The rabbit hole that is the Rx8 rear end swap.
 

So after posting this thread https://www.rx7club.com/2nd-generati...ng-me-1141127/ I started down the path of putting an Rx8 rear end in my '89 GTUs. Mistakes were made along the way that I'll detail when I get to them. Right now I am probably 60% to 75% done but I'm feeling good about it. I think I may pull this off. With that let's get started.
First off I got an Rx8 rear end and mount off Ebay. Though through the course of all this I never took a picture of the mount.
https://cimg8.ibsrv.net/gimg/www.rx7...ecb835d38b.jpg
https://cimg0.ibsrv.net/gimg/www.rx7...4c78a302c3.jpg
As you can see from the pictures I also ordered some TII half shafts as well. They stick out due to the RX8 rear being narrower than the TII rear. I have a plan on how to deal with it. Pretty much what Higgi did on his car. I'll have to make or get made something to relocate the oil seals.

I at one time thought I could fab up something that used the factory Rx8 mount.
https://cimg3.ibsrv.net/gimg/www.rx7...f9fc9e4e92.jpg
I quickly decided against it, but decided to use the center as the start of my template, and came up with this.
https://cimg3.ibsrv.net/gimg/www.rx7...df5435c4f3.jpg
I tried to make it so that the centerline between the upper diff mounts line up the centers of the output shafts with were the FC shafts were. Hence the reason for the offset.
That's all for now I'll try to post the rest of my progress over the next couple days.

So at this point I really just a vision in my head. A basic idea of what I am trying to build. I have bought the metal I think I need. The key piece being the cylinders that the diff mounts go in. I found a cylinder with the I.D. I needed and got a section of it. Here is what I ended up with to get started.
https://cimg5.ibsrv.net/gimg/www.rx7...cc39043ddd.jpg
https://cimg6.ibsrv.net/gimg/www.rx7...af5dd9a0ba.jpg

Next I decided to cut out the center to go around the top and drill the first two mounting holes.
https://cimg4.ibsrv.net/gimg/www.rx7...86892e7aae.jpg
https://cimg5.ibsrv.net/gimg/www.rx7...499707fd8f.jpg
I was proud of my work so far but this step was my first mistake. More on why later. Remember I was designing as I went along, but that wasn't the culprit as much as my inexperience as a welder.

Next up I added the piece to mount to the other two studs( though technically I'm using bolts) And also the pieces I made for the lower half of the mounts to sit against.
https://cimg8.ibsrv.net/gimg/www.rx7...06ae7ba959.jpg
https://cimg0.ibsrv.net/gimg/www.rx7...03b49a7585.jpg
I had also cut spacers out for where the bolts go through. All tacked up and feeling good.

Next up I welded everything up so far. This is when I found my mistake.
https://cimg9.ibsrv.net/gimg/www.rx7...2c467f9ef4.jpg
https://cimg0.ibsrv.net/gimg/www.rx7...33317a675c.jpg
If you look close at the right side of the lower view you may be able to tell it's not level anymore. The heat from where I welded the two pieces together caused it to bow. Too far in to stop now so I kept on going hoping it wouldn't cause a problem but being irked by it at the same time.

So next I boxed the front in to strengthen it up and added some metal back where I removed more than I needed Also discovered I miscalculated the height of the part the mounts pressed into so I added some metal in there. Apparently I didn't take a picture at this point showing how bad it warped but it wouldn't slide up on the mounting studs due to the angle it put that diff mount in. I need to get this done and haven't the time to start over so I cut on three sides and removed some metal and bet it to where I thought it should be and tacked it for a test fit of the mount only. Slide right up the studs as it should. I plated over my cuts on the sides. for added strength. You can really tell how bad it was by the angle coming from the boxing of the center. Mechanically I think it's fine. Cosmetically it doesn't look so good.
https://cimg5.ibsrv.net/gimg/www.rx7...d48b36e99f.jpg
https://cimg7.ibsrv.net/gimg/www.rx7...af151477a6.jpg
https://cimg6.ibsrv.net/gimg/www.rx7...25e1602f53.jpg
https://cimg1.ibsrv.net/gimg/www.rx7...7b4556bc26.jpg
I may have bent it too much but it mounts into the car just as it should. This is my prototype I guess. It really bothers me but I need to get the car back together. I seriously consider driving it for awhile and building another improved version based on what I learned on this one. Then again it will be out of sight out of mind once installed. Other than I've put it here for all the world to see. If I do it over I think I will wait until I have learned to TIG and then I can make it pretty.

