Today on the highway I took off in 3rd gear and then the road had some bumpy spots. The tires kinda hopped and I heard a strange noise from the rear end. At first I thought it was an exhaust backfire, but something didn't seem right, then when I tried to go around some corners it made an awful binding/clunking sound. It drove straight just fine, but going from a stop and going around corners was clunking/binding pretty bad. So I got it home and put it on the lift, drained the differential fluid and found 2 chunks of metal on the magnet. Now a few questions/options I've gone over in my head.

1)Can my differential be rebuilt? The case is fine. Cost? Do it yourself job for mechanically inclined?

2)Where is a good place to find T2 differentials? Since I drag a lot I think this is a better way to go. I found one that said it was run out of fluid and needed rebuilt. How hard is this? Cost for rebuilding? Do the clutches need replaced often? And how much should I expect to pay for a good T2 differential?

3)KAAZ rather expensive last resort for me

4)Anyone seen/heard anyone run this differential? hear anything about them? I generally try to avoid ebay items with moving parts but if it's something that will hold up then it could be an option.http://cgi.ebay.com/ebaymotors/OBX-H...Q5fAccessories

 

If it drives forward fine you probably tore up the torsen worm gears. This part of the diff is not rebuildable, but fortunately you can get a decent stock one or T2 diff and drop it in.

Rebuilding a stock T2 diff is no different than any other clutch-pack diff. The part that requires the most care is setting the ring/pinion backlash and contact pattern when you install it in the differential case, but I think you should be able to leave that alone when you re-use your existing gearing. The thing about running out of fluid is that if it burned up the bearings and clutch packs, it's rebuildable. If the low fluid also pitted or wore out the gearing, it might not be worth it.

 

if it is just the worm gears just weld it up run it like a spool. lol

i actually happen to have a s4 tII diff laying around right now unknown mileage ill let go for 250 shipped fyi or i know japan2LA has them.
as far as backlash and contact its pretty straight forward just look up any how too for that they are all pretty much the same. alot off offroad guys have writeups for that. read em and if you are pretty mechaniclly savy youll be able to determine if its something you can handle or not.

z

 

If you are looking for the cheapest/least labor intensive route, just buy another FD torsen from Fritz or Japan2la, bolt it up and go. I think I got quotes from both of them for around $300 for the full diff when I was in the market. Instead I went the Kaaz route with a 4.77 ring and pinion and would do the same thing if I had to do it again.

T2 is a nice swap, but if you have to rebuild the clutch pack and replace the bearings, you might want to check the prices first. The clutch pack bought from Mazda will cost as much as you might pay for the diff.

Dan

 

Only 2 chunks of metal?

I just changed mine last week and found 4 chunks. 3 were stuck on the magnetized drain bolt. The other just dropped out with the oil. Car still runs pretty normal. I have no idea how long those chunks were in there. Last time I changed the diff oil was 3 years ago.

 

Plenty of low mile Stock FD3S Diffs

Complete diff $350.00 plus shipping

unbolt your driveshaft, unbolt the PPF, unbolt the diff mount, pull the axles out, drop the diff complete... bolt the new low mile used one in and add new gear oil..

Done..

 

proz07 i might be getting in touch with u about that differential. It seems like for my use it might be the best way to go. Was it still working and everything when it was pulled out or from wherever it came from? Any idea how long those clutches last?

I'm concerned about going back with another FD differential considering I already broke mine so I'll probably break it again. I have differential and tranny braces + solid diff mounts but i'm also nearing in on 500 HP so I need something that will hold up.

The Kaaz is nice but at 800+ it's a bit on the high side for my wallet. Do the T2 differentials bolt up just like the FD one? Anything that I will have to change out or change over? Do I swap cases or anything? Thanks for the good info so far guys.

And yeah only 2 chunks, must have been some important ones. Or maybe there are some still dangling in there haha. All i know is it makes an awful noise.

 

Ok read up on the TII diff swap. Sounds like I pull my torsen out and stick the s4 TII parts in it's place. The backlash adjustments sound tedious and time consuming especially for someone that's never done them. Can anyone confirm whether the backlash needs to be set if the same gearing is going in? I think I read it still needs to be adjusted.

How much do new bearings run? And what other new parts should I get? I read about some collar that should be replaced also. Anyone else replace this "collar"? I'm still trying to get all the details sorted out to decide how to go about getting this fixed. Anyone know any good shops in the central TX area somewhere between houston auston and college station that could adjust the lash without charging me a crazy amount?

 

^Try talking to Rotary Performance - rx7.com

 

Alright i sent them an email

 

I recommend against doing a TII diff unless you really need it, like for track use. Any TII diffs in the US you'll come across will likely have a lot of mileage and abuse. Rebuilding it properly, as stated, costs a pile o' money and is tricky to do. Not only is building the core diff itself tricky, but setting up the diff right is quite difficult, and if not done right you can tear the diff up and be back where you started.

The FD's stock diff is damn good - it really holds up for a VERY long time, it's strong, and it works great in corners. Yours breaking is pretty out of the ordinary.

Buy a good low mileage diff (those JDM diffs are a damn fine deal, too), bolt it in, and get back on the road. Put some good Redline gear oil in there and forget about it.

IMHO, go the TII diff route if you're SERIOUS into drag racing or making a truckload of power. KAAZ if you're serious into road courses. Otherwise spend the money on something your car REALLY needs.

Dale

 

Well I'm pushing 500 hp and have some bfgoodrich street slicks. Will the stock differential hold that kind of power with the occasional drag launches?

 

Post # 6

 

Well I purchased proz07's TII differential and will be doing the swap. I would like to do everything myself, I never trust anyone else to do it right even shops but i might change my mind if it seems overwhelming once I dive into it.

