Talked with multiple people at the DC Pro Solo that have had their transmissions blow up. Granted these are autocrossers so the usage is probably a bit more aggressive than normal, but it seems most of these cars have failed within 10k miles of use and I've never heard of previous gen cars breaking a transmission with stock power levels.
Yes, it's a concern and it's not just the autocrossers. I don't know if it's a bad set of gears or a design issue. They have very little fluid capacity, which doesn't help. Interestingly, the Fiat has a different trans.
You could break the original NC 6-speed in about 200 laps with a stock engine. The 3-4 shift fork would snap.
Being a Miata, though, you know the R&D team is very aware of it and on it. Especially with the Global Cup series starting. We found out about the NC weakness with the first three prototype MX5 Cup cars in 2006 at the 25 hours of Thunderhill, and the news had made it all the way to the top of the technology pyramid at Mazda before the checker was thrown.
From some pictures and statements people far more educated than I on transmissions, it looks like second gear has a tendency to explode. It's hard finding facts through all the conjecture, the pictures definitely show a gear that decided it didn't want to stay one piece anymore. It's been interesting to watch, over on m.net I guess it happened to a member or members family member while he was test driving one.
Edit to add. Facts thread with Pictures They are trying to compile actual facts and issues into one thread. An industry 3rd party tester has offered to investigate with broken parts or up close HD photo's. Someone has spoken to Mazda development, boilerplate response but they're trying to figure it out.
So far, mostly second gear, but there are 2 confirmed cases where it was 3rd gear.
I saw someone selling a fully prepared, don't remember the class, but he was advertising it with a brand new transmission because his destroyed itself.
Wonder the number of people having issues on street tires versus r compound rubber
I hope they fix it soon. I'll be interested in getting the RF as my next DD when the time comes.
Any idea on how many cases there have been? Is it possibly related to early production cars?
They're all early production at this point :)
Mazda will be on this, have no doubt. Factory engineers are no dummies. There will be fires lit under derrières and Mazda will be analyzing the parts that came back. A self-appointed forum organizer pestering Mazda for answers won't get one because it's still in R&D.
In reply to captdownshift:
Street tires, but today's good street tires are comparable to last decades r comps.
Not trying to be negative, but if the original NC transmission had issues, when did they resolve the problem? Was it when it became common with the S2 RX-8 for '10?
WAAAAAAAAYYYY back when I first started driving, both my dad and grandad advised me to avoid buying the first production run of a car model design change; to wait for the second production year. This is a perfect example of why. There are always unknown issues with a new design, and the worst issues are usually fixed by the second year.
Snrub wrote: Not trying to be negative, but if the original NC transmission had issues, when did they resolve the problem? Was it when it became common with the S2 RX-8 for '10?
It only showed up under hard track use as far as I know. I'm not sure when the design of the shift fork changed, I'm sure a thorough perusal of parts fiches would tell.
It wasn't until the second year of production that the engines started to fail 
Another example of why not to buy a car in the first year of production.
I had an ND in the plans for later this year.....not now, I'll wait a year or two.
NickD wrote: Another example of why not to buy a car in the first year of production.
Yup, and if you'd waited for the second year of production for the NC you would have got the bad engines
It's not a great thing, that's for sure. But Mazda's on it, and there will be a fix. If they're so fragile, you should be able to pop one pretty quickly and get the update under warranty We'll have ours on track all instrumented out the wazoo to see what's going on with things like temperatures. We've heard rumors from reputable sources that the rear end might be an issue as well, mostly due to the 600 ml fluid capacity of the diff.
Expect more of this kind of thing as automakers chase efficiency. The ND Miata is proving to have a remarkably efficient drivetrain in terms of power loss, but maybe they went a bit too far.
Snrub wrote: Not trying to be negative, but if the original NC transmission had issues, when did they resolve the problem? Was it when it became common with the S2 RX-8 for '10?