Here it is in the car and also the interference of the PPF mounting studs and the subframe.
https://cimg4.ibsrv.net/gimg/www.rx7...44ee3852ff.jpg
https://cimg1.ibsrv.net/gimg/www.rx7...2341fc03e7.jpg
https://cimg3.ibsrv.net/gimg/www.rx7...115849d766.jpg
Well that's my progress so far. On to the Oil seal spacers, front diff mount and custom driveshaft.

Nice work on the mount, definately easier approach than mine. Rigidity is questionable on both of our designs, lets see whose will break sooner :D
As shown in my build, piece with upper mounting studs to be removed, or you will have to do more work on the subframe. Since i find rx8 diffs cheaper/easier, i decided to chop off the diff.
Part of my build was dictated by already existing custom driveshaft, you are more flexible in that part.
Good luck!

Thanks. It seems right where you need to retain some rigidity is where you have to cut out material to mount it to the case. I guess I could make some bracing that bolts to the rear cover like the Ronin Speedworks 8.8 kit. I'm going to use as is and see how it holds up. will try to figure out how to improve on ver.2.0 if this one breaks. Since I'll be n/a for awhile yet I feel good for the moment.
I was actually going to message you and ask what mount it was you used for the front mount.

For front mount i had some old bushings laying around (might be stock rear control arm bushings which i pressed out to replace them with energy suspension ones)
I will be testing my design soon with GT35R + semi pport, so it might break sooner :D

Thanks. Still sorting out what I'm doing for the front mount. Thinking about using the Ford front diff mount from the 2000-2005 Explorer and using that front bolt location that is currently in the way. If I could knock it out in order to use a longer bolt.

Great progress! You're much more amdaring than I am in this project, and it looks like your efforts are paying off.

Dak, can i ask why you are putting a rx8 diff in your rx7?. is it for the ratio, or for the LSD?

Both. I explained it in the other thread but here's why. My cay is the GTUs which had a Viscous LSD and a 4.3 gear in the rear end. All other USDM cars except the automatic had a 4.1 rear gear. Spider gears in my LSD went bad I believe. Was going to put a Torsen from a Miata in it but they have went up in price to between $400 & $500 after shipping. Since I plan to go turbo one day and make more power than the 7" ring gear may survive I thought of getting a TII rear end( it has an 8" ring gear) but I didn't want to switch to the 4.1 gearing. Mazdatrix sells a 4.3 gearset but it's $826 on top of what a TII rear end would cost. That is the only source because the FC uses a long pinion gear unlike the FD and Rx8 which use the same style short pinion and I discovered Rx8 rear end could be had for a bump over $300 after shipping and the 6speed was a LSD but. So I decided to try to put one in my car. Gear ration is 4.44 though, but if I don't like it auto Rx8 rears with a 4.3 can be had for under $300 shipped. They aren't LSD. Not sure of the final cost but it will be more than the Miata Torsen but should be less than the gearset alone for the TII rear. I've currently hit a snag as the quote I got back for my oil seal spacers was twice what I had hoped. I am going to quote them somewhere else next week. It has been a little bit more of a project than I initially thought but for me I think it will be worth it in the end.

Dak

Have you opened your rx8 diff ? If u can, please do it and take pics of internals. I have opened bunch of rx8 diffs and none of them was Torsen, they were all SuperLSD.

I have not but I think it is a SuperLSD. If you look through it with the axles out it has a metal bar in the middle like the SuperLSD.

I believe Torsen in RX8 is fairytale.
Btw...all SuperLSDs in RX8 i have opened had crack in th friction gear.....still working

Maybe I'd better open it up and take a look at it. At least it looks like Torsens out of a 3rd gen are still around $300 on Ebay.

It's been awhile. Put my garden out and the MX track finally opened due to some Covid-19 restrictions easing up so it's hindered my progress. I knew if I didn't get it done before gardening season I'd get delayed. Anyway here are some updates.
I re-quoted the oil seal spacer from a company called Xometry and they came in priced more to my liking so I ordered them. Haven't installed them yet as I'm waiting till I open the diff up to check to see that nothing's broken inside.
https://cimg9.ibsrv.net/gimg/www.rx7...dd068a3b99.jpg
https://cimg3.ibsrv.net/gimg/www.rx7...bcf69a337d.jpg
I decided to use the Ford Explorer mount. Mounting it vertically didn't pan out so I did it similar to hiGGi.
Here' the mount
https://cimg0.ibsrv.net/gimg/www.rx7...06d856df6f.jpg
https://cimg9.ibsrv.net/gimg/www.rx7...3217a8c52c.jpg
I was going to give a better update but have discovered I haven't gotten all the pictures I need off my phone so I'll try to get them off in the next day or so.

