3.9 gears on a LSD

[rztmartini]

I have a 3.9 open diff (R200 out of a Z) and a 4.375 cLSD (R200 out of a skyline) and i am trying to put the 3.9 gears on the lsd center section. is this possible, because the 39 tooth ring (left side in the picture) is thicker than the 35 tooth ring on the LSD (right side). As you can see from the pic, the flange that the ring bolts to is a lot thicker because of the two halves of the lsd unit vs. the open carrier.

 

so my question is this: is it possible to swap gears? or where can i get a lower gear ratio that might fit on the lsd (S13 or other)?

 

[zguitar71]

There appears to be a spacer making up the difference on the 35 tooth gear so leave it off of the 3.9 gear and it should be fine. You might need 1mm spacers for the bolts because the 3.9 has 10mm bolts and the 4.37 has the 12mm bolts, search it and you will find good info about it. As far as a lower ratio, 4.37 is low already, did you mean Higher?

[JMortensen]

There appears to be a spacer making up the difference on the 35 tooth gear so leave it off of the 3.9 gear and it should be fine. You might need 1mm spacers for the bolts because the 3.9 has 10mm bolts and the 4.37 has the 12mm bolts, search it and you will find good info about it. As far as a lower ratio, 4.37 is low already, did you mean Higher?

No spacer. The LSD has a 2 piece case, and the seam where the two pieces meet is what you're seeing.

 

The lower the gear ratio the smaller the pinion head and so the ring gear gets thicker so that they both meet. American cars have something called "carrier breaks" where the offset of the carrier is different so that you might need one LSD for gear ratios up to 3.55 and then another carrier for gear ratios of 3.73 and up. This is so that they can use less material in the construction of the gears themselves. Japanese manufacturers don't do the carrier breaks nearly as often, which makes it easier to figure out what parts you need, but makes the ring gears thicker and thicker on the lower gear ratios.

 

If you set the two carriers on end and measure from a table to the back side of the gear, you'll see that they're the same and there is no carrier break. The 4.37 gear will work on the LSD because the pinion head in that diff is smaller.

 

EDIT--I missed you flipping off the LSD the first time... good stuff.

[rztmartini]

ok sounds good. I was actually trying to put the 3.9 onto the lsd carrier, the 4.375 is what came on the lsd. the ring gear looked too thick, and the pinion gear is actually bigger (10 tooth) than the 4.375 (8 tooth). thicker carrier flange (2 pieces of the lsd) + thicker ring gear + bigger pinion gear = too much?

 

EDIT - Nevermind, I see that the entire carrier is offset to account for the two pieces of the LSD. pictorally, the two measured distances are the same. Thanks!

 

[JMortensen]

I see you figured it out, but there is another issue you touched on here which should be explained while we're at it. Not all 10 tooth pinions are the same. If you have a 3.70 and a 3.90 with 10 tooth pinions and 37 and 39 tooth ring gears, the 3.70 pinion head will be larger than the 3.90 pinion head in diameter. The tooth pitch will also be different. In short, keep the ring gear with the pinion gear, NEVER swap one ring for another in a differential (even if they're the same gear ratios, it's a bad idea as a ring and pinion are a matched set that are "run in" with each other on machines before installation).

 

To confuse things it is possible to change the pinion head size arbitrarily and run a more severe pitch on the teeth, but in general the ring gear gets thicker as the ratio gets lower. Maybe Nissan did that on the 4.375 to keep the pinion head stronger. You see warnings against really super low gear ratios sometimes because the pinions break.

[rztmartini]

one thing to note: the carrier shims had to be rearranged in order to get the ring and pinion to mate correctly. on one side, there was a thick and thin shim, and on the other was only a thin (actually three different sizes). the left side had to be switched with the right side in order for them to mesh and be able to get the bearing caps on.

 

this is my first differential rebuild/teardown, so i dont know exactly how much play is acceptable, but after swapping the gears, the amount of play seemed very small, perfectly acceptable to me. i painted the gear to check mating surface contact, and there was a good amount of smear. i guess the real test will be when it is sent down the quarter mile.

 

the only snafoo (sp?) was that the bolts that hold the ring gear. on the 3.9 ring gear, it uses 10mm bolts, where the bolt holes that are in the LSD carrier and the 4.375 ring are 12mm. i used a bushing (12mm OD and 10mm ID) to take up the slack between the bolt and the hole. is there anything wrong with me doing this? so bad news for someone wanting to swap 4.375 gears into thier R200 open diff, cuz it wont fit. possibly because those gears came from a newer car, and used beefier hardware.

[JMortensen]

one thing to note: the carrier shims had to be rearranged in order to get the ring and pinion to mate correctly. on one side, there was a thick and thin shim, and on the other was only a thin (actually three different sizes). the left side had to be switched with the right side in order for them to mesh and be able to get the bearing caps on.

 

this is my first differential rebuild/teardown, so i dont know exactly how much play is acceptable, but after swapping the gears, the amount of play seemed very small, perfectly acceptable to me. i painted the gear to check mating surface contact, and there was a good amount of smear. i guess the real test will be when it is sent down the quarter mile.

 

the only snafoo (sp?) was that the bolts that hold the ring gear. on the 3.9 ring gear, it uses 10mm bolts, where the bolt holes that are in the LSD carrier and the 4.375 ring are 12mm. i used a bushing (12mm OD and 10mm ID) to take up the slack between the bolt and the hole. is there anything wrong with me doing this? so bad news for someone wanting to swap 4.375 gears into thier R200 open diff, cuz it wont fit. possibly because those gears came from a newer car, and used beefier hardware.

Situation Normal All F'd Up = snafu

 

You really need to measure the backlash. Should be .005" to .007" if I remember correctly. Nothing wrong with the bushings, I did that myself.

[Whittie]

I have both 4.375 and 3.9 R200 VLSD as well as a 3.5 VLSD. We used shims on them (the 3.9 and 3.5 from memory) just used a metal pipe that was 10mm inside 12mm outside. Basically what you did.

 

Have been racing the car for quite a while now with no dramas.