S13 Open Differential to S15 Helical – Install Notes

by 92_240sx

There are many installation/how-to articles out there, so I won’t do the write up. I have some tips though, so I’ll just cover those. I’ve never worked on one before. I have s13 with open diff and swapped the internal with s15 Helical.

First, I bought the diff on eBay last year. I choose ‘By ship’ delivery option and it took about a month for me to receive it from Japan. If you are not in a hurry, that’s way to save some money.

I drained the fluid first, spray PB Blaster and got all the bolts and nuts loose before I take the diff out from the car. Like bolts for the cover, filler plug, etc… I figured it would be easier that way since I don’t have a vice.

Output shafts and driveshaft bolts were not easy to loose. I used 2 wrenches and hit one with rubber mullet several times to let them loose. Also I had to turn output shafts/driveshaft to access them bolts and nuts. I used parking brake to lock and unlock output shafts, and put the gear on neutral to turn driveshaft. I didn’t remove stock rear sway bar.

I’ve found this bucket is very useful when I was working on it once it was out of the car.

You will see 2 holes, and you can put a flat head screw driver to open the cover. Then used paper knife to clean up the gasket.

When I tried to install s13 ring gear to s15 diff, it won’t go all the way in. It got stuck in the middle, so I put all 10 ring gear bolts on and tighten them little by little in cross pattern, so they can pull the ring gear. Torque spec is 112lb, and make sure to use threadlocker. I bought 10 Nismo ring gear bolts, so didn’t have to worry about fabricating sleeves. This bolt has 13mm shank on 12mm thread. I think original s13 bolts would have worked just fine as long as you torque them down right. This step is impossible without the air gun or table vice. I went to the shop I know, and used their table vice. It took about 20min to move the ring gear from old diff to new one.

Side seals wouldn’t come out when I tried to pry them out with flat head screw driver. I used back of breaker bar and they came out real easy. When you install them, use 1-3/4′ socket. Lip will go inside of socket and you can just hammer the socket to install new seals.

As far as output shafts go, I measured open diff again just now and distance from circular clip to outside is 55mm on passenger side and 60mm on driver’s side, and those are same numbers that I got from s15 helical diff, so LONG output shaft goes DRIVER’S SIDE and SHORT output shaft goes PASSENGER SIDE.

I was very concerned about the shims, but it was easy. There are total 3 shims. 2 thin ones and 1 thick one. Just make sure to keep them in order when you install everything back. Bearing races and shims wouldn’t go in all the way, so just like ring gear, I had to tighten these 4 bolts little by little. Eventually, they went all the way in with no problem.

I counted splines carefully, and s15 output shaft has 30 and s13 open diff output shaft has 29. Lengths are same. One is short the other one is slightly longer(about 0.5 inch?). Output shaft went in fairly easy, but I didn’t know what goes which side since lengths are different. I measured the depth on both sides, and they were same. I had to measure depth of ring clip inside to figure out.

I used gasket instead of silicone sealant. You know silicone gets messy sometimes. Besides, it looked like Nissan originally just used gasket, so I did the same. I left the diff overnight after I filled it, and didn’t see any leak. I guess since the fluid is heavy(75W-90), it should be okay to use just the gasket. I used about 1.5 quart of Mobil 1 Synthetic gear oil 75W-90. Mobil 1 doesn’t make 80W-90 BTW.

I also got the new breather valve. Part number for the breather is 38322-P6400. It’s about $8 from the dealership. Used a flat head screw driver to hammer it out and rubber mullet to put a new one in. Make sure to put some all purpose grease on side seals and breather valve. They’ll go in easier that way.

When everything was out, I cleaned the cover, bolts, nuts, output shafts and the housing with de-greaser. Spray painted the housing with 500 degree high temp DupliColor rattle can spray paint($4.99 from Autozone). It came out really nice and the diff looked like original stock.