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so how difficult is it to tear the rear-end apart and get it back to gether right withought messing up the alignment of the ring & pinion?
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You didn't say anything about what it is doing wrong, or what rearend you have or or ...
...so there is gonna be a lot of guess work here.
There really isn't much inside an ARB. They are elegant in their simplicity. A lot like an open diff, with the addition of a cage and lockiing ring to lock the spiders to the carrier and a pressure ring to operate it all.
I am going to make some assumptions here.
1) That it does not engage the locker.
2) That you have an AMC 20 rear end.
I am going out on a limb and guess it was either run on low air pressure or a couple springs broke and broke some teeth on the locking ring.
The good news is - ARB is very good about having replacement parts for the internals. And they will sell repair parts direct.
Also ARB has their manual online.
Here is the one for an AMC 20 3.08 and up.
As far as getting it apart and back together.
I am going to make a couple more assumptions.
1) You have a decent assortment of hand tools
2) You have good basic mechanical skills.
3) You just have to pull the existing carrier and repair it and reinstall.
If so - As long as you are very careful and keep track of all the shims to make sure they get back where they came from, you should be fine. The relationship between the ring and pinion doesn't change just because you took it apart.
If you have to get a new carrier (the housing) then all bets are off. You will need to set the gears up again, just like it was all new parts. And at that point, you may as well put in a bearing/ mstr install kit.
Special tools wise, the only thing I can think of off hand, is the locking bolts that hold the carrier halves together. It sticks in my head they they are a funky size - like 6 MM 12 point and you will want a good quality wrench to break them loose if they have never been apart.
While you have it apart, replace the o-rings on the acuator end. When you order the o-rings. Order 2 sets. It is easy to pinch one reinstalling the seal housing and you don't want to wait another week for new o-rings.
(Ask me how I know this and why we stock ARB o-rings now)
And be VERY CAREFUL with the copper tube running from the air ring. (ARB calls it the "seal housing") they are easy to crack.
Last item. When you find out what broke. Try to play dectective and decide WHY. Maybe it just broke, but more likely there was some other problem. Air pressure. Contaminated fluid. Low or improper fluid.
You don't want to do this again because you didn't fix the original problem.
Other than that - use your head, take your time and with a decent shop manual for the rear you are working on, you should be fine.