Here is one of the mount on the diff.
https://cimg3.ibsrv.net/gimg/www.rx7...dcd4ad321b.jpg
Mock up of the subframe brackets
https://cimg7.ibsrv.net/gimg/www.rx7...746db17efe.jpg
https://cimg9.ibsrv.net/gimg/www.rx7...c80b7bb83c.jpg
And the final tacked on to the subframe. I still have to weld it and add some support pieces.
https://cimg5.ibsrv.net/gimg/www.rx7...298cbb2b1a.jpg
https://cimg3.ibsrv.net/gimg/www.rx7...88d6bdb1e7.jpg
https://cimg1.ibsrv.net/gimg/www.rx7...f22b7b2267.jpg
Here's one of my mocked up driveshaft for length measurements. Drive shaft is on order and axles are here so I should be in the home stretch. Just a little more welding and getting everything painted.
https://cimg6.ibsrv.net/gimg/www.rx7...a5e0529e96.jpg

Hey I finally opened the diff up. From my image search it looks like a SuperLSD. No metal pieces in the bottom of the case. It looked good when I had it open but I didn't know exactly what to look for. When I have the cover back off to put the sealant on it I'll look again. Is it real noticeable if it's broken?
Here is a picture.
https://cimg9.ibsrv.net/gimg/www.rx7...8d43c9dda2.jpg

Not easy to spot, i believe u have to take it out and apart to find the crack in friction ring.

Great. I guess I'll just roll the dice and run it. They're still all over Ebay for cheap. If it were to fail and any pieces damage the ring & pinion.

Here's the diff ready to go in. Turns out I got a couple measurements wrong on my oil seal spacers. First I wanted a press fit into the diff. They were loose. If you look close you might see the JB weld. Second they were too wide and the stub shafts wouldn't snap in. I took them to work and used the lathe to move the faces I needed moved. They probably could have stood to be moved another .050" more so the cone shaped part of the oil seal wouldn't fully compress but they snap in so I moved forward and installed them.
https://cimg9.ibsrv.net/gimg/www.rx7...48a9d3516b.jpg

My drive shaft finally came in as well.
https://cimg8.ibsrv.net/gimg/www.rx7...e1ba12aca9.jpg

Well it's always something. So I installed the diff and the driveshaft and Houston we have a problem. The ears on the drive shaft that the U-joint mounts in hits the bottom of the shifter housing on the transmission. It uses a conversion joint like this thread https://www.rx7club.com/2nd-generati...shaft-1081644/ said and also which is what Mazdatrix uses in their 3" aluminum driveshaft. At this point I'm just deflated. I was so close. Anyway I'm not sure why the guy in that thread or Mazdatrix doesn't have problems. Only thing I can figure is their shafts look like a slightly different design for those ears with a more aggressive/ more angle on the chamfer. That or the yoke sticks out of the trans more getting it out from under the tail housing. I don't think so though cause using the stock L2 measurement like that thread would put it right under the tail housing. I think I am going to break out the grinder as that seems to be my only option. Shortening the shaft enough to get it out from under the tail housing would have it sticking out from tranny by over 1". Here is a picture of the joint on the diff side.
https://cimg9.ibsrv.net/gimg/www.rx7...0849be9636.jpg

Nice work!

Thanks.

Well after bolting everything up and looking at it more I've come to the conclusion I'll have shorten the driveshaft. I can't remove enough metal without getting into the groove for the retaining ring. Also I talked to Mazdatrix and that's how they build theirs. Maybe I'll get to drive it soon. I think this driveshaft is the last piece of the puzzle. Of course I haven't put my axles in yet.

I have already put some miles (and smiles) on mine, but not full boost/WOT yet...so far, all good :)

Took mine on it's maiden voyage today. Seemed to do good. Revs pretty quick. Don't know if it's noticeably quicker than the 4.3 since I haven't drove it in like 8 months but way quicker than the 3.73 in my pickup. I discovered my passenger rear wheel bearing is shot while I was changing axles so still sounds like I have a big mudder tire on that side. I don't think I have the vibration in the chassis I had before though. I noticed in your 13B-REW thread where yours cracked. I discovered that due to having to remove so much metal to clear part of the diff housing that I don't think I'll be able to use the diff as a jacking point anymore(not on this version anyway). I don't think there is enough enough metal there to support the weight of the car. I guess I didn't take that into account. I guess I'll count this one as the prototype. Starting to think on improvements for version 2, but for now once I fix the wheel bearing I'm just gonna drive it for awhile and see how it does. I will post the rest of my pics in the next day or so.

congratulations on your success. i admire the work you put into making this happen.

