6strngs

Hi all, I'm actually using this differential in my miata but I signed up here because I think this forum will be able to better answer my question. So, I have never taken a diff apart but it's one of the few things on a car I've never taken apart so I'm attempting to rebuild this diff by myself. So, the reason for the rebuild is I've noticed it handles a lot differently from when I first got it, only does peg-leg burnouts, if one tire lifts off the ground the other just free spins the one in the air. I first measured the breakaway torque by jacking one wheel off the ground and put my torque wrench on the axle nut and measuring how much torque it required to start spinning the wheel and I found this to be only 13 ft lbs. Next, I removed the rear end and dissassembled everything and measured all the friction plates and thrust washers with a micrometer. The desired spec for the thrust washers is 1.60mm and I have one at 1.60mm and the other is 1.59mm. The spec for the friction plates is 2.00mm, all 4 of the inside splined plates and 2 of the outside splined plates are right at 2.00mm, the other two outside splined plates I have a 1.99mm and a 1.97mm. The minimum spec is 1.90mm so I am very very close to being perfectly in spec for everything. The only two issues I've identified are, when it came apart, the plates were arranged Outside-Inside-Inside-Outside when it is supposed to be Outside-Inside-Outside-Inside. The other issue I noticed is that 3 of the 4 pinion gears on the spider rotate freely and slide onto and off of the spider easily while one took some effort to remove and does not spin freely, there is some light scoring on the spider on that one leg.

So my question is, with everything being so close to spec, how could my breakaway torque be so low? Would the fact that it was assembled incorrectly affect this? And how much of an issue is that spider gear, should I be looking to replace it or could a machine shop smooth it back out so it could be used? Lastly, if the sequence of clutch plates does not have an affect on breakaway torque, then my best option would be to add shims, correct? If so, how much should I be looking to shim it by if I am trying to achieve a cold breakaway torque of ~120-130 ft lbs (so that it will "break-in" to 90-100 cold and will warm up to 110-120, I am using the car primarily for drifting so I want it tight) I have the diff apart in my garage at the moment with no clue where to go from here so any help is greatly appreciated

misterstyx69

iTrader: (142)

This is so technical I got got lost after the word "help"!
You would be best suited to get to the 1st gen section by gaining posts to enter that section...and the INFO on how to do that is at the top of the INTRO section.

cab91089

iTrader: (3)

Just throwing this out there but you can just swap in a Torsen LSD from a Miata and be done with the entire thing. It'll last longer and with older clutch type LSD's they tend to lose their strength with old age. The Torsen catches right off the bat but I'm not sure how good it will work for drifting. People have swapped the Torsen LSD from the Miata into an FC, in fact there is a write up on how to do it on here in the 2nd gen FAQ.

6strngs

Haha, yeah, I understand there's probably only a handful of people on this forum who've attempted this. I've searched as much as I could but most of the rebuild threads I've found most people had a higher starting breakaway torque than me, and their thrust washers were ususally worn below spec so they were able to repair theirs by adding oversized thrust washers. I'm confused by mine because if I add all the numbers together I am only 0.05mm below the desired spec.

Oh well, guess I'm off to learn all about rx7's until I can post in the appropriate forum :P Probably a good thing since I know almost nothing about rotaries but I want an rx7 as one of my next cars. I actually almost bought an FD rolling shell yesterday if the guy hadn't sold it already.

6strngs

Yeah, but I've already spent $500 on this rear end (my car is a 91, so I had to spend $250 on a 94+ housing and then $250 on this diff) and the torsen's usually go for $600-700, not to mention they aren't the greatest for drifting. I actually did a lot of research before deciding that a clutch type was what I wanted. If I can't get this diff working again for a reasonable cost I'll be looking to getting a Tomei 2-way clutch type which sell new for ~$800, can't justify spending $1100 on a kaaz since I literally just got done building a turbo kit like last week so money is pretty tight at the moment.

6strngs

Update: So I took those spider gears to a local rear-end specialty shop and right away the guy there was able to tell me that the reason why that gear had siezed is from one wheel spinning independent of the other, most likely from the plates not providing enough clamping force, most likely from worn clutch plates. He recommended I replace all the plates and discs but when I asked him for an estimate of that he told me he most likely wouldn't even be able to get the parts. I know I can get the plates from mazdatrix but for the price I'd rather put that towards a more aggressive aftermarket diff. When I told him about how my plates were arranged he confirmed that the way that they WERE in there would cause a less aggressive lock-up, so he recommended I take some emery cloth to the spider gear until it went on smoothly again and then re-assemble the plates in the order that they are supposed to be in. Like so:



So that's what I'm going to do. If the breakaway torque is still low I'll just put some shims in it (between the thrust washers and conical spring, items 2 and 3 on that diagram) hopefully that'll fix my problem and this diff will last me for a while longer.

cab91089

iTrader: (3)

Ah I understand. Yeah the Torsen LSD is pretty expensive I didnt really think of cost when I posted. Goodluck with the problem!

6strngs

Ok, so I completely re-assembled the diff yesterday and measured breakaway torque at 30 ft lbs, better, still not great. So I put one shim in with a .28mm thickness and re-assembled and it came out to 32 ft lbs... It should have been much higher from a shim that thick. So I figured that was as good as it was going to get, re-assembled and then dropped off at the rear end shop to have the backlash and bearing pre-load set. I didn't realize until today that when I made those breakaway torque measurements I had the bolts for the ring gear in snug but not torqued, which will affect the readings, so, it could potentially (hopefully) be much higher than that. I will measure again once I get it all back in the car. Have to go buy the RTV and gear oil and then get back to work again... Going to be using a NON-synthetic 75w-140 based on the recommendation of the owner of the rear end shop. He said the thicker oil will help it lock more with the worn clutches. I asked if redline shockproof might be suitable and he told me that stuff is awesome for 1/4 mile drag cars but is not meant for sustained high-heat conditions and would be terrible to put in a diff that sees any highway driving or another sustained high-heat condition (like drifting, lol)

Also, I think I've been wrong this whole time and the diff I have is actually from an 86-89 non-turbo rx7, not the 84-85. I'm not 100% sure, but pretty sure haha

6strngs

All back in the car and with the ring gear torqued, still only 33 ft lbs... At least it's almost triple the strength it had before It does feel better, but still not as aggressive as I'd like. Oh well, guess it's time to start saving for a Tomei or Kaaz...