Post update: a few months ago I made this mod on my car and I am very satisfied with the results, after 2 trackdays and intensive road use there have been no problems and the transmission works perfectly. The car makes 400hp and 500Nm of torque and I have installed a ceramic double disc clutch, therefore the transmission must withstand greater loads. So you can imagine the level of power this mod can achieve. It is the first Miata to make this mod, it has never been done before and it really works.
The countershaft modification was done in Neat Gearboxes, see my post where I explain the assembly procedure:
Aisin AZ6 6-Speed Gearbox “CIRCLIP MOD”

Good to know. Have you done it yet, or has anyone taken you up on your offer?

Who wants to request the Circlip measurements send me a DM

Very interesting. I've got a spare 6 speed laying around, I might have to go this route...

ncjake I’d definitely like to get this done. How else can I get in contact with you to work out the details?

Sorry for the bump and probably a noob question, but does this mean that these transmissions can be swapped into NA instead of NB 6 Speed ? By changing front case, or even without changing and just getting an adapter plate?
As you can guess, 6 speed is very hard to get here, impossible to legally import 2nd hand car parts but other transmissions on the list can be found, even order a brand new BRZ trans (probably for the cost of a kmiata e46 swap kit)

You will have the same problem since you should also disassemble the transmission and make the modification of the circlip to reinforce the countershaft. As far as I know, the transmission of the RX8 changes the bellhousing and the main shaft is not compatible, so you would have to change these 2 components, I do not know the adaptations that should be made in the Silvia or in the IS200

it is ok, I will already be happy to move away from the glass cannon 5 speed, I am targeting more or less what I can get from a TD04 so, 300 bhp at best. Rear is already 1.8 torsen, only weak link is tranny. I guess there would have been swap kits etc. produced already if it was an easy swap, but I wonder how difficult it would be. Labor and machining costs are acceptable here, just do not want to get into fabrication too much.

no, the BRZ transmission does not swap into the NA/NB.

It really would not be worth doing.

Where and what is the name of the shop?

Hows that 4th gear holding up? That is many powers.

Awesome to hear it!


And for final clarification, the only thing done to the trans was the circlip mod?

Also, a tidbit of info in reference to the RX8 compatibility, *the bell housings can be swapped between the two but the tail shaft is much longer. The shifter would be about where the trunk release is in an NB. Looking at Alldata, I figured the gears could be swapped around but not sure if the output assemblies/tail casings are interchangeable. I picked up a $200 RX8 trans last year and these were my findings before deciding I was in over my head and returning it.

Has anyone in the states had this mod done? I'm looking for a shop to do this to my 6 speed while it is apart.

Any machining workshop can help you, all you need are the measurements. Send me a DM to know more...

I contacted Jacks Transmissions and they said they could do the 4th gear modification. They're familiar with the AZ6, because they build the FRS/BRZ trans. I also asked them if any of the BRZ modifications (carbon fiber synchros, gear sets, shift forks, bronze shift rail bushings, etc.) can be applied to the Miata AZ6 and there is basically nothing interchangeable between the two. The term AZ6 just defines the basic architecture of the gearbox.
They could rebuild the box with OEM parts, perform the 4th gear mod, and micropolish or WPC treat the internals. So I got disappointed and have given up on the stock trans given that my complaints with it are: it shifts like stirring a bucket of bolts (even with MR shifter), locking out 5th gear on track, and reliably holding 300ft-lbs.

Question.... Do we know the reason for locking out 5th gear on track? have this problem... an for now havent tested yet is to put a oil cooler so that the gearbox doesnt get to hot.... thats the one solution i could think.

The reason is that the 3rd and 5th gears are too close together, so sometimes you can engage the 3rd gear. If you move the lever beyond 5th you go to the reverse gear rail and it doesn't engage. Look at my last trackday and 33 seconds from the end you can see how this happens:

most probably then my problem is the engagement of reverse gear rail is there any trick we can do to block somehow the reverce at least in track... maybe a stoper at the gear leaver?

Correct there are gear levers with this function, this will be my next improvement:
https://cae-racing.de/CAE-Ultra-Shifter_1651