First, both 46mm and 54mm give the same protection so the choice really depends upon spring rate or ride height.
1. For stock springs at stock ride height, use the 54mm progressive.
2. If you want to keep stock springs but get a firmer, sportier ride use one of our high-performance 58mm kits. Call for a consultation.
3. If you have put in stiffer spring or lowered it from stock height, use the 46mm linear.
4. If you have gone stiffer than 250 lb/in front spring, we recommend 36mm linear front with 46mm linear rear.
5. We are finalizing our recommendations for the NC (2006+ Miatas).

I have a 1990-2005 Miata / 2006+ MX5 - what should I do to it?

***The wiper seals on modern dampers make dust boots less critical than in the past. I have direct experience leaving boots off Bilstein and Koni but recommend boots for Tokico and KYB. I don't use boots on any of my cars, which have Bilsteins.

Here are some great resources for information and discussions on suspension, tires, alignment, etc.

• Miataforum.net - Search the archives for 'Lanny alignment' (street-oriented) or 'Icehawk alignment' (sport-oriented)
• Icehawk's FAQ - includes extensive information on setups for Tein products
• Ocean Ragtop's Suspension FAQ - he went from stock to stiff and came back to something in between. Excellent reading!
• Lanny Chamber's original Miata alignment page

Now, I will focus on suspension tuning which is the most important topic (and the least understood). Here is a guide to help you choose what dampers to use for your desired spring rates.

Stock springs, Racing Beat, Eibach Pro-kit, Tein S-tech (lower cost to higher cost)
• Monroe
• KYB GR2
• KYB AGX
• NA Bilstein (aftermarket HD, OE R pkg)
• NB Bilstein (aftermarket HD or Sport, OE Hard S or Mazdaspeed)
• Koni Sport
• Tokico Illumina
• Revalved Bilstein (HD/Sport/Rpkg)

Flyin' Miata (FM) springs and standard Ground Control 375/250 springs with adjustable perches.
• NA (1990-1997 Miata) Bilstein (aftermarket HD/ OE R pkg)
• NB (1999-2005 Miata) Bilstein (aftermarket HD / Sport)
• Koni Sport
• Tokico Illumina
• Koni RACE
• Revalved Bilstein (HD/Sport/R-pkg)

