Rear Axle Swap - FB w/ disc into an SA
 

So I have a 1983 rear axle with disc brakes that I want to swap install into my 1980 RX-7. I know there will be some differences with the master cylinder, and it looks like there are some other differences. I am hoping someone may have some experience and can point me in a direction of what all needs to be, or should be done.

Some of my questions are as follows.

1. Are the connections at the M/C the same? Booster & brake lines.
   
2. Does the proportioning valve need to be replaced? Assuming so. Are the brake line connections the same?
   
3. The brake hose between the chassis and axle, I believe it is different? Can it be solved with an adapter if it is? Sizes?
   
4. Does the booster need to be changed?

Thank you in advance for any guidance.

thread pitch on the line fittings differ between the sa and fb.

Nothing changes at the Front of the car.
All you will need an "Adapter" soft line off the rear hardline (the one running from the front of the car to the back, at the axle) to mate to the hardline on the rear FB axle. The thread pitches are different and this "Adapter" soft line has the correct pitches at each end. I believe Racing Beat and /or Mazdatrix still sell this line.
You will also need to use the FB hand brake cable ENDS (the 2 that are attached to the axle) and attach them to the SA cable assy's front section in your SA.
Good time to replace/update your soft lines all around with SS.

Stu A
80GS (with FB discs)
AZ

For reference, FB thread pitches are M10 x 1.0 and SA's are M10 x 1.25. The SS lines can be found at Mazdratrix but I believe they only come in full kits - not certain about that though. RB put up a bulletin last fall about making some SS lines for all the first gens but I haven't heard anything about that since. I need to do SS lines on my car too, so if you find anything, please do tell.

I'm not sure about the proportioning valve, although I believe someone just did some reworking on one recently and would probably know. I'm fairly sure the booster should stay as is regardless of if you end up swapping masters.

I see the kit your talking about at Mazdatrix. Looks like what I need for the entire car in away. As for the differences in thread pitch, do you know if they are different at the master cylinder? Because that will make the job a lot more work.

Thanks for the advice.

I would assume that the masters match the rest of the car, so yes, that would make them different. Honestly I wouldn't touch the whole brake system to just swap the rear axle. Like 7aull said, if you can find the line for the rear axle with the two different ends, that would be the cleanest and by far simplest option. If you can't find the hose, there are plenty of adapters that you could use to accomplish the same thing.

My suggestion would be to get the SS line kit for the SA's from Mazdatrix and then hunt down an adapter than can fit between the axle hard-line and the soft-line coming from the body. That would require the least redoing of parts and be the cheapest option too. Redoing the hard-lines is really a pain to avoid if you can, particularly the one that goes between the two rear brakes (ask me how I know...).

There are a few here that support keeping the drums unless, and sometimes even if, a GSL-SE swap is available for the LSD. If you look at the numbers, on these cars the drums actually have a slightly shorter stopping distance, although disks dissipate heat more efficiently so, pick your poison. Not feeling like opening that can of worms though, so I'll just leave it at that. I can't remember which years went to the larger axles but it was somewhere in the 83+ region.

Hey, you may have already found this, but I believe this is exactly what you would be looking for: Mazdatrix Brake Line Kit (rear-end swap friendly). No adapters necessary I would assume, although I might suggest calling to make sure the axle-to-body hose has the two different fittings.

I did see this and plan on purchasing. I emailed them and confirmed it was what I need.

So I think I still want to go down the route of swapping the whole axle and rear brakes out. I plan on making this an auto-x car and just think it would be worth my effort.

I spoke with the guys who sold me the car and the rear axle. He did the same swap previously on his ‘79. From what he remembers, he deleted the factory proportioning valve and distribution block and inserted a universal adjustable proportioning valve. He said the stock master worked fine for him.

I think this this can be done pretty easily. The challenging part at the moment is locating brake line adapters for 10x1.25 fittings. It does seem to be an odd size. If I can find some unions and tube nuts I think I can make this happen pretty easily.

