NB (M2) shock top on NA (M1) Tokico Illumina

There is no standard recipe for the mod. It's all about fabrication. Please note that this write-up is for reference only. This write-up applies to Ground Control adjustable coilover made for NB and custom spring length. NA stock spring or aftermarket spring not intended for adjustable coilover needs more fabrication since upper spring seat from NB shock top sits higher which means a longer spring required. However, the longer NB spring will not directly fit NA shocks since NB lower spring perch is not flat. It's all about custom fabrication for your specific setup.

This write-up is not a full recipe for my installation and I do not recommend this exact method because I can think of an easier way to do it. Different aftermarket shocks have different shaft dimensions. This adds to the difficulty of having a cookbook recipe. The fabrication involved in my method is complicated and could have been simplified. This was meant to be a temporary setup as I have been planning to weld on a fabricated housing for a spherical bearing, totally eliminating rubber on the shock top mount. If I were to do it again with rubber setup, I would have done it with an easier method without compromise (more on it later).

NB shock top by itself does not come with any rubber cushioning to provide for lateral shock shaft movement throughout suspension travel. Therefore, the NB shock top used in this mod has to be sandwiched by a top and a bottom donut before being clamped down by the NA dust boot metal plate and top metal cover plate. Since the shock shaft will be able to move relative to the NB shock top, there must be some measure to prevent the 2 metal hitting each other. CAUTIONS! If there's not enough lateral movement for the shock shaft thru'out suspension travel, shocks internal seals will fail in short order! I used a metal sleeve bored out to perfectly fit over the Illumina shock shaft while its outside diameter perfectly fits a section of my dead compressor hose (a friend rested a live soldering iron on it). With the outer skin of the compressor hose peeled off, the diameter of the inner compressor hose fits perfectly over the NB shock top center hole.

Please note that the following pics were taken due to requests. I don't want to disassemble my entire setup just to take some pics so that's why I just took off the top for pics. Sorry folks!

Here is the setup with the top cover plate removed. Top donut in my dirty hand. The donut comes from a shock bottom eye rubber sliced into four. Luckily, the centre hole of the donut is exactly the same as the NB shock top centre hole. Keep in mind rubber stiffness determines shock shaft lateral movement capability so therefore again this is another reason I cannot write a cookbook recipe for my mod.

A closer look at the metal sleeve (brass color), and the compressor hose wrapping around it.

An even closer look at the sleeve and compressor hose.

Here is how the top donut fits over the compressor hose.

Here is how the top cover plate fits on top. This is the factory NB top cover plate installed upside down due to the possibility of top cover plate edge hitting shock top edge.

As you can imagine, tightening the shock shaft nut clamps down the top cover plate against the metal sleeve; therefore tightening torque is not as critical to shock shaft lateral movement as in factory NB setup. However, that means that the metal sleeve height relative to the top/bottom donut height is critical. Here is another reason I do not recommend this method due to the complexity. For safety, a second nut (as used by original Tokico setup) is used to prevent loosening.

As promised, here is what I would have done it again if I were to use rubber cushioning. Please note that it may or may not work since I have not done it and the different dimensions of different aftermarket shocks may present a different set of problems.

I would buy the factory top rubber donut and slice it into halves horizontally. I would rest the half with the protrusion into the NB shock top center hole, just like the factory NB setup does. The other half of the top donut would go underneath the NB shock top. The threaded length of my Tokico should fit everything in. I would not need the metal sleeve or the compressor hose. The protrusion of the NB factory top donut does the job, just like the NB factory setup does. The top cover plate would have to stay the right side up to make sure the shock shaft will be centered. In my current setup, this is not necessary because the metal sleeve and compressor hose do the job. I would use double nut to lock down everything. Due to the lack of metal sleeve, NB factory setup requires specific torque on the lock nut to provide corresponding shock shaft lateral movement. Due to the reduction of rubber use in my proposed method, I would have to use a different torque number than NB setup. And I would have to do the lateral shaft movement comparison with a factory NB setup side-by-side to determine the torque number.

That said, since SCCA now allows spherical bearing on miata shock shaft, that means if I am to redo my mod I will definitely use spherical bearings, especially because I have welding machine at home.