Not sure on the MIata, but on the RX-7s I have used a core plug (freeze plug) 27 or 28 mm if memory serves. less than a dollar at Napa.
Chris "The Cat Killer" Childs
Angry Sheep Motorsports
810 417 7777
angrysheepmotorsports.com
IT,SM,SS,Touring, and Super Touring
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