Rebuilding the R180 on a Subaru STI: Stack and Torque

What Rebuilding the R180 Changes

The unit in this rebuild is the plated STI rear, the clutch type sold under the Sure-Trac name. A Torsen does not use this plate stack. The finished measurement is one 2006 STI. A 2004 R180 was treated as the same case. Ring-and-pinion tooth contact and pinion preload still follow the service procedure. The decisions that decide whether the diff locks are the plates, the cone springs, and the clearance left in the case.

The 65 to 103 ft-lb band is the initial torque tied to a correct STI-kit clearance. The 98 to 155 ft-lb figure is a scaling of that band for the Nissan disc swap, not an axle-nut measurement. Cone springs, also called Belleville washers, are the part that goes soft. On a used STI rear, rebuild before it goes back in the car. The plates are not the usual reason a high-mileage unit feels open.

Rebuilding the R180 Plate Stack

Measure the internal case depth, then subtract the assembled ramp thickness. What remains has to hold two spring plates, four friction discs, and six friction plates, and still leave 0.06 to 0.24 mm. A smaller remainder is a tighter diff. On this 2006 STI the case was 86.64 mm and the ramp assembly was 65.99 mm, so the room was 20.65 mm.

Those plates add up to 10.00 mm. With the springs and discs, the stack is 20.50 mm, and 20.65 minus 20.50 is 0.15 mm, the middle of the window. Kit friction plates come in 1.60, 1.70, and 1.80 mm. On this kit each one measured within 0.01 mm of its mark. The new discs were 1.70 mm, the same as the original discs. The new springs were 1.85 mm, thicker than the original springs.

The spring in the kit is 1.85 mm every time. One rebuild entered 1.5 mm, the old spring thickness, and the formula then called for a taller plate stack than the case could hold. Breakaway went past what a normal torque wrench could show. For a different case whose ramp assembly measured 65.70 mm, the plate mixes that fit were four 1.60 mm with two 1.80 mm, or two 1.60 mm with four 1.70 mm. The second mix was the one preferred there. Those counts belong to that case depth and that ramp. They are not a substitute for measuring the case in front of you.

A stack of all 1.70 mm plates, with clearance calculated at 0.06 mm from separate measurements, would not break at 150 ft-lb. After 30 minutes of figure-eights it still took a guess near 200 ft-lb to move. That guess was not a torque-wrench reading. The plates had not been measured as an assembled stack.

Plate Order

The original order, and the order in the rebuild kit, puts a friction disc against the ramp and two friction plates against each other. The ramp face wears, and each side is left with about 3.5 working friction faces instead of 4. The 60° ramp instructions reverse that so a plate faces the ramp. Thicker plates go on the outside, which is how this differential came apart.

From one spring plate to the other, the order used here is: 1.70 mm plate, disc, 1.70 mm plate, disc, 1.60 mm plate, the center, 1.60 mm plate, disc, 1.70 mm plate, disc, 1.70 mm plate. Coat every plate and every disc with 90 weight before it goes in. Reuse the thrust washers. A dry stack reads tight and is not the breakaway number you are building to.

The 45° and 60° ramps are machined to the same thickness, so the clearance math does not change with the angle. The reversed plate order is the 60° instruction. A 45° rebuild still uses the measured clearance and the kit’s own order.

Ramp Angle

A 60° ramp needs less force to climb than a 45° ramp, so the diff reacts sooner to throttle. On this 2006 STI that showed up as a sharper lock in a tight corner. The new plates were also easier to hear. The rear disadvantage is a diff that locks sooner than you wanted. There is not a long list beyond that.

The N14+ rear is a 2-way, the same layout as the GDB plated rear, with three angle pairs. The front STI and Cusco units are where the 1-way and 1.5-way choices live.

Initial Torque

Initial torque here is the breakaway at the axle nut, with one rear wheel lifted. It is not the most precise test, and it is not pinion preload. Pinion preload is the rotational drag at the input shaft. On this rebuild a trigger-pull gauge that tops out near 12 lb covered the upper end of that spec, and the shaft landed in the middle of the range.

The gravel, street, and competition rows are planning bands. Cusco’s gravel suggestion is lower, about 35 to 45 ft-lb, and Prodrive Group N cars ran about 55 ft-lb. Cusco’s out-of-the-box figure for a GDB rear MZ limited-slip is 87 to 101 ft-lb, inside the street band. Do not average those into a new target. Pick the band for the tires, then measure the nut.

One later case, with plates that looked unworn, measured 0.223 mm of clearance. That is the loose end of the window, close to no preload. A shim between the spring plate and a friction plate was added to reach 0.123 mm. Breakaway after that shim was not measured.

Nissan Discs in the Stack

A shorter rebuild keeps the original ramps and adds two Nissan parts so the pack has more friction faces. On each side, one 1.7 mm friction disc becomes a 1.9 mm disc, and one 1.65 mm friction plate becomes a 1.65 mm spring disc. Stack height grows 0.2 mm per side, 0.4 mm in total. A spring disc on each side adds clamp. Friction faces go from 8 to 12, and that change alone is estimated to raise initial torque by about 50 percent.