Thanks. It ended up being a little bit more of a project than I anticipated.

So here's a few more pictures. Up first is the driveshaft solution. Ends up with about 1.5" of pull out. It's really about.100" to .250" more than recommended but it seemed to be the only option I had.
https://cimg5.ibsrv.net/gimg/www.rx7...3cb402009a.jpg
Here is the area of the subframe I had to cut out to clear the flange the driveshaft bolts to. Unfortunately I didn't take a picture before I semi boxed it back in.
https://cimg2.ibsrv.net/gimg/www.rx7...afbb8de5d8.jpg

And after I ground down the welds for clearance and cleaned up the sub frame some for painting.
https://cimg0.ibsrv.net/gimg/www.rx7...be2a1cea44.jpg

Picture of the front diff mount bracket before painting.
https://cimg1.ibsrv.net/gimg/www.rx7...651bb2f2d5.jpg
https://cimg2.ibsrv.net/gimg/www.rx7...00148b7f15.jpg
Apparently I didn't take any pictures of the painted subframe before I put it in the car.
And fully assembled.
https://cimg1.ibsrv.net/gimg/www.rx7...a9eb5e4814.jpg
On a side note I see replacing a fuel line in my future.
https://cimg5.ibsrv.net/gimg/www.rx7...eda850246d.jpg

congrats!

just few cents
- on my setup, i am already pushing 1,3bar of boost on semipports and diff and mount is holding fine after first crack, definately feels better than oem FC diff setup mounting - about 500km on it so far, mostly power runs :D
- next version of side diff seal spacers/adapters will need to be o-ringed in my case, as oil is slowly seeping between adapter and iron housing....its tight fit, but in my case corrosion in the iron is not helping to seal it properly

nice job! I'm curious on this for the opposite reasons (FC bits in an rx8, long story) how did you do the axle portion? do the rx8 axles spline into the FC rear hub?

what combination of parts did you use for success?

also xometry is our competitor check us out at forecast3d.com instead I'll help you out!

Maybe my mistake of making my adapters too lose will end up a good thing. Since I had to epoxy them in with JB Weld maybe that will keep me leak free. I've only got about 70 miles on mine so far. Once I fix my wheel bearing I'll get some more miles on it.

Thanks. I used TII axles. Since the Diff itself is inter changeable between the TII, FD, and Rx-8 the TII stub shafts snap right in. I just had to make those spacers to relocate the oil seals since the Rx-8 rear is narrower. As to the hub. According to Rockauto the spline count is different. The Rx-8 has 31 and the Rx-7 has 28. You could probably make a custom axle like Ronin speedworks does on their kits and put an Rx-7 end on an Rx-8 axle.

I'll check you guys out. It's gonna be awhile but once I learn to weld aluminum eventually I want to put this FD UIM I have on my N/A LIM and I'm gonna need some flanges made to use in my adapter. Oh I probably need a standalone first too to get the full benefits I hope to gain of a less restrictive intake while still keeping the "supercharging effect" of the VDI intake. Oh I've been kicking around if there is a way to make a bracket/ adapter to adapt Rx-8 rear calipers to the FC as well so that may be my next project.

thanks for the details, I was beginning to wonder some frankstein combination of parts to support the next rally car platform (which will likely be dead engine'd s2) I may not need the seal spacers like you as I plan to keep the rx8 diff in the rx8 and use the FC rear suspension so potentially if the lengths are correct I would just need to swap the spline side from an rx8 to an rx7 style and this SHOULD work but general fabrication. but I'm still pretty early on that project because the rally FC still works great :)

we do some machining but mostly do a lot of 3d Printing (metals and plastics) so if you get a wild design we can always print it out of metal in a way that can't be machined.

I've been working on Z32 rear calipers on the FC for a brake upgrade but running into some clearance issues, I want to avoid any sliding caliper system.

No problem. I was kicking around the RX-8 caliper mainly because it seems the FC ones are NLA and it looked like they used the same style of parking brake setup along with (according to Rockauto) the rotor width being close to the same.

I have used rx8 rear calipers on FD (together with rx8 brake cables) and it works fine. No so much for FC.
For my old drift FC i had adapters for 2 sets of 4 piston FC calipers

https://rx7.cz/gallery/?Qwd=./2010-0...v=thumbs&Qis=M

one last question, overall length wise how different is the t2 axle to the rx8?