I can't think of any rear drive Mazda that didn't have transmission issues. Well, the NA/NB seemed to be okay, but that was because they benefited from ten-plus years of development in that design in the RX-7s. And the 1.6's trans was the same as a contemporary RX-7 with 60% more power and 40% more weight. Stronger, actually, thanks to its different drop gear ratio.
Keith Tanner wrote: Expect more of this kind of thing as automakers chase efficiency. The ND Miata is proving to have a remarkably efficient drivetrain in terms of power loss, but maybe they went a bit too far.
The Audi guys are whinging that the new 6-speed trans (01X, I think?) isn't as strong as the old 01E. Well, no, but it also has maybe half the internal losses, because the 01E and the 016 it was based on had a severely overbuilt geartrain. Overbuilt geartrains are fine for enthusiasts who want a 800hp Audi, but they are a great way to waste fuel for the other 99.5% of the people who drive them.
After reading that thread, people seemd to be surprised that the trans broke Just Driving Along.
The damage may have been done days or weeks earlier, and all it would need is a little extra bump at just the right time to cause the failure to happen. EVERY trans I have broken was immediately on shifting. I stripped 2nd gear once by accelerating gently across the road, I stripped the drop gears once by shifting into 5th on the Interstate.
The 2nd-gear trans, at least, had been making an unnerving ticking/clanking noise for about half a week prior to the failure 
Noticed it after packing everything up at a rallycross and was talking to Chris (cghstang) with the engine idling and we said, what's that noise? Well, we'd found out. But none of the others gave any indiciation of imminent failure, they was just there one shift and gone the next.
ONE of the images that I could see in that thread was interesting in that the remaining teeth on one of the gears looked like it had worn through a layer of surface hardening. But none of the other gears had pitting, and it may have just been a trick of camera angle and reflections. Oily gear teeth are surprisingly hard to photograph well.
Knurled wrote:Snrub wrote: Not trying to be negative, but if the original NC transmission had issues, when did they resolve the problem? Was it when it became common with the S2 RX-8 for '10?I can't think of any rear drive Mazda that didn't have transmission issues. Well, the NA/NB seemed to be okay, but that was because they benefited from ten-plus years of development in that design in the RX-7s. And the 1.6's trans was the same as a contemporary RX-7 with 60% more power and 40% more weight. Stronger, actually, thanks to its different drop gear ratio.
Actually, early NAs would have an issue where the shifter would leave the pattern, often resulting in a car stuck in reverse with a shifter that moves more or less freely. IIRC, the fix/workaround is easy, but I've seen CL Miatas in this state listed for very little money.
That probably explains the "d00d, you ever driven a manual trans?" gearshift when I try to get my LE into 2nd, especially on a cold trans. No auto-x or track use either.
I do find the whole transmission to be a bit of a disappointment compared to the ones in my NAs or the RX8, but I guess the above discussion explains a few things.
Any competitive issues with the 99-05 6 speeds (AZ6)?
Nope.
In reply to Mike:
On early na, 5th and reverse forks would jam together shifter is stuck in 5th but trans is in revers. Easy fix, pop out the speed sensor i think, big plug on the passenger side and jam a short flathead in and pry back and forth until you hear a clunk. Did that a time or two on the side of the road so i kept a big adjustable and short flathead in the car at all times.
Sucks the nd fix isnt as easy haha
chiodos wrote: In reply to Mike: On early na, 5th and reverse forks would jam together shifter is stuck in 5th but trans is in revers. Easy fix, pop out the speed sensor i think, big plug on the passenger side and jam a short flathead in and pry back and forth until you hear a clunk. Did that a time or two on the side of the road so i kept a big adjustable and short flathead in the car at all times. Sucks the nd fix isnt as easy haha
If only I could figure out how to get mine to stop locking me out of reverse... Used to be rare, it's happening more particularly when I let the car sit few days.But 5th has been making not nice noises on engagement for months, I think I'm just due for new fluid and a turret rebuild kit.
Keith shrugs it off but its pretty unacceptable for this to happen,I mean its not like mazda has no clue what a portion of the cars buyers do with them.
I'd be more than a little pissed if I dropped that kinda coin and major driveline parts started failing so early.
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