Aisin AZ6 6-Speed Gearbox “CIRCLIP MOD”
 

I present my compilation of a couple of videos where I show the whole step-by-step process of rebuilding the gearbox.

A solution to prevent major gearbox failure in the Miata NB 6-speed gearboxes, and the sister Aisin AZ6 gearboxes found in Nissan Silvia S15, Lexus IS200, Toyota Altezza, GT86, Subaru BRZ and the Mazda RX-8 series 1.

Essentially the issue is the same every time: more-than-stock torque loads put excessive lateral force into the 4th gear “circlip” (actually a pair of spacer C clips retained by an outer snap ring) which causes it to chip the retention groove. This leaves 4th gear to freely float across the countershaft and inevitably come out of alignment with it’s drive gear and causes major failure. Modifying the countershaft you can reach a torque of 500Nm, capable of more than 400hp.




Here we go again! I have decided to publish all the images and measurements so that other people can make this improvement. All measurements have been made with a micrometer and with images of each measurement. With this you can go to the machining workshop to make the modification.

https://cimg0.ibsrv.net/gimg/www.mia...e027705632.jpg
Here you can see the difference between a stock countershaft and a modified one

https://cimg6.ibsrv.net/gimg/www.mia...cd6de2eb4f.jpg
Neat Gearboxes Circlip vs OEM

https://cimg7.ibsrv.net/gimg/www.mia...010f71c77f.jpg
Top view

https://cimg8.ibsrv.net/gimg/www.mia...5ea1e1769f.jpg
Side view

https://cimg9.ibsrv.net/gimg/www.mia...fd49e5e271.jpg
It is made up of 2 halves with a curved interior.

https://cimg0.ibsrv.net/gimg/www.mia...71681fdccc.jpg
Notice that the notch for the retaining ring is not centered

https://cimg2.ibsrv.net/gimg/www.mia...d0c9518ff6.jpg
Width measurement 7.28mm

https://cimg3.ibsrv.net/gimg/www.mia...87ed63e468.jpg
Thickness measurement 3.97mm

https://cimg5.ibsrv.net/gimg/www.mia...bb0374b2d8.jpg
Overall size 42.50mm

https://cimg2.ibsrv.net/gimg/www.mia...0f09605fb4.jpg
Retaining ring width 3.50mm

https://cimg3.ibsrv.net/gimg/www.mia...4ebc9ac245.jpg
Retaining ring thickness 1.07mm

https://cimg8.ibsrv.net/gimg/www.mia...6df593d4f2.jpg
Countershaft machining depth 28.2mm

I hope that all this research helps all the enthusiasts eager to continue learning more about these wonderful convertibles.

List of parts used?

Sam from Neat Gearboxes is the guy who can do the mod. I can give you the reference of the countershaft but the circlip will have to be made by Sam. I have taken precise measurements of the modification and the circlip but at the moment I cannot reveal this data.

I only have made the videos to teach the disassembly and assembly process.

500Nm of Torque in the NB 6-speed Transmission

I am 100% going to do this to my 6 speed do you know of anyone in the US that can do this modification though? I saw that neat gearboxes was located in AU and I would be worried about my package getting lost in the mail and long lead times.

Thanks! I emailed them

Good luck with Walter. I've not been able to get them to respond to me. They have been dark for over a year at this point, restructuring or something.
JO Motorsports in... I think SC says they can do this modification. Spec Miata shop.

nothing from walter or JO and jacks transmissions says they no longer offer this service

Supposedly Walter will only do business through dealers now. You need to find a dealer for them and put your request in through them. I know Dynotronics near San Antonio, TX is a dealer for them.

Any word on if anyone stateside can complete this mod? Seems like a great way to eliminate the need for e36/e46 trans in high power turbo builds.

sadly no I don't have any companies left to message. They all after a month/ months either didnt reply or said no. I am about to just bite the bullet and send it to neat unless someone has another option.

Anyone in the UK doing this?

FYI, I'm currently working w/ a company in the midwest completing this modification. I'll try to remember to reply when I receive back the countershaft. The are making multiple sets of thrust washers since I mentioned this is a common issue amongst the trackday Miata community.

