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NOTES ON DOING AN R31 DIFF CONVERSION

A fairly common upgrade for RWD Toyota owners is to fit the differential from an R31 Skyline. Below are some important notes that anyone doing the conversion should consider.

 

 

Conversion disadvantages

Housings and weight: Unlike the diffs used in most Toyota's which have a cast iron diff center bolted to a fabricated housing the Skyline diffs which are made by Borg Warner have a complete center section that's cast steel and steel axle tubes welded in place. This means that the diff itself is very heavy for its size and changing the diff center requires a much more difficult procedure to remove the pinion.

Caliper / handbrake issues: The brake calipers used on these diffs have a mechanical / hydraulic combination. The mechanical over-ride is for the handbrake and these are known to be very problematic.

Stud type: Although the studs are the same diameter as those use on a Toyota they have have a different thread pitch. Therefore unless the studs are changed the vehicle will have two different types of wheel nuts.

Gear ratios: There is only a limited range of gear ratios.

LSD availability: See "LSD options" further down.

 

 

Conversion advantages

Width: For some people it may well be possible to not have to shorten one of these to be of use, or to only have to shorten a single side.

Stud pattern: Using a 4 x 114 PCD these are already the same as all but early AE / KE models

 

 

Width

Flange to flange the R31 diff is close to 1470mm, for comparison an ae71/ae86/ke70 diff is 1410 and 1370 for a TA22

 

 

Shortening

The Skyline diff is an odd shape, while it has one short and one long axle the diff pinion is already offset to the direction of the longer axle by 30mm. So while it's possible to use a second shorter axle in the longer side and cut down the housing it's never suggested to do so. This is because it would put the pinion even more out of center, resulting in an 80mm offset, this is so much that the tail shaft might even hit the floor pan of the donor car.

 

Also due to the axles shape the minimum it can be shortened is 50mm, otherwise there wouldn't be enough material to re-spline after being cut down. A way to get around this is to only shorten the side of housing that holds the short axle and then use a second long axle to be cut down to any length needed. For even narrower cars the short axle can be put in the long side and then the short side reduced even further. For a TA22 there is an ideal solution, as just said a long axle in the short side will take the diff down to 1420 and getting the short axle re-cut and spline -50mm will bring the housing right on the original 1370mm. This also puts the center of the diff to exactly to where the original Celica housing had it.

 

 

Brakes and calipers

While the solid rotors fitted to the R31 are a decent diameter the calipers, or more specifically the integral handbrake mechanism is woefully bad. Unlike some of the newer split disk over drum designs the skyline uses a mechanical over-ride of the hydraulics much like ae82 and ae93 corollas. The biggest problem with this design is that to adjust the mechanism the piston has to be rotated at 180 degree intervals in and out. Unlike the corollas with their 90 degree adjustments this often means that the calipers pad's won't fit over the disk or use most of the actuation of the lever to just make the pad touch the disk.

There is no easy solution for this however the calipers off a Holden (GMH) VS Commodore can bolt right on that are a plain hydraulic design with handbrake caliper. Previously here at SQ Engineering two customers brought spare calipers and brackets where the brackets where machined and then welded end to end. This then held the calipers opposite to each other with one for the main brakes and the other activated by a hydraulic handbrake, this although crude was a brutally effective drift modification.

 

 

Breather

The original breather on the Skyline housing has to be moved as it ends up being where the spring seats needs to be. So a simple drill and tap of the housing solves this problem. Both housing are readily weld-able so the old breather hole is easy to fill.

 

 

Wheel studs

While the stud pattern is right the thread pitch on the studs is wrong. Nissan use a 12 x 1.25mm pitch while Toyota use the more course 1.5mm pitch. While at first someone could just run mismatched wheel studs there is always the risk of a mix-up and the wrong nut being used. The way to fix this is to knock out the original stud and fit something new. Conveniently the Nissan's use a 12mm hole in the flange unlike the much larger Toyota design. One option is to fit the studs NS380 from the company Niceproducts. However they do require the drilling out of the original flanges to 13mm. The flanges are a hardened metal and are extremely hard to drill out by hand without causing destruction. In this workshop the axle was fitted to the base of pedestal drill, clamped in off the bearing collar and then drilled through.

 

 

Ratios available

Note: while the crown-wheel and pinion can be used from a Pintarta the diff center and axles are incompatible.

 

- Skyline manual: 3.7:1

- Skyline automatic: 3.9:1

- Pintara: 4.1:1

 

 

LSD options

While the Skyline Silhouette had a factory LSD fitted it was of a cone design which had a very limited life and locking ability. Even if it could be serviced it would not be worth the effort to do so. Aftermarket centers are available such as from Kaaz. Spool centers are sold cheaply.

 

 

Pinion flange

Bizarrely enough the pinion off a Borg Warner made S-series diff as fitted to a corolla or some sprinters will fit straight on to the R31 nose. Just undo the pinion nut and slide it off.

 

 

Handbrake cables

It is possible to fit the original hand brake cables from a corolla/ae86 to the Nissan calipers. Originally the arms on the calipers do not have anything to hook a cable on, however by chopping the small hook off the drums from the old diff the lines can be hooked up the same way. After this however the cables are much too long, this can be fixed using only a pipe as a spacer welded to the position on the caliper where the cable is held on.

 

 

Hydraulic lines

The hydraulic lines on the diff housing itself need to sourced from the diff that was previously fitted in the car. This is because they are the right shape to fit around the brackets which also have been swapped. Where possible also swap the small clamping tabs over to hold them in place. The ends of the metal lines can be hand bent to fit into the Nissan calipers.