Hello, I'm new here and I'm having trouble finding an open rear differential for my 1980 RX7 GS. I'm looking everywhere and can't find anything. Having to call salvage yards etc. I was wondering if anybody knew of a rear differential (open or LSD) that'd fit in there smoothly without having to do any different modifications to the axles or drums. I've seen forums on here talking about differential modifications involving a whole new set up and I'd like to avoid that route (if possible). It's been a headache for me, my car has been in the shop for over a month now. The old open rear differential is kapoot. Any advice? Thanks.

(You'll see in the pics below the only issue I have is an axle problem now, or an easy differential replacement. If anybody knows of either 2 simple fixes, please reach out. Thank you.)


This is the rear differential I bought on ebay that doesn't fit the axles to the carrier, the axles are too short.


This is the info that the seller told me about. Looks like it'd fit. Didn't think about the axle length, I was more worried about it fitting in the housing. The seller isn't accepting returns so if anybody wants it, it's for sale.

 

Here's one on ebay

 

So the 24 spline SA axles make the diff internals hard to come by. This post explains it a bit: https://www.rx7club.com/1st-gen-gene...nd-use-1151793

Whats the issue with your rearend?

Also the SA diff is a 3.9 not a 4.1 or whatever you posted about. I would see if you can find either a good used one or a LSD for an SA and swap it in there.

 

Thank you, this really helped out. Do you happen to know if the 79 (where the part came from) and 80 have the same axle length? That's the only thing I'm worried about if I order another center chunk.

 

I recently had some work done on it at a shop, and with that work I had rear axle seals ordered (in all honesty I just threw them in there bc I was ordering new bearings from Atkins for my new rotors, so I thought what the hell, they've probably never been replaced anyway). They replaced the improperly installed seals (rather they gave out or whoever installed them previously didn't install them flush), then they drained and refilled it with differential fluid.

After I got my car back it sounded like there was broken glass and screws thrown into a blender coming from behind me. Took it back to the shop and they were trying to diagnosis the problem. They couldn't find anything except think however long I've been driving my car, it's been on 1/4th of the differential fluid it's supposed to be on, and with the seals being broken previously, maybe some dirt or whatever else got in housing and messed everything up. They say they don't know. So they drained the fluid, ran a magnet through it, and found metal shavings and a few metal chunks. Bad news right?

So now I'm on the hunt for a new differential chunk. My mechanic said it can be a 4.1 ratio bc he can set it to that rather than the original 3.9, just as long as it fits in the housing. Well the one I ordered fits in the housing, it's just the axles are too short now. My car didn't have this issue before I took it to the shop mind you, I drove it all the way from Nevada to Florida with no issues. I just wanted the front rotors, bearings/seals, brake pads, brake shoes, and a straight pipe from my headers to my exhaust replaced from this janky flex pipe that was installed instead. Now these past 3 weeks I've been having to research rear differential options.

 

The 1979/1980 are really hard to find brakes parts for. Nobody makes the rear drums. With this said, I'd switch to a 1981-1983 (small axle) or 1984-1985 (big axal non-GSL-SE). With the 1984-1985 (non-GSL-SE) you'll need a new driveshaft. This will give you the chance to go with disc brakes.

 

Riggs vintage mazda on ebay will prob have what you need.

riggs store link

I got a 3.9 from them for about 200$

Be VERY picky about the bearings. don't ask how i know

I am going with SKF this time around. bought and fried lower end bearings....you really gotta look at load, rpm operating capacities and such. bearing are not even close to being similar in quality, and the price difference is worth it.

 

if it's a 79/80 drum you will need small axle type rear carrier.

I have a set of drums form my 1980 in a bucket in my attic I been meaning to sell if anyone needs em.

 

I found a salvage yard on car-parts.com that has a 79 carrier assembly available, everything is still in the car with only 84k miles on it. I just need to know if the 79 and 80 have the same axle length so it'll fit in my 80. Do you know if they're the same length?

I also saw you said you upgraded to SKF bearings? Are you saying you recommend those over the OEM ones? Bc my mechanic basically broke down everything that's in a differential that he'd want to inspect before slapping the "it's good" sticker on it before recommending buying one. Understandably though not every seller is gonna give a photo album of the intricate part you wanna buy. But if I can buy those bearings for cheap after the differential comes in I might go with that option.