Jack's Transmission in CO services the FRS/BRZ/GT-86 community; since their 6MT is AZ6-based, they have the same issues. Jack's will only do the modification as part of a full rebuild (I asked if I could send just the countershaft assembly).

These countershafts have been discontinued -- not sure how widely known that is. I'm having this done mainly to preserve the shaft b/c it was difficult to source a good one. (Once installed, mine shouldn't see more than 160 ft-lbs at the wheels.)

So, any feedback on this modification? In fact, the box can withstand these 500nm? My gearbox man who has already dismantled several damaged 6-speeds said that there is no way that this mod will fix anything, in fact no gearbox that he disassembled was damaged the way that this modification was supposed to prevent.

Can the share the failure modes he has seen?

It sounds like unverified information... There are many more people with satisfactory results having made this modification than negative opinions, what happens is that there are still not many success stories in Miatas since most have been done with Nissan or Toyota. Sam from Neat Gearboxes wrote to me 2 years ago when I asked him for references, before hiring his services and this is what he replied:
"Tthe clip modification works for the Miata / MX5 6-speed transmission, we have done at least 5 or 6 Miatas.
While I don’t have any lab testing or anything, we do have over 60 of these in the real world being used since 2015. I have seen one break in a BRZ with 303kw@wheels. And in fact, it was not the clip that broke but 4th gear itself. Other than that, I don’t think I’ve heard of any breaking with our modification."

What's the approx $ to do this mod, parts and all?

In 2021 it cost me AUD$450 to modify the c clip and AUD$638 a new countershaft bought from Mazda Australia plus shipping. You can also remove the countershaft of your transmission and send it

Thanks, that's not bad at all.

Yeah, I am wondering if I should get my spare box done, then I could thrash the SE will a bit less regard to the health of the gearbox. I might wait until the engine rebuild/new turbo dyno results are in and see if a broken box is on the cards.

I would strongly consider doing it. NB 6-speeds are only getting more expensive. I'm having mine modified since getting a new countershaft will be very challenging.

@mkrazit

Have you had the mod done? any feedback? Would you have the source for the parts needed to do the mod?

I wish I had a spare 6 speed as I've been wanting to rebuild a 6 speed, study them for anything that can be done to improve the reliability. This mod sounds interesting.

Thanks for @ mentioning to bring me back to this. I haven't formally reported back because I don't have a modified countershaft in-hand yet. The shop I'm using successfully modified a shaft, but that shaft had corrosion in the shaft gear teeth, thus I reassembled only to check clearances (no issue).

Good news is I expect another modified shaft back by December which WILL be assembled into a rebuilt box w/ new bearings and syncros. I'll share details when I have that shaft back.

Guys I just updated the post :dealwithit:

Looking forward to the follow-up. I have a set of gears and another box sitting around I want to build for a spare, but I might do this mod and make it my primary box.

Here's a pos-cat for posting the machine work information!

I just noticed something about your measurements above. Is there an issue with the curvature of the components against the flat measuring surface of the mike? I suspect so, at least if absolute precision is required of those measurements? Or is it a case of close enough for practical purposes?

For the OD it’s fine, but yes for the inside curves your absolutely right, it should be measured with a ball-anvil micrometer

Well, it's December, and still no modified countershaft. Was told they would have time mid-Nov to do the shaft, haven't been contacted for payment so I've been trying to get an email or call back since. Thrust washers are already made, shaft just needs machining and shot-peening...

What is the shop you are using for the work? I am in Indiana aswell

So I have the countershaft back from being modified but have been occupied w/ OTM maintenance -- the reason I haven't updated. (Photos below showing machined and shot-peened shaft, hardened thrust washers, Spirolox retaining ring, and 4th gear w/ clearancing for wider thrust washers.) Next is this gearbox rebuild.

Team Rip Engineering in Kalamazoo, MI. Quality and price are great. Delivery? Shaft delivered to them 21OCT, I picked it up 22DEC. It takes a few days for them to modify the shaft and set thrust washer clearance -- many other priorities shoved this to the side. If more people sent their shafts to be modified, this might become more of a priority for them. PM me if you have further questions.
https://cimg3.ibsrv.net/gimg/www.mia...5f220c7a20.jpg
https://cimg4.ibsrv.net/gimg/www.mia...2c4c83c6b7.jpg

I've got a spare 6-speed I'd like to have rebuilt