Aisin AZ6 6-Speed Gearbox “CIRCLIP MOD”
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From: Canberra, sort of
I guess more people aren't doing this simply because it's difficult to find a shop to do it and if your box goes then you're having another built instead of grabbing a junkyard box (in the case of the ZF swaps)? I suppose it also doesn't increase the power handling by a ton (considering the gears will still strip teeth), so anyone who wants to push past 400 WHP is probably just going to grab a stronger box.
I wish there were more people who had experience/feedback running a box with this mod. Assuming it does work it seems like a great way to bulletproof the drivetrain for people wanting to stay around/under 400 WHP.
ETA - I did reach out to TRE to see if they're still doing this in the states.
I wish there were more people who had experience/feedback running a box with this mod. Assuming it does work it seems like a great way to bulletproof the drivetrain for people wanting to stay around/under 400 WHP.
ETA - I did reach out to TRE to see if they're still doing this in the states.
My interest in the NEAT internals has cooled - there has been no indication from them of progress, let alone a result. After my car is back on the road running happily, I will be exploring the mod for my spare box with a view to swapping it into the car and turning up the boost.
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That's about where I'm at Gee. My current box leaves a lot to be desired as far as synchros and overall noise, but I figure if it's getting opened up I might as well do something to try and prevent it from blowing a couple hundred miles later. I also have no experience with gearboxes so I figure a spare might give me some room to breathe as I figure things out.
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From: Canberra, sort of
There is someone doing the mod here AIUI, not that it helps you Muricans.
My interest in the NEAT internals has cooled - there has been no indication from them of progress, let alone a result. After my car is back on the road running happily, I will be exploring the mod for my spare box with a view to swapping it into the car and turning up the boost.
My interest in the NEAT internals has cooled - there has been no indication from them of progress, let alone a result. After my car is back on the road running happily, I will be exploring the mod for my spare box with a view to swapping it into the car and turning up the boost.
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From: Canberra, sort of
Herring input shaft mod
Found by SimBa, this may or may not be a variant of 'The Circlip Mod', but at least we here have another option/source of expertise in these boxes, and for me, one that is a lot more convenient than NEAT 
I'd post it here, but can't find how to do it - if someone can either tell me how to, or do it themselves, they will have my thanks.
EDIT. Deciding to strike while the iron is hot, I called Todd Herring. His mod above is not the circlip mod, but complementary. He recommends both, and can do them as part of a gearbox overhaul for c$2k. That is good for 350hp/260kw (wheel/flywheel not stated) when the gears themselves fail structurally. Higher than that he recommended the PAR gearset ($7-8k installed), knows of half a dozen or so cars running them successfully, though I know of one very unhappy customer.
He described the key constraint being tyres, race rubber - especially race rubber with aero - where the resistance to tyre slip/spinning maximises the stress on the gear teeth.
I'd post it here, but can't find how to do it - if someone can either tell me how to, or do it themselves, they will have my thanks.
EDIT. Deciding to strike while the iron is hot, I called Todd Herring. His mod above is not the circlip mod, but complementary. He recommends both, and can do them as part of a gearbox overhaul for c$2k. That is good for 350hp/260kw (wheel/flywheel not stated) when the gears themselves fail structurally. Higher than that he recommended the PAR gearset ($7-8k installed), knows of half a dozen or so cars running them successfully, though I know of one very unhappy customer.
He described the key constraint being tyres, race rubber - especially race rubber with aero - where the resistance to tyre slip/spinning maximises the stress on the gear teeth.
Last edited by Gee Emm; Sep 4, 2025 at 07:46 PM. Reason: new info
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From: Phoenix AZ
Start with this (spaces added to make it appear as text) [ y t ]
Then the string of gibberish letters from the link h t t p s : / / y o u t u . b e / This Part Here -->AUkQfFvRMdg<--This Part
and then end with this (minus the spaces again) [ / y t ]
All on the same line of course. Starting tag (whatever they call it) in hard brackets, the string of gibberish identifying the specific youtube video, then the ending tag in hard brackets.
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I debated posting that here. IDK Why I didn't. If nothing else it seems like another way to beef up the transmission before the gears go the way of a meth addict and start losing teeth.
I think at this point I'm in the camp of the circlip mod and the welding mod to be a "while your in there" type of thing, but we'll see how I feel once my 6 speed lets go or I pull it apart.
I think at this point I'm in the camp of the circlip mod and the welding mod to be a "while your in there" type of thing, but we'll see how I feel once my 6 speed lets go or I pull it apart.
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From: Canberra, sort of
I am delivering my spare box to Todd Herring on Saturday. I will report back when it is in the car, if not sooner.
He says the SE gearbox has shot peened gears and is definitely stronger, presumably that means MSMs too, I can't see why ADM would be that different to ROW. When my box comes out to be replaced by the overhauled/modified 'spare' I will have a decision to make as to whether to do it too, and then maybe sell the spare or retain it as a spare again, though the odds on needing it will be much longer.
He says the SE gearbox has shot peened gears and is definitely stronger, presumably that means MSMs too, I can't see why ADM would be that different to ROW. When my box comes out to be replaced by the overhauled/modified 'spare' I will have a decision to make as to whether to do it too, and then maybe sell the spare or retain it as a spare again, though the odds on needing it will be much longer.
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Cool stuff. I take it you mean the 00se gearbox? Wonder if 10ae is the same..
