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To begin this story, we must first witness what some hard driving on a 460+ WHP 440+ WTQ 2JZ will do to a 7.5" diff, even with a Truetrac and solid spacer kit (and the big truck input flange for 1310 U joint):


Yes, major ring and pinion destruction. Bear in mind I have 18x10 rear wheels with very sticky 285/30 rubber, and my subframe has been modified to give me around -0.75 degrees rear camber, so traction is a big factor here - a lack of it is probably why the 7.5" holds up to big power in other cars, as once you are in higher gears the torque multiplication from the transmission is so much less.

So, I went in search of and had delivered to me this large lump of unobtanium, thanks to Yahoo Japan auctions and Jesse Streeter.


Yes, it is the elusive 8" G code diff from a GX71 Cresta/Chaser with 1GGTE. The same diff is also offered in MZ12 Soarer with 6MGE and manual trans. It's pretty rare, and this later version has the 6 bolt side shafts that are the exact same as the MA70 8" diff. In fact, other than the case and cover, the rest is all exactly the same as a MA70 unit. In a pinch the MA70 cover even fits but it has a couple strange holes for mounting studs on the back this diff doesn't have. Some older versions of this same diff have a 4 bolt side shafts with a larger PCD than the 7.5", but I prefer the 6 bolt as if one gets damaged you can swap in one from a mk3.

The real benefit to this diff is that it is almost a drop in for the mk2, as the GX71 and MZ12 share the same suspension design and diff mounting. However the actual main rear subframe is a tiny bit different, we will get to that later.

Oh I also pillaged this from a FRS diff - its a high bias T2 Torsen LSD. Total cost after offloading the diff carcass and innards without the LSD was less than $300, and it was less than 60k miles!


It bolts in to the G code diffs, as the FRS uses a version of the G code diff the same as the IS300 (just has smaller pinion bearings, but even uses the same gears).

So first I used the damaged housing from the little incident up at the start (the only salvageable parts from that diff were the cover, Truetrac, solid pinion spacer, input flange, and output flanges) to jig the diffs up and compare the mount points. My biggest concern was that the 2 mount points at the back for the smaller rear subframe, were in the same position relative to the 2 most forward top mount points near the input flange.


So the position actually looks really good, So good in fact that the rear upper mounts, as well as the forward 4 mount holes only need the one washer welded inside the subframe on the right hand side as the 8" mounts are actually a tiny bit narrower in this location. The 4 more rearward mount holes for the main subframe are all over the freaking place, so the first thing to do after removing the subframe is cutting off the back half of the diff mount, and welding that one washer to the inside. I also welded some washers to the outside of the mount holes as doubler plates to reinforce them as one had some cracking around it that I welded up. I actually bolted the new diff in place with these washers before tack welding them - some of the old holes were a little widened out by many diff installs so I wanted to make sure I had it right. I also welded that one washer to the inside of the right hand side mount.


Next step was to cut the back half of the mount into 4 sections, one for each mount hole, and weld a doubler washer onto each mount hole the same as the forward part of the mount. Then trim and bend them with some test fitting on the new diff, before bolting them up and welding back onto the subframe. The only place I had to add material was between the 2 top mount holes, as the mounting pattern is actually closer to the front 4 holes and no taller, but slightly wider than the 7.5" diff.


Final product looks pretty good! Final steps here are to remove the diff to weld and clean up the underside, as well as heating and hammering the subframe right above where the input flange and U joint are - the one we will use is even larger than the stock size on the 8 inch so you will need lots of clearance. I already had the larger flange so I could use it to determine how much clearance I needed. Then re test fit the diff, and sandblast and paint the subframe. Biggest benefit with this diff (even though it isn't quite bolt in) is that the position is the same as the old one, just an equal amount wider on each side (14mm per side). It will make our life easy later.


Now the first test-fit in the car, with the scabby diff pre-rebuild. This is to measure for driveshaft and CV axle lengths, and to confirm that the rear antiroll bar still fits (I can confirm that it will fit with stock, Addco, and Whiteline bars). I did find an interesting crack starting from the hole at the top of the rear diff crossmember! Probably from driving this car with lots of boosted horsepower for many years.


The hole serves no function so I welded a plate over and welded the hole up from underneath, then repainted the subframe and reinstall. This is something more people should probably check for as the hole is a bad stress concentration.


Now on to that crustacean like diff. Not sure how many years it spent out in the rain, but after several hours of disassembly, cleaning, bead blasting, and painting it is ready for a rebuild. I have already installed the ring gear onto the Torsen by heating the ring gear in an oven at 300F for 60 minutes or so.


For the diff build, I used the rebuilt kit from Driftmotion with new 29 spline ring and pinion from Nitro Gear. It includes nice seals (the side seals are the updated version used in the IS300 and FRS/BRZ diffs and the pinion seal is from a Tacoma to match the larger 29 spline input flange), Koyo bearings, a solid pinion spacer instead of the dumb crush sleeve, the brand new 29 spline ring and pinion in 3.73 ratio with a set of pinion shims, as well as a double drilled 29 spline pinion flange and matching larger nut. I got new lock plates for the ring gear bolts, new fill and drain plug with washers (I got the later model drain plug with the huge magnet), a new vent, and new cover bolts. I also had to get a decent selection of side shims for setup. Luckily all the parts other than the seals and input flange are mk3 diff parts. And of course the T2 Torsen LSD from the FRS diff, but you can use the LSD of your choice. The diff setup was pretty easy with a new ring and pinion making for a pretty clean pattern check. Backlash was set to 0.005" and good preload with 175 ft-lb on the solid pinion spacer.


