When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
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.
Here you can see the difference between a stock countershaft and a modified one
Neat Gearboxes Circlip vs OEM
Top view
Side view
It is made up of 2 halves with a curved interior.
Notice that the notch for the retaining ring is not centered
Width measurement 7.28mm
Thickness measurement 3.97mm
Overall size 42.50mm
Retaining ring width 3.50mm
Retaining ring thickness 1.07mm
Countershaft machining depth 28.2mm
I hope that all this research helps all the enthusiasts eager to continue learning more about these wonderful convertibles.
Last edited by jrmiata42069; 11-10-2023 at 12:54 PM.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
09-12-2021, 12:40 PM
Aisin AZ6 6-Speed Gearbox “CIRCLIP MOD”
10