Unfortunately that I don't know. I never had any Rx-8 axles and I didn't take a measurement of the overall length of the TII axle and stub shaft. I relocated the oil seals by0.640" per side after I modified my spacers. I got my original dimension by taking measurements of the rear end widths. Google says the Rx-8 is wider so I'm guessing it's more than my 0.640". Sorry. Maybe hIGGI knows.

So now that I've put about 550 miles on it I thought I'd post an update. Probably be the last one until either something breaks or I make version 2 of the mount. I'm not in any hurry so it'll be awhile. I replaced the wheel bearing and installed my DTSS eliminators. Then of course the the brake fluid leak I forgot about came back with a vengeance. Was leaking where the rod for the parking brake went into the caliper. Luckily Rockauto got some passenger rear calipers in so I snagged one and fixed that. Everything seems to be good now except for I have some vibration when not under load at speed. Like if I put the clutch in at 60mph or so for example or if I'm coasting with the clutch out but no real accel or deccel load. It is smooth under acceleration and for the most part during decceleration. Not sure if it's from the amount of pullout on the driveshaft as the company who made it warned it could cause some vibration or if it's normal from using the competition diff mounts. I did have a small leak on one of the axle seals but it seems to have went away. I've had some full throttle pulls from a roll through first and second but no hard launches and all's held up well. I'm pretty sure it will for now as I'm probably not making much above stock power levels.
As for the gearing after more driving I think I can tell a difference. I like it but I think I may eventually go back to the 4.30 gearing. I think I am going to pick up another Rx8 rear from an auto while they're still cheap so I'll have a set of 4.30 gears around, and I want to pick up a torsen diff from a third gen while they're around and not too expensive since it sounds like this super LSD will eventually break those ears off the cones. Back on topic. It's hard to explain the gearing. From a pure acceleration standpoint I like it. From a daily driven stand I find my revs are a little higher than I was used to in a given gear. When in town it's taking some getting used to. On the high way it seems happy turning those higher revs so I find myself driving faster. Sometimes it feels like it's being held back and just wants to be unleashed. It only threw my speedometer off by 5mph, reads faster than I am going, but it already was 2 or 3mph off. Too soon to tell about fuel mileage. First check was 19mpg which is normal for this time of year. Second was 16mpg but there wasn't much highway on that check. Mostly short trips and city driving. That's all I guess until I have something else to report.

The factory limited slip is a Tochigi Fuji Super LSD, it’s a 1-way function similar to a Torsen, but a cone engagement clutch style

when strange metal chunks/pieces start coming out with the oil then you know it needs to be swapped out

https://www.rx8club.com/new-member-f...y-diff-270191/

be careful, the auto 4.30 diff is a different open-type design and won’t accept a limited slip, uses different axle stubs, etc.
.

Are you saying you can't put a LSD like the torsen from a FD in the Housing of the auto RX8 rear end? Hmm that might slightly change things in that I'd have to move the 4.3 gearset from the auto into my current rear housing. As far as the axle stubs the TII, FD, and Super LSD from the Rx8 are all splined the same and the FD and RX8 use the same oil seal. So if it's just a matter of the splines in the diff itself I'd be throwing it away anyway.

yes, the RX8 diff with LSD is internally interchangeable with the FD (I posted an RX8 LSD up FS on here about 13 years ago), but I’m not entirely sure that the Auto diff can do that. I’m just warning you to be careful and not just assume. The 4.3 R&P can be used in the LSD diff, but whether the bearing carriers, internal clearances, etc. are the same is a question mark. They have two separate parts diagrams for each


https://cimg5.ibsrv.net/gimg/www.rx7...29a3c69349.gif
.
https://cimg8.ibsrv.net/gimg/www.rx7...6369a1bbec.gif
.

I had noticed that there were two diagrams and wondered what the differences were other than the diff itself. I looked on Jim Ellis and they show the part# for both housings and carrier bearings to be the same. I haven't cross referenced the other parts but the last time I looked the complete auto rear ends were still so cheap if I could only use the ring and pinon and had to swap it to my manual rear end I'd still save nearly $400 vs. buying a 4.3 gear set from Mazdatrix.

This is fantastic, where did you source all the metal material from?

Thanks. The square, rectangle, and round tubing I got from Metals Depot. Metals Depot® - Buy Metal Online! Steel, Aluminum, Stainless, Brass
I got some flat mild steel from Ace hardware, but Lowe's has it too.