i got off work and had all my stuff ready to do an oil change on the car. i'm actually small enough i could get to the drain bolt without lifting the car (and remember, it's lowered). I got a 19mm wrench on it and couldn't budge it. Ok, no problem, out comes the 3/8 ratchet. no luck. I hate to raise the car and take the right front tire off and get my half drive and about 2ft worth of extenions out. I still couldn't get it to move. really getting mad now, i bounced my weight on the driver until i heard a loud snap sound. i figured i either broke an extension, my driver or the bolt. It actually came out and the oil drained. Of course there was no effin crush washer on it. the bolt was a little rounded and i needed a crush washer while i was out so i went to the parts store. they looked up the part and i was on my way. i just didn't look right to me though so i eyeballed the one i removed with the new one. completely different. it wasn't until then that i noticed it. apparently someone screwed up the aluminum drain pain threads so they ran a LARGE tap into it and put some type of threaded adapter on the original drain bolt. a crush washer wont fit over it that i can tell. i called the guy and expressed my concern and he said it was that way when he bought the car. it didn't leak when i bought it and i've yet to refill it. i'm going to try and coax it back together and see what happens. looks like my only alternative is a new oil pan. i'm not happy about this....
There should be some kind of washer that would work. Maybe you could try one of those rubber washers that is used inside a garden hose end. Or, I remember buying an assortment of oil drain washers form Autozone once. The bag contained several sizes, some plastic and some metal ones. I would try anything I could before replacing the oil pan. I have heard they are a "B" to remove. Also, once removed it may not seal back like the factory seal, so you will be looking at more problems.
When I first got married my wife's car was a Geo Storm. She always took it to a quick lube place to change the oil. I decided I would start changing it and I couldn't get the drain plug off for the life of me. So, I took it back to them and made them loosen the plug but not change the oil. I was pissed, and they did it for me and didn't charge me a penny. I have always changed the oil in all my vehicles since then.
I can fit my arm under the Miata also and get to the drain plug. I still have to lift the car though because there is not enough room under it to get the wrench with extension to move the bolt.
I know you will find a solution. Good luck.
i got off work and had all my stuff ready to do an oil change on the car. i'm actually small enough i could get to the drain bolt without lifting the car (and remember, it's lowered). I got a 19mm wrench on it and couldn't budge it. Ok, no problem, out comes the 3/8 ratchet. no luck. I hate to raise the car and take the right front tire off and get my half drive and about 2ft worth of extenions out. I still couldn't get it to move. really getting mad now, i bounced my weight on the driver until i heard a loud snap sound. i figured i either broke an extension, my driver or the bolt. It actually came out and the oil drained. Of course there was no effin crush washer on it. the bolt was a little rounded and i needed a crush washer while i was out so i went to the parts store. they looked up the part and i was on my way. i just didn't look right to me though so i eyeballed the one i removed with the new one. completely different. it wasn't until then that i noticed it. apparently someone screwed up the aluminum drain pain threads so they ran a LARGE tap into it and put some type of threaded adapter on the original drain bolt. a crush washer wont fit over it that i can tell. i called the guy and expressed my concern and he said it was that way when he bought the car. it didn't leak when i bought it and i've yet to refill it. i'm going to try and coax it back together and see what happens. looks like my only alternative is a new oil pan. i'm not happy about this....
-mikec
Also go back to the parts store & see if they can come up with one to fit. Sound like the previous owner didnt do anything to the car. Keeps telling you it was that way when he got it. I also have a oil pan off a 99 that should fit there both 1.8 end. Also get some Rhino Ramps to pull your car up on makes life alot easier working on Miatas or anything else. You can also go to a hardware store & look thru those pull out boxes they have to see it you can find something that will work.
i thought about running a bead of silicon around the base of the bolt. i guess just anything to keep the bolt from getting up against the aluminum pan.
Steve's right, removing the oil pan is a PITA in a big was. Either the engine has to be pulled or the subframe needs to be dropped. No way to swap it while it's in the car.
I pulled the pan off of my old engine before I shipped it back to Track Dog. I picked up another 1.6 pan to swap it out. Even with the engine on a stand, it was still a pain. There are 2 seals and quite a bit of sealant to keep the oil in.
Is the bolt able to be reused? If so, I would try a washer first. Like Steve recommends, maybe a rubber or aluminum. Just something to seal it up and make sure to not over tighten the bolt.
I've also used blue Loctite as a sealant on an oil pan drain bolt without a crush washer, but not on an aluminum pan. I imagine it would be fine on there though.....just don't use the RED Loctite by mistake!! 