375/250 up to 500
• For the NA Miata, the OE Rpkg / 93LE Bilsteins are very strongly rebound damped and even weaker than stock Showas in compression. They make for a rather bone-jarring ride on rougher surfaces with significant jacking down. Some people find those shocks 'capable' of handling stiffer springs but this is not for the faint of heart.
• The aftermarket NA Bilstein HD (no Sport available for the NA) has less rebound damping at low speed but more high speed rebound damping. This is the shock used in Spec Miata racing. The HD is deceptive - it feels pretty comfortable on moderately smooth surfaces but on larger bumps where you experience high shock velocities, the damping ramps very rapidly and there is EXTREME jacking down! We do not recommend using HDs with stock springs unless you get a revalve, live in an area with good roads or don't mind a sometimes-smooth, other times-rough ride.
• On the NB Miata, the aftermarket HD / Sport valving is essentially the same. If memory serves there's a little more rebound on the Sport but this is minor. The main difference is a Sport has 1" less shock travel which makes it a better choice when running a lowering spring so the spring is less likely to come loose under full droop. The amount of damping present is better suited to stock or slightly stiffer springs but the low speed bump and rebound ramp fairly quickly which gives a 'Sport' character as advertised, but can compromise comfort. Using the aftermarket Bilstein HD or Sport with springs of 375/250 may leave the driver feeling underwhelmed when the suspension is being pushed hard. It'll suffice for a less demanding application and can be revalved if needed to match springs rates/driving preference. One major problem, in our opinion, using NB shocks with NA cars is the longer body length which reduces available bump travel that the Miata sorely needs. Use a soft / short bump stop like a 46/46 with an NB HD/Sport, or a 36/46 for a more performance-oriented application.
• The OE NB shocks are a study in going from comfortable to firmer to very stiff (Showa to Hard S Bilstein to Mazdaspeed Bilstein). We have a video here explaining the differences. The latter two (Hard S and MSM Bilstein) may feel stiffer than the aftermarket NB HD or Sport but actually have less mid and high speed damping which is vital for control on larger bumps. Unfortunately, putting stiffer springs on any NB Bilstein can be a disappointing experience if you live in an area with broken pavement.
• Koni Sport/RACE - For autocross/track purposes, off-the-shelf Sports have enough rebound damping to handle up to a ~550 lb/in spring for autocross/track purposes. This will not be the most comfortable street setup due to insufficient compression damping to prevent jacking down (video).
  In the past, it worked well for me in several years of daily-driving (GC 550/350 on OTS Koni Sports, 1.5 turns front, 1 turn rear). The NB Konis have less rebound damping than the NA and make a reasonable replacement shock. Generally, run any Koni Sport as soft as you can to reduce jacking down and promote grip.
• Tein Flex - generally good road manners, reasonable (though fairly weak) compression force build-up however too much mid and high-speed rebound which causes the car to 'suck down' (jack down) on mid- to high-speed bumps. Overall, good compression damping and design for a twin tube. Adjustment ONLY affects rebound!
• JIC Magic FLT-1 (I have not tested this setup. It uses a twin-tube instead of a monotube as with FLT-2).
• JIC Magic FLT-2 - Very good control, but, again, too much rebound damping. You'll want to run these at nearly full soft for the street, IMO. Lots of spring, not much travel (90mm of shaft stroke vs. 120 for Bilstein HD). More a track-oriented setup.
  Adjustment affects low-speed rebound AND compression.
• Koni RACE - many autocrossers/track folks like this setup, I have also heard some track drivers describe vagueness after a number of laps. Based on the Koni Sport design, it's a twin-tube with shortened body and comes with a Koni MCU bump stop. LOTS of rebound damping so you can throw really high spring rates on this setup.
• Revalved Bilstein - my personal favorite for many reasons. Internal parts are widely available and share components with oval track/off-road racing (durable). Low entry cost, bodies can be shortened to any length needed, revalved to whatever use or spring rates you wish, has great repeatability and control due to the larger shaft and internal design than every twin tube. Comfort can also be extremely good by not using excess rebound damping. A true underappreciated gem in our entry-level motorsports world (circle track racers have used Bilstein products for decades). Due to the steel body it is heavier than aluminum-bodies shocks such as XIDA.

Now that you've got good rubber on the road and are able to control the chassis, you might want to consider:

Chassis stiffening and other ride quality modifications:
• Replacing the factory bushings (no longer effective after ~60k miles) with new ones or a polyurethane bushings
  if you're adventurous. Search Miata.net for more information on this topic. Some links to get you started:
• Stiffening the chassis and adding rollover protection via a roll bar. If you do this, make sure to get the proper tye of padding to prevent head injuries. Again, Miata.net is your friend.
• Stiffening the chassis via frame rail braces (longitudinal) or a butterfly/cross brace. I installed the Flyin' Miata frame rail braces on my '91 turbo car and it made a tremendous improvement in chassis smoothness. I could now hear but not feel the suspension working. Pretty amazing! For a 90-93 with very little bracing, this can be your most effective stiffening mod.
• Engine bay triangle brace. I have never used one, but reliable friends and customers have indicated they do stiffen up front chassis.

Lower shock eye bushing for NA Miata Bilstein - 93LE, 94-97 R-pkg or 90-97 HDp/n E4-GV1-Z044A02
Pro Shocks spanner wrench, for 2.5 inch spring perches, available through Summit Racing, p/n Z902:

How can I touch-up the paint on my Bilsteins?

Plasti-kote Ultra Enamel School Bus Yellow #1116

Robert H. has created an excellent procedure with detailed photos and videos, based on the 'San Diego Miata Club method.'