand and fall out of it slot on the shaft. If you were to install the black washer with the large bore facing up, the clip would eventually fail, and your shock mount would fail as well. Make sure you install that washer with the large diameter side facing downward so it goes over the small circlip and holds it in place.
The oem rubber bushings are not the same. One has a slight shoulder on one side that fits into the ID of the oem top plate mount that you must reuse. The rubber bushing with the shoulder goes on top of the black metal washer with the shoulder facing up. Next you put the oem mounting on (make sure it slides over the shoulder on the rubber bushing), then the second rubber bushing, then the silver-colored metal washer (about 3 mm thick) provided by Tein, followed by the top nut. Top nut gets torqued on to about 16 ft lbs. The top of shaft has 10 mm flats on it so you can hold the shaft from turning while torquing the nut.

The rest is pretty much the same as shown on all the You Tube videos that demonstrate installation of shocks on Miata MX 5 NC models.
I will add that as shown in the Tein instructions, I set the spring preload spacing at 40 mm and made sure to tighten down the adjuster ring and lock ring together. When I finished and set the car back down, I measured 116 mm from the front pinch weld jacking point to the ground and 127 mm in the back. The majority of online posts recommend between ½” to ¼” higher in the rear (about 13 mm to 6 mm). My measured difference of 11 mm seemed fine to me and provides a nice stance to the car.
On the first test drive I immediately noticed the difference. The car was much firmer and responsive. I don’t do track days, so I cannot speak to that, but I really liked the new feel and handling. Redline 360 advertised the Advance Z as being a bit more aggressive than the Street Basis Z, but still comfortable for daily driver. I agree completely!
Overall, I am very happy with this upgrade and the price for these shocks was pretty reasonable at just under $600 with freight and tax.

I am enjoying the Miata roadster and planning to add a Hard Dog M3 Sport rollbar as the next upgrade! My wife insisted it get a roll bar, and who am I to say no to that!