Factory order on one side, at the stock thicknesses used for this swap, runs from the pressure ring through a 1.7 mm disc, a 1.65 mm plate, a 1.7 mm disc, a 1.65 mm plate, another 1.65 mm plate, and a 1.8 mm spring, then the case. Two plates together leave the third disc with nothing to work against. After the swap, that side is a 1.7 mm disc, a 1.65 mm plate, a 1.9 mm disc, a 1.65 mm plate, a 1.65 mm spring disc, and the 1.8 mm spring. The original pack on the 2006 STI did not match every one of those stock thicknesses. Measure the parts you have.

If the STI kit’s 65 to 103 ft-lb window is the starting point, the same clearance with this disc swap is estimated at 98 to 155 ft-lb, or higher. That is the tarmac end of the table, for sticky tires. Doing only this swap on a high-mileage 2004 R180 still leaves the old cone springs in the case. Thickness and the extra spring disc bring some torque back. Worn plates will not match a full kit.

Not every Nissan R180 part fits a Subaru case. The large difference is the axle: Nissan left and right splines are not the same. These two part numbers are the swap. The rest of a Nissan differential is not a Subaru parts bin.

Assembly and Break-In

Side-bearing races are the easy piece. The bearings themselves need a press, or a careful cut if a press is not there. A bearing puller kit is the tool for the outer race in the side plate. On this 2006 STI a shop pressed the bearings. New side shims went in at the same thickness the case already had, and input-shaft preload landed in the middle of the range. Replace the axle retention clips if they leave the side gears. Line the case up on the match marks.

The bolts that the service procedure seals with threadlocker used Loctite 243. Three-Bond red is the same class as blue threadlocker. The bolts under the ring gear on one rebuild were Phillips M6x12 and needed a PH3 driver. A worn Phillips head means drilling them out. Those were replaced with Torx heads. Paint the ring-gear teeth and read the contact pattern before the diff goes back in the car. The pattern on this gear set sat inside the acceptable band.

A new pack gets about 30 minutes of figure-eights, then fresh 90 weight. Buy 2 liters so the first fill and that immediate change are both covered. A third liter covers another change somewhere between 500 and 3,000 miles. Then lift the right rear and read the axle-nut breakaway. A correct kit stack should move inside the 65 to 103 ft-lb window. A wrench that stops at 150 ft-lb with the wheel still locked means the stack is too tall or the plates were never measured together.

What the hardware list covered

• In the kit and beside it. Eight ring-gear bolts, a carrier gasket, side plates with seals, side shims, side bearings, a breather, 90 weight, threadlocker, and yellow contact paint.
• A separate job. Pinion bearings and their preload shims. Parts and labor jump when those are worn.
• 2015 money, as an example only. The rebuild kit was estimated at about $350 to $400, with one import closer to $260 before shipping. Other hardware was about $100 to $200. The 60° ramp kit on this rebuild was about 320 EUR shipped.

Order of work

1. Measure the case and the ramp. Clearance is case depth minus ramp thickness, then minus springs, discs, and plates.
2. Use the new 1.85 mm springs. The original 1.5 mm springs are worn even when the plates still look thick.
3. Measure the six plates as a stack. Leave 0.06 to 0.24 mm. Trusting the stamps is how a diff locks solid.
4. Set the plate order and oil every face. Keep the thrust washers. Point plate tabs at the pressure ring.
5. Close the case on the match marks. Threadlocker on the bolts the service procedure names. Confirm the axle clips are in.
6. Set side-bearing preload and read the gear pattern. New shims can stay at the old thickness only when the input shaft is in the middle of the range.
7. Figure-eight for about 30 minutes and change the oil. Then read the right rear axle nut.

FAQ

What does rebuilding the R180 change?

The clutch plates and the cone springs inside a plated STI rear. The measured car is a 2006 STI whose side bearing had failed. A Torsen does not use this stack.

How tight should the plate stack be?

Leave 0.06 to 0.24 mm after the springs, discs, and plates are accounted for. The 2006 STI landed at 0.15 mm. Tighter than that locks the wheel. Looser than that feels open.

What should the axle nut break away at?

The STI kit window is 65 to 103 ft-lb. This 2006 STI measured 85 to 90 ft-lb at the right rear nut. That test is not pinion preload.

Do 60 degree ramps need a different stack?

No. They are the same thickness as the 45° ramps. They climb with less force, so the diff locks sooner.

Can the original cone springs go back in?

No. The kit springs are 1.85 mm. The original 1.5 mm springs are the part that loses clamp. Entering 1.5 mm in the stack math builds a diff that will not unlock.

Will Nissan plates fit a Subaru R180?

Two parts are specified for the disc swap: 38426-E4610 and 38433-RS610. Plate tabs face the pressure ring. Nissan left and right axle splines do not match, so the rest of a Nissan differential is not a Subaru part.