The OS Giken diff is one of our favorites with a smooth action ramping up to a positive lock which is both easy to drive but puts the power down hard. The OS Gikken unit has a strong and light forged case which is both stronger and lighter than the stock diff. Less rotating weight means more power to the wheels. It also has more clutches for longer life, harder locking, and less temperature generated. The OS diff has 4 spider gears vs the OEM two which with the forged case makes the diff pretty bulletproof.
The most revolutionary thing with OS diffs is these pressure rings that have negative springs that counter the wedging action of the spider gear cross-shaft cams. By countering the wedging action, the springs ensure a smooth and linear ramp-up of locking that puts the power down and makes these diffs more predictable to drive. You can also tune the ramp-up by changing the spring rates. For more details on these diffs and how to tune them, check out this video.
Since we are switching to a deeper 0.767 sixth gear vs the stock 0.861 we souced a 4.55 genuine Toyota final drive from Battle Garage to help with our acceleration. The stock final drive on an AE86 is 4.3. To reduce friction we WPC treated the gear. The little 4AG doesn’t have much so we are trying to reduce drivetrain losses as much as possible.
For our little NA motor, the 6.7″ OEM ring and pinion is enough but it is probably too weak for turbos and maybe even a BEAMS engine swap. However, Jackie Ding is racing a K-swapped AE86 in GLTC with this diff with no problems. For more strength, a 7.5″ MA40 Supra rear end just about bolts in or there are kits to swap a Ford 8.8″ diff into the car but bigger diffs mean more weight and friction so if you are NA like us, don’t overkill it!
Battle Garage provided us with this billet sleeve that replaces the pinion crush collar. Under hard use the factory crush collar tends to start deforming which messes up the pinion depth on the ring gear, leading to ring and pinion failure. This part prevents this from happening.
Battle Garage has this handy kit that has all the parts to rebuild your differential and axle assembly. When OEM Toyota parts are no longer available Battle Garage and sourced quality Japanese OES parts for the kit.
15 comments
Open lug nuts in Southern California on an AE86…Mike you’re braver than I am!
Are all J160 gears compatible with each other? I have an NC with the J160 which also has an absolutely horrible bog on the 3-4 shift… can I mix and match ratios out of all the cars they came in? Previously I’ve only been looking at NC’s and RX-8’s… but will gears from the 86 twins and S15 work too? And aftermarket gears designed for those transmissions? I know this might be a “duh” question, but I’m having a pinch me moment about it… might finally be able to get gearing I like!
Also, are these transmissions really good for 300 ft/lbs of torque? seams a little ambitious, no? Reason I’m asking is I’ve been toying with the idea of putting a Honda J series V6 into an RX-8 but the transmissions grenading itself with “crazy V6 power” has been one of the discouraging factors… How much torque would you realistically trust to this transmission for a street car with a decent amount of track use?
From messing around with S chassis cars and the FR-S it seems like the 300 ft/lb thing is about true, Once you exceed that kaboom. at least on track driven cars. On cars like our supercharged FR-S, the transmission seemed happy and reliable at 250 ft/lbs.
We exchanged 6th gear with an S15 one so the other gears can probably change. This one Australian company has a reliability mod for the box. We never had this chipping problem, more like 3-4 loosing the teeth,
https://neatgearboxes.wordpress.com/2014/04/16/nissan-silvia-s15-6-speed-gearbox-circlip-mod/
We’re talking crank torque, not torque at the wheels, correct?
more gear ratios to play with potentially makes things more interesting… time to spend hours looking up all the ratios and mix and matching them to find the perfect spread, haha..
Thanks for the link! Did you chip the teeth with the 250tq FR-S?
Hope you meant NB – the NC and later RX-8s are an inhouse Mazda top loading transmission.
There’s some variations, I think the NB has smaller diameter shafts, certainly a different size output shaft. At some point I’m going to get ahold of a Subaru version and see how much can interchange between them.
From many sources:
Vehicles that use the J160
Toyota Altezza AS200 and RS200
Mazda MX-5 (Miata)
Nissan Silvia
Mazda RX-8
Lexus IS
Toyota 86/Scion FR-S/Subaru BRZ
Correct; but only the 1999-2005 (NB) Miatas and the 2004-2008 RX-8 used the Aisin AZ6/J160 gearbox. The 2006-2015 MX-5 (NC) and 2009-2011 RX-8 used a completely different transmission for whatever reason; if you look up photos the differences aren’t subtle. More subtly though, while the Miata and RX-8 Aisin gearbox casings bolt to one another, the Miata version has some dimension differences on the output shaft and yoke. I don’t know if the Miata version is an oddball or if they’re all subtly different on internals.
Good to know!
well thats a shame… but thanks for the detailed info Dani!
Any chance you know if the 09+ RX8 transmissions can handle more power over the earlier Asin units?
Supposedly the NC transmission holds more power than the Aisin transmission; they used it again for the Fiat 124 Spider and they have a better reputation for hard use than the Aisin transmission does. They all have the annoying 4th gear ratio though; I think the Fiat 124 just has a different input to countershaft gear pair than the NC and RX-8 as the 3/4 gear ratio split is identical.
For the record, I’m coming from all of this from wanting to put an RX-8 front case on a Subaru version of the trans to get rid of that 4th gear (and a closer ratio 6th for track use) behind an NA 13B.
There’s a version of the RX8 in the UK (think 09+) that has a taller 3rd gear (1.536) which helps quite a bit with the 3-4 spacing. There’s a guy over on the Miata forum that did it and says the difference is huge. You do end up with a little bit longish 3rd, my IMHO thats easier to overcome being a lower hear and lower speed. With the long 4th and all of the wild 150whp my NC had it was starting to hit the aero wall right around the time you bogged into 4th gear.
I plotted out all the ratios, and I think my ideal would be the long 3rd paired with the 86 4th gear. If that was a thing… which its not…
What are the ratios for these transmissions?
US NC MX-5 and S2 RX-8 (the Mazda inhouse gearbox) are 3.815, 2.260, 1.640, 1.117, 1, 0.843. The US NB Miata with the Aisin box is 3.76, 2.269, 1.645, 1.257, 1, 0.843 and the S1 RX-8 with the Aisin is the same except that 4th gear is 1.187:1. I have no idea why.
I’m seeing the UK NC MX-5 having 3.709, 2.190, 1.536, 1.177, 1 and 0.832 which, yeah, seems a little better. It looks like the JDM NC had the same ratios as the US ones.
That is a pretty big jump!
I also committed to an Aisin AZ6 (J160 is the Toyota nomenclature for that transmission) in my AE86, in RX8 guise for my 13B swap, as well as an FB RX-7 rear axle from my GSL-SE for the sake of a little extra power handling, and in my travels I’ve found out that there is supposedly a machining operation one can do to… the countershaft, IIRC(?), which allows you to install a heavier duty version of a circlip that’s somehow involved in these transmissions grenading at around that 250-300 lb-ft mark with any sort of shock loading. There’s only a couple of transmission builders that advertise doing this modification, but if you search for “AZ6 circlip mod”, it’s not a humongously difficult thing to find more information on. Said modification is advertised as allowing the transmission to handle approximately an additional 50% of the torque, but I am skeptical of that figure, as that seems like something Aisin would have figured out and made a revision for in that design in the roughly 25 years or so that it was produced in various forms.
As far as I can tell, the machining operation is effectively taking a ball end mill and continuing a slot that’s all ready in the shaft until it reaches the root of some splines, as opposed to said slot just kind of ending in the middle of them, but my memory on those details are sort of rusty; it’s been a couple years since I last really looked into it.