I found this thread for rebuilding the TII diff and all the correct ways to set the clearances. https://www.rx7club.com/1st-generation-specific-1979-1985-18/differential-rebuild-tutorial-711712/
I'm not sure but I think the guide is for an s5 TII differential, (1) will it be essentially same guide for the S4 going into the FD case? (2)Are the bearings the same through the years or will I require different part numbers for FD differential bearings?? The ones he purchased were from napa and much cheaper then the dealership.

It sounds like provided I put all the shims and caps back exactly how they came out that I won't have to set the pinion depth. But the backlash and bearing preload will have to be set correctly. (3)Is the backlash set with shims? Or just the adjusters? (4)In the guide above he mentions adjusting the carrier bearing preload and shows a 12" tool he made with 2 bolts welded to it spaced 57mm apart, I can't tell from the pictures where the tool goes to tighten the carrier bearings. Will this same tool work in the s4 diff?

(5)Is a pinion collar the same as a Crush Sleeve? Someone said to replace the pinion collar because it's a one use item, when reading the above thread it mentioned the crush sleeve should be replaced. So I got the idea they were the same thing. (6)Will I even be messing with the crush sleeve?

I think that covers pretty much everything at this moment if anyone has more input it's greatly appreciated.

 

I didn't notice originally but that guide is for a 1st gen LSD rebuild. Is it basically the same for the TII or are they very different in their designs? My 6 questions in my above post still stand though.

 

Basically the same. You could probably use a guide for a Ford 8.8 LSD if you want.

Just curious: what oil were you running in your current diff?

Dave

 

Neo 75 90. With a lucas additive.

 

Don't use the additive. Additives are for clutch-pack LSDs like the T2 diff, but I don't think they help the Torsen operation. (Torsens rely on the mechanical ratio of worm gears to limit torque across the axles rather than friction, thus there is no need for friction modifier).

Neo 75w-90 is a good oil, but it's a bit of an all-purpose designed also for use in transmissions with synchronizers.

If you're still considering another stock diff (I would, because I'm cheap and the stock Torsen is so wonderfully streetable) you might get better performance using a differential-specific oil such as Redline Shockproof series without any additive.

 

3 comments/questions:

1. Why do you think NEO75W90 would be deficient in the diff?
2. IMO, if his worm gears failed, it was probably due to the cumulative damage sustained from wheel hop, not just from wear.
3. Why do you specifically suggest Redline Shockproof? I know you never do anything without a good reason.

Dave

 

1. If it's the Neo 75w-90 HD, it's their least exotic diff oil. While I don't believe it's insufficient (I'm running it in my diff atm) there are more exotic diff oils out there. For those guys who are running power above stock the few extra dollars (and it's literally a few dollars) is a no-brainer. Neo 75w-90 RHD and 70w-140 HD come to mind, 75w-140 if your heaviest abuse comes from road racing. In the Redline product line, the shockproofs stand out to me as the premium for rear-end gearing. Take any particular product recommendation with a grain of salt - it's extremely difficult to make real comparisons or pick the "perfect" gear oil, and very, very few people in the world actually have hands-on experience and test data that can really show the truth. Even the most die-hard gearhead or engineer will (should) admit they either have vast experience with just a couple products or shallow experience with many.

Regarding the "additive", if it's a limited slip friction modifier, it is probably compromising the lubrication a bit. Base gear oil is too slippery for clutch packs, but in a Torsen (nothing but gearing) you want the slickest oil possible.

2. I agree. However, tooth bending stress (the general cause of tooth breakage) is based partly on friction. Less friction, less tooth stress. Under shock load, the friction and wear is driven by the performance of the EP additive package. Multi-purpose gear oils usually compromise the EP package a bit to work in a synchronized gearbox or for compatibility with clutch packs. Since the stock torsen diff contains no clutch packs or synchronizers to hold us back, then the most extreme EP formulations are open game and arguably the best option.

3. Shockproof is not my ultimate recommendation, just one that's readily available and is built for EP performance. The downside to Shockproof is that it's highly tacky and should probably be changed more often since it contains solids, and solids almost always break down sooner than the base stock. Most major brands have some gear oil that's specifically for racing differential applications. My only caution is that you may compromise cold performance (stick with a 75w- or 80w-) and lose a touch of HP if you go with a high viscosity racing weight oil.

Dave

 

I used the lucus oil which was advertised compatible with all differentials and mixed with synthetic oils, i used it because i didn't quite have enough neo after filling my tranny also. It didn't take much of the lucus to top it off though. I don't think my gear oil had anything to do with the damage. I'm going with the tii clutch style cause if it broke once it will break again. I rather not be replacing differentials on a regular basis especially since my thirst for more power is never ending.

 

I definitely understand.

If your power level was staying in the same place, I might argue the T2 is overkill. But for more power with shock loading, it makes a lot of sense.

I assumed you were talking about lucas limited slip additive. The additive is not oil, and can compromise the performance of the base gear oil. But it sounds like you just mixed some lucas gear oil with the Neo. That should not have done much if anything.

 

Thanks for the answers. Always good to see the reasoning behind the statements.

Dave

 

Yeah i don't think the lucas did anything either. It was a thicker oil which I was alright with cause 75w seemed a bit on the light side for the rear end.

Question: when the TII differential gets here, is there a good way to test how well it locks up before swapping everything over while it's out of the car? If I can test and see its a bit on the weak side I'll probably disassemble it and check the specs on the disks or order a couple of the thicker disks. I won't be doing a full rebuild if I can avoid it just a little something to insure it clamps good the first time.

 

I had a quick look a the FSM rear axle section, and Mazda does not show a way to test the diff without taking apart and measuring the discs.

http://www.cochran-racing.com/FSM/s4...REAR_AXLES.pdf