Replacement
Transmission
Well, my original tranny finally gave up the ghost after several years of good service. Added power from the turbo and my occasional overly-spirited driving style certainly contributed to its failure. 2nd gear was turned to toast. Replacement trannys for the miata can be found pretty cheap. I located one from a salvage yard out of a '95 that had 20k miles.
The '95 unit (on the right) certainly looked MUCH cleaner! It bolted right up fine. All transmissions from '90-'97 hook up exactly the same. I chose the 95 because it was low mileage, good price (about $300 shipped to the door) and I heard that in '94, Mazda made an improvement associated with 2nd gear... the gear that failed on my '91 tranny. So... it's a slight upgrade.
Much more info on miata transmissions and other drivetrain issues can be found at from Randy Stocker's site.
Oh, and for changing the tranny fluid, I use Redline MTL. The drain plug requires either a 24mm socket or an adjustable wrench. The fill plug is square in shape and can be removed with a 14mm (I think) open end wrench.