And what condition are your drums in? I haven't removed the surface rust from mine yet so I don't know how bad the damage is but they're still running. If yours are for sell I'd definitely take a look.

 

Not to throw more research tasks at you, but I have an 81 diff you can have for free. Just cover shipping from SF. Don’t know if it will work with your 79, but if any of the more expert folks here can tell me how to check I can take measurements or post pics if it helps explore if it will work.

Dont know mileage on the diff. If it’s original to the car it has 180k on it. Was working fine when I removed it. Only removed it because I swapped to a disc / LSD rear axle.

 

By all means take as many pictures as you can and send them my way. I appreciate the help greatly. I see a lot of talk in the forums about converting to GSL with disc rotors or GSL-SE upgrading the whole housing, but not much info about GS interchangeable parts. I'm trying to have as minimal part hunting to do to get her running again, then I can look into modifications in the future haha.

 

81 diff I have to give away is still in the axle. Lets see if anyone here can help us work out if its possible it may fit your 79, and if so what pics would help turn possible into definite. So... you more experienced folks on this site... Is there a chance an 81 diff will work in a 79, and if so what info / pics do you need to check details?

Diff I am offering came off an 81 FB. Small bearing axles, drum brakes. Smaller bolt center diameter drive shaft. Currently consists of diff still in the axle body. No axle shafts or brakes. Can we tell from what is left it my 81 will work in Door's 79?

 

It'll fit.

 

yes, they are the same. you should grab that asap....just don't let em know you're in dire need and charge you an arm and a leg.

I guess you could just slap it in if the mechanic says the bearings are in great shape. If you change the pinion beaings you'll need a new crush sleeve and bolt from mazdatrix. oem bearings should be fine on yours if its a rotary. just a bit more coin, and machine shop to press bearings etc....would be much easier to slap one in if it looks 100

I have not even looked at the drums in years, but they looked fine when I put em up there. I'll take a look soon, as I am up and down in the attic for stuff during the holidays. car had ike 110k, so they should be good....but I'll look.

 

Door, if you are willing to give it a try message me your address and I will get you shipping cost.

 

All the fb axels are the same length but dont swap them from side to side

 

..

 

I'll definitely call that salvage yard again Monday. I'll look for those parts too on Mazdatrix, I'm familiar with them. And by OEM bearings do you mean the ones that it already came with or I should order OEM bearings? Bc if SKF has better bearings I'd buy from them instead.

And sweet, if the drums look nice I'd be definitely interested. Get back to me whenever you take a look. Thank you so much for all this information, it really helps.

 

Removing the diff to send to Door. Removed the 12mm hex head bolts around the outside of the housing, including the sneaky longer one half way up the side. However it’s not coming off. Gave it a few good whacks with a rubber mallet but not budging. Is there something I missed, or do I just hit it harder. FSM skips over the removal bit.

 

hopefully the axles are out or it aint coming out lol. other than wacking it straight on, left to right or up or down. she'll come loose

 

Axles are out. I wailed on the diff housing this morning with a rubber mallet then a heavy hammer, and it ain’t budging. I see a black sealant seam oozed out and dry all the way around that looks too rough to be factory, so any bodies guess what was used. Hitting it so hard that am at the point of damaging the housing. Already missed one blow and put a dent in the part that bolts to the drive shaft.

Door… next thing I can think of is hammering a pry bar between the axle housing and the diff housing. This could damage the face of the diff housing, maybe make it hard to reseal. Are you planning on using your own diff housing, so damaging the one I have is OK?

 

perhaps a wide, sharp chisel can be used as a wedge?

I'd only hammer it in at the top half since the seal is mainly for splashing fluid there.

 

How about some heat after the diff fluid is removed?

 

Officially gave up. Heat via a blow torch then two handed test-your-strength style swings with a big mallet. No movement at all. Starting to wonder if it’s welded on but don’t see any evidence.

Recommended Riggs to try next. Can take a while as they have to search their junkyard, but they get the job done and have never failed me.

 

I have one from a 79 with 45k miles on it. Nothing wrong with it when pulled. Have another with unknown miles looks mint. Have pictures of interested.