Replacing the Miata Radiator

Place your basin under the drain hole. You may want to put the car on ramps or jack it up since most catch basins, once in place, leave little room for you to open the drain. Using a Philips head screwdriver, open the drain and the coolant will begin to run out slowly. Next, remove the radiator cap to allow the coolant to come gushing out.
Disconnect the upper coolant hose and overflow hose. The overflow hose is right next to the radiator cap and is easily pulled off. The upper coolant hose will require loosening the hose clamp with pliers. Try not to damage the hose clamp - you will want to reuse it.
Remove the plastic air intake. It's held in place by a large hose clamp on the air filter housing and another large hose clamp on the throttle body, and a 12 mm bolt toward the middle (see photo). Unscrew both hose clamps and remove the bolt. Also remove the two chrome plated acorn nuts that hold the chrome pipe onto the valve cover. There is one more hose clamp that must be opened near the throttle cable. Lift out the air intake as a unit (see photo).
Disconnect the electrical connectors on both fans. Pull the wire harness away so the radiator is clear.
Remove the two 10mm upper fan bolts from each fan.
Loosen the two 10mm lower fan bolts from each fan.
This is the most difficult part of the job. First, access is difficult. Second, the bolts are probably corroded and will be hard to move. The good news is that they don't have to be removed completely in order to remove the fans. Take a look at the photo of the fans - you can see at the bottom that the bolt holes are slotted and the fans can be lifted out once the bolts are loose. You can get to one of the bolts through the trap door. The photo (looking down toward the bottom of the radiator) shows where the four bolts are. Once the bolts are loose, the fan simply lifts out of the engine compartment. Make a note of which one is for which side. The fit is slightly different for each fan.
Remove the lower coolant hose. Its easiest to remove one side only - the side connected to the radiator. You may need to access the hose clamp via the trap door.
Remove the two radiator bolts - one in each corner. Note: Some cars may have two additional bolts at the bottom, as well.
(Note from Stephen Shaper: On the passenger-side fan, each bottom bolt also attached a bracket for the oil cooler line/s. These bolts could not be easily accessed/removed--indeed, the inboard one was impossible--without first removing the splashguard. And removing the splashguard was quite easy ±5 minutes/8 bolts.)
Carefully lift out the radiator from the engine compartment. Again, be very careful not to damage the cooling fins - they are very delicate.

OK, now that you've got it out, you can continue with whatever job you needed to remove it for. While its out, check the condition of the radiator and the hoses. Replace any hoses that are dried or cracked.

Installation

Installation is the reverse of the removal. The radiator bolts get torqued to 14-19 ft-lbs.

Mix up a solution of 50% coolant and 50% water. If you are in a climate that will not drop below freezing before your next coolant change, you may want to change the mixture to 30% coolant and 70% water for more effective engine cooling. Refill the radiator with the solution and start the engine. After warming up to operating temperature, allow the engine and radiator to cool completely. After it cools, top off the overflow container with your coolant mixture until it reaches the "FULL" mark.

Be careful with your old coolant. If any dripped on the ground, be sure to hose it down. Coolant is highly toxic to animals. Remember to dispose of your old coolant responsibly!

Note from Jon L. Jacobi

Just changed my radiator and have some additional advice to add to the garage article. Mainly just to remove the bottom tray to reach the bottom fan nuts. This is almost a necessity on an AC car due to the pipes running behind the radiator and said nuts but it just makes everything easier and only adds a few minutes to the process.

Note from Nicholas J. Carroll

I've found that mine (and probably a number of other after market radiators) did NOT use the stock railings that the stock radiator slides out of. Instead, they use integrated rails.

It's not quite as simple as that, though. Once these rails have been removed from the car (an easy enough task, once you've got the radiator out), some hardware may need to be transferred from the old rails to the new rad: one rubber and brass bushing and one pin and rubber bushing from each side.

To remove the rubber/brass: pop the brass cap out, then work the bushing out of the metal bracket. If the rubber is difficult to move, lubricate it with some power steering fluid.

To remove the pin/rubber: This is easiest with circlip pliers or the flat tip of a certain can opener on a Swiss-army knife...remove the circlip from the small end and pull the pin out.

Installation is, as always, the reverse of removal (but on the new radiator!). You'll really want those circlip pliers to put the circlip back on...anyone who has already rebuilt their clutch master cylinder should already have one ;-).