Well, here we go. You really can't get there from here. You have to go back there to get there so you can get here. So let's start there.
In that beginning, ARB made RD55B. 31 spline, for a 9 inch, robust design, etc. Then they rested, got drunk, woke up, needed money and said...Hey! we are selling a shit-pot of these 9 inch 31 spline ARB's but we need a 35 spline solution. And the RD99 was born. But crap! Those shafts are huge, and need a 3.25 bearing. And bigger side gears. Someone drink another Foster's and hand me a pencil and eraser. In order to fit the larger side gear, the collar engagement depth was reduced from .150 to .075. Because... "good enough." Problem is, when you couple .075 depth with bevel tooth you get less than .075. Add some case flex, and some horsepower, and they slip. Not often, sometimes not even noticeable, but yeah, they cam themselves out. So what to do?
Simple. Increase air pressure. But by now, Rock racing is happening, and the 9 inch is the axle of preference, and yeah, they actually slip a lot. But only when you need them. And they snap right back. But race teams are complaining, and everybody is mad. Because when they slip, and then grab, other things start to break. So the fix was to increase the collar depth. Another Fosters, and another pencil. Let's go half again back. So now the engagement depth is .120. But with a longer sliding collar, and a deeper gear, you need more physical space. OK, we'll move the air piston to the opposite side, and put in a giant spring apply, and call it a "competition only" solution. And the RD99CE was born.
Now the RD99 is in existence, and we need to cost reduce and why would we have two completely different designs for the 9 inch. That's just silly. Let's upgrade by downgrading. We will take the bulletproof, excellent, capable RD55B and replace it with the vastly inferior RD119. And no one will complain, because no serious person would ever run a 31 spline axle, and if they do, we can recommend the RD99 and the world is fine. Anyone that lived this knows is BS, but here we are. (where is
@nvrstuk he lived it)
So the evolution of the drive axle from 1-1/8 to 1-1/4 to 1-5/16 to 1-1/2 brought us to 1970. (yeah, I know the Dodge power wagon got 1.5 inch axles in 1957, and the Dana 60 got 1.5 inch axles for the hemi, but those are 23 spline, and you don't get the good stuff until 1970 in the Super Track Pack Dana 60. The fact that every car under 10 seconds in the 1/4 mile is running 35 spline on 28 inch tires seems to be lost on the Bronco crowd. So yeah, keep your 31 spline shafts. Fact is, that for rock bouncing on 33's and 1/4 mile ET's in the 9's, you want 1.5 inch, 35 spline shafts.
Now before you say: "but James...didn't they reduce the engagement on the RD116 versus the RD35 on the Dana 60?" I have to say: "Don't care, I run the old RD35's" And yes, they did, but the collar and gear is about twice as big. So apples and oranges.
You are lucky. The RD55B is superior. And if you are stuck with 31 spline axles, you should keep it. And if it ever leaks, you can use a standard Aerospace Viton quad-ring in the ARB o-ring slot, and it should last forever.
Now what was the question?