I really should re read this thread, but does this address the shift lockup issue that some folks (Emilio) have reported that happens in high heat? Iirc aluminum synchros expanding and locking up? Or is that a diff issue?
I really should re read this thread, but does this address the shift lockup issue that some folks (Emilio) have reported that happens in high heat? Iirc aluminum synchros expanding and locking up? Or is that a diff issue?
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From: Canberra, sort of
It will be an overhaul and the two mods - the one described in the vid on the input shaft, and the circlip mod. Base cost c$1500AUD, but depends on what he finds when the box is opened.
I didn't ask exactly what the 'overhaul' included, this box has ~120k kms and seemed ok when I acquired the donor car, so I am hoping minimal additional work.
Fire, no the ADM version of the MSM the ROW got is called the SE, mine is a 2004. The spare box getting the treatment is out of a 2003. Shifting issues not mentioned in my enquiries, I will ask him on Saturday.
I didn't ask exactly what the 'overhaul' included, this box has ~120k kms and seemed ok when I acquired the donor car, so I am hoping minimal additional work.
Fire, no the ADM version of the MSM the ROW got is called the SE, mine is a 2004. The spare box getting the treatment is out of a 2003. Shifting issues not mentioned in my enquiries, I will ask him on Saturday.
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Joined: Apr 2010
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From: Australia
Speedtek announced on facebook 2 days ago they had a 2 piece countershaft solution in the works since NEAT has gone quiet.
We’ve been receiving a lot of questions about the delay on our AZ6 6-speed transmission sets, so here’s the full story.
During production, our engineering team made the decision to move away from the traditional one-piece countershaft design, which uses a bushing inside 4th gear and a circlip to retain the gear There is no actual bearing supporting 4th gear in the OEM transmission,its operation relies solely on oil film lubrication, assuming sufficient oil makes its way inside..
In real-world high-load testing, we confirmed that this configuration has a critical weakness: under load, 4th gear can shift outward, breaking the circlip and damaging the reverse gear it seats against, ultimately leading to failure.
To ensure our gearset meets the durability standards we stand behind, we paused manufacturing, scrapped the batch of countershafts originally produced, and went back to the drawing board.
After more than 200 combined engineering hours, we now have a completely redesigned and far superior solution: a proprietary two-piece countershaft featuring K-type needle roller bearings in 4th gear and a secure mechanical locking system that prevents shaft separation. This design eliminates the failure point entirely and delivers the reliability expected in a true motorsport-grade gearset.
This new countershaft is unique, and we will be securing patent protection to safeguard the design.
Thank you for your patience and support as we bring you the most robust aftermarket race-spec transmission gearset available for the Nissan S15 Silvia, Toyota GT86, Subaru BRZ, Mazda MX-5 & RX-8, Toyota Altezza, Honda S2000, and more.
Our latest update is 4-5 months until we finish the batch to suit all the variations.
www.speedtekautoracing.com
During production, our engineering team made the decision to move away from the traditional one-piece countershaft design, which uses a bushing inside 4th gear and a circlip to retain the gear There is no actual bearing supporting 4th gear in the OEM transmission,its operation relies solely on oil film lubrication, assuming sufficient oil makes its way inside..
In real-world high-load testing, we confirmed that this configuration has a critical weakness: under load, 4th gear can shift outward, breaking the circlip and damaging the reverse gear it seats against, ultimately leading to failure.
To ensure our gearset meets the durability standards we stand behind, we paused manufacturing, scrapped the batch of countershafts originally produced, and went back to the drawing board.
After more than 200 combined engineering hours, we now have a completely redesigned and far superior solution: a proprietary two-piece countershaft featuring K-type needle roller bearings in 4th gear and a secure mechanical locking system that prevents shaft separation. This design eliminates the failure point entirely and delivers the reliability expected in a true motorsport-grade gearset.
This new countershaft is unique, and we will be securing patent protection to safeguard the design.
Thank you for your patience and support as we bring you the most robust aftermarket race-spec transmission gearset available for the Nissan S15 Silvia, Toyota GT86, Subaru BRZ, Mazda MX-5 & RX-8, Toyota Altezza, Honda S2000, and more.
Our latest update is 4-5 months until we finish the batch to suit all the variations.
www.speedtekautoracing.com
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From: Canberra, sort of
Hmm; sounds expensive. Link didn't work for me, these people are local? Herring reckons his mods take it to the point teeth fail, I think that will be enough for me. For now 
Found their website, they are Melbourne based. Website says a MX5 6-speed dogbox will be ready mid 2025, that will be interesting. Oh wait, it is end 2025 and page has not been updated and no stock available ...
I presume that, and the related Toybaru box, are what the first sentence refers to. So, is the new 4th gear mod for the dogbox, or a mod to the standard box, or both? On current performance I will not be putting a large slab of $$ up front, even with the sizeable discount on offer.
Found their website, they are Melbourne based. Website says a MX5 6-speed dogbox will be ready mid 2025, that will be interesting. Oh wait, it is end 2025 and page has not been updated and no stock available ...
I presume that, and the related Toybaru box, are what the first sentence refers to. So, is the new 4th gear mod for the dogbox, or a mod to the standard box, or both? On current performance I will not be putting a large slab of $$ up front, even with the sizeable discount on offer.
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From: Canberra, sort of
I found the attached online, and while it is topped by a Mazda logo I make no claim as to its accuracy or provenance. It does support the statement quoted though.
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Aug 25, 2015 06:48 AM
