So you're staring at a crimped rear Miata Bilstein that you want to get revalved - what do you do? While shortening the Bilstein HD is an option which I briefly outline in the first video, you may need to keep close-to-stock body length for various reasons.
In those cases, you could instead use a few of the motorsports style Bilsteins, such as the aluminum threaded body / Schrader-equipped AS2, the steel body / Schrader-equipped SN2, or the non-threaded / non-Schradered SZ series. Both the SN2 and SZ (digressive) shocks accept a sleve and spring perch for a 'coil-over' spring application such as the Miata has.
Very likely, for current and upcoming FCM Elite builds where we'd normally select the 90-97 Bilstein HD, we'll be switching to the S5Z-KIT and its related coilover hardware (threaded sleeve and spring perch).
To make the spherical-bearing supplied SZ damper work with a Miata shock mount, you'll need to machine the shock shaft to the NA Miata's profile. This modification to the shaft profile will reduce droop about 15-20 mm which is fine as long as the body length is comparable to the stock Bilstein. The S5Z body, from measurements in the Bilstein catalog, looks to be very similar to the standard Miata rear Bilstein HD.
cart.bilsteinus.com/Portals/0...
qty 2 - S5Z-KIT (5" stroke 46mm steel body digressive piston), 16.25" extended, 11.25" collapsed (eyelet to eyelet), $113 MSRP ea.
qty 2 - B4-BOA-0000117 46mm coil-over kit, 2.5 inch (for SZ steel bodies), $62.75 ea.
Total $351.50.
You'll also need to address the fit of the factory Mazda M12 x 1.5 lower shock bolt to the now-larger-ID 1/2-inch (~12.7 mm) spherical bearing vs. the ~12mm bushing. I suggest using some 0.007 inch stainless steel shim stock to do this. I give more info about the shim dimensions and clearances in the third video.