I have another question on the front brakes and hubs. There is a guy on CL selling an ‘84 non-gsl front setup. I’d like to do this change to upgrade to the larger bearings and better selection of struts/coilovers for possible future upgrades. Will the the ‘83 and later setup allow me to reuse my existing front calipers? It looks like brake pads are the same. So I’m thinking the change for calipers might just be the thread pitch for the brake lines.

Also, is the wheel lug pattern is the same?

If the rotors and hubs are one piece it is 4x110

all first gen rx7s that are not gsl-se have a lug pattern of 4x110.

Just coming across this thread ; I didn't see anyone mentioning the difference in the driveshaft / pinion mounting flange if you are swapping complete rear end disc to disc. 1979 through 1982 used the same size , 1983 they went to a bigger flange . If you use a 1983 or later driveshaft
you will have no problem .Good luck with the swap .... the LSD is a good improvement .

I just wanted to take a moment to let everyone know how I worked this out.

I bought a Summit adjustable proportioning valve and eliminated the OE one. I was able to bolt the Summit one in the original location. In able to eliminate the valve and install the new one, I had to make three brake new lines join everything. One, for the front right circuit joining the OE line to the M/C. A second one between the M/C and Adjustable p-valve. The last one joining the rear chassis brake line to the p-valve.

The easiest thing to do was buy some 3/16” premade brake tubes, 12” in length. I bought them with 3/8-24” tube nuts on them. Since that’s what the p-valve came with and the unions were easily to come by. I cut one end off to length, and shaped the lines as needed. And then flared the side I cut. The OE chassis lines were cut to accommodate 3/8-24 tube nuts. Then used unions to splice in everything with the chassis. For the connections at the M/C, I bought some M10x1.25 tube nuts. It came out pretty clean and took me about 2.5 hours.

Below is a diagram to make sense of it all.

I’ll update the post once I get the car driving and report back how well it works out.

https://cimg3.ibsrv.net/gimg/www.rx7...db6ac6cfc5.jpg
https://cimg4.ibsrv.net/gimg/www.rx7...6f458d32eb.jpg
https://cimg7.ibsrv.net/gimg/www.rx7...8c72e6d41a.jpg
https://cimg1.ibsrv.net/gimg/www.rx7...34515a2138.jpg
https://cimg4.ibsrv.net/gimg/www.rx7...d3ab188f8f.jpg

Nice write-up Yolo. Did you do this because you expect to need a different balance of braking with the weight difference from the furd motor?

I am pretty sure you have to change the balance because the drum and calipers in the rear use different brake line pressures to get the same amount of braking force. I believe if you didn't change the bias, you would have way too much rear brake because the calipers need less pressure.

Ah, yep. I brain farted that one... I got a GSL / GSL-SE proportioning valve for this purpose (disc to drum swap). I guess I overthought it when I saw the Summit part Yolo installed.

The brake fluid bias going to rear disc is the main reason. It seems a lot easier to source this than hunt down an OE valve from a later year with rear disc.

There were several reviews on Summits website for rear disc conversions.

I do like how i have control over that bias.

Does anyone know how to tell the larger bearing rear axles from a small bearing ones?

I was told the rear axle was from an ‘82, which would make it a small bearing axle (I believe change was in ‘83?). I have now ordered two sets of axles seals from two different vendors/manufacturers and they are both wrong (both giving me the same size seal). This makes me think I might actually have the big bearing axle.

The seals I received do not fit over the bearing retainer on the axle shaft (where the seal rides). So at least the retainer is larger.

If if I go back in my memory. Granny had me measure the bolt pattern on the rear differential flange for the driveshaft. And if I remember correctly, the replacement LSD one was different than what came in the car.

An outer bearing or axle diameter measurement would be helpful.

Thanks in advance!

Large bearing axle:
Axle bearing id 35 mm
Axle bearing od 72 mm
Axle bearing width 17 mm

Thanks for the information. Good news! I have the larger axle. Your information all lined up with what I have. I also found some other information that helped. It has 26 splines and the pinion flange is larger with through holes (not threaded).

Ill go ahead and reorder some seals.

Thanks.