Finally the cover and side shafts are installed. One minor modification for the T2 Torsen is required, as the FRS and IS300 stub shafts are 6mm longer, I just welded a 6mm tall stack of washers that fits inside the spline to the end of each shaft and sanded the end nice and flat. The Torsen isn't splined past the retaining ring, which locks in properly. This keeps the side shaft dust shields from rubbing on the case and seals which will damage them. I used black FIPG (my personal favourite) on the cover.


Now the final install of the diff into the car after filling with Redline 75W90NS. I like using fluid without LSD additive for Torsen LSDs as it tends to increase the bias, and they run smoother. This diff holds almost 2 quarts of fluid which is a big improvement over a Truetrac equipped 7.5" that barely fits 1 quart. It fits well and looks fantastic, I had to double check the clearance around the input flange for the larger flange yoke and U joint, good thing is you could always hammer a bit more and re-paint at this point if you had to.


This is the driveshaft I had made. I wanted to go with the Driftmotion R154 to 1350 U joint slip yoke as it has removable caps allowing the single piece driveshaft to be removed with the diff still in the car (this was a major issue with my previous setup). It has a 1350 U joint in the front and a 1330 U joint in the rear attached to a 66mm x 66mm flange yoke for a Tacoma (Neapco N2-2-1904) which fits the larger bolt pattern on the new 29 spline input flange. Diameter is 3.5" and length is 45.5" from the axis of the front 1350 joint to the face of the rear flange yoke (this is with a R154 on the mk2 automatic transmission crossmember, which is the standard setup for the R154 swap). The rear U joint, flange yoke and input flange is a very tight fit with the top of the crossmember and could take some additional hammering for interference free fitment (the whole driveshaft is a pretty tight fit in the car). I got a new set of 11mm driveshaft mount bolts as well. You could use the 60x60mm pattern on the flange with 10mm bolts and the same 1310 U joint flange yoke as the large 'truck flange' on the 7.5", but I wanted a bit more strength here (The driveshaft for a 7.5" would also need to be shortened). You can even get a 29 spline flange that has only the 60x60 pattern, which would eliminate the need to clearance the crossmember near the input flange as it is a smaller diameter.


Finally for the CV axles, I read up on Don's thread here: Supra vs Cressida axle length

I contacted Driveshaft Shop but ended up deciding to design my own adapter plates. They are 20mm thick and use ARP nuts to attach to the flanges, and have a lip that fits into the counter bore to keep everything centred. Then they are clearanced for the CV joint grease caps and have the proper metric threaded holes for the CV axle M10x1.5 fasteners. They are made from 6061-T6 high strength aluminum and are hard coat anodized - I plan to make them available for both this diff swap (would work with a properly positioned mk3 diff as well) and for the stock diff (the adapters for the stock diff stubs are thicker so you can keep the same CV axles if you want to upgrade to an 8" diff in the future). Driveshaft shop supplied the 370mm long CV axles (total length with my adapters is 410mm) based on high strength Porsche 930 CV joints and 14.5" long axle bar, as well as the fasteners to attach the CV axles to the adapter plates.


I did need to clearance the bottom edge of the spring perch on the control arms to get the adapter plate onto the wheel hub flange. Torqued the ARP nuts to 60 ft-lb with ARP lube, I designed the plates with enough room for a 12 point socket in 3/8" drive.

Last step is to install the CV axles with the included bolts. Torque is 60 ft-lb again with ARP lube, had to do a couple passes as the covers compressed down a bit. They have good clearance loaded and unloaded, and still have around 3/4" of plunge left over with the axles straight (just slightly above normal ride height for my setup).


After driving on this setup I am very happy - feels super solid, everything is tight, diff is quiet and the T2 torsen has good torque transfer and is nice and smooth. Plus the extra fluid volume will help a lot with head dissipation (along with all the extra fins on the diff housing). The 8" G code diff is very strong and will handle anything I can throw at it.

Not a cheap or easy upgrade, but a good one for people with lots of power and grip, let me know if there are questions! Here is a link to a build parts spreadsheet with some part numbers etc.

 

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Highly improved, more so impressed with your presentation. The time you took to document and take pics throughout. 🤜🤛
 

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Epic!
Probably best post here in a long while.

Man, we've been talking about this swap for decades, about time someone did it!

Did you weigh your car yet?
 

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Discussion Starter #5
Haha no. I guess an extra 75lb or so over the previous setup with a truetrac and 1 piece 3” driveshaft with 1310 joints both ends. At least the weight is on the right end of the car for this one!

so with a full tank of fuel and no driver I am probably around 2900 lb, as it was like 2820 previously.
 

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I thought we established your car was heavier then that lol Mine is 2800 with 15l of fuel and no driver ;)

But yeah, this should help with weight balance! You might be like 51/49 now!
 

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Discussion Starter #7
Maybe it was 2820 with no fuel? I forget now.
 

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Discussion Starter #10
I have to say, the new drivetrain has been solid so far. Almost at 500 miles so time to do a fluid change in the diff after gear break-in and then I can really give it the beans. I do also need to use a dial gauge to eliminate the runout on the CV shaft ends a bit on the adapters as there is a bit of a high speed vibration and the adapter plates don’t keep the shaft ends exactly concentric due to the clearance required for the grease caps.
 
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