When talking about rear ends I keep hearing about 2 series vs 3 series vs 4 series carriers, housings, gears, posi etc. I'm trying to understand how all the different things relate to each other. About all I fully understand is that 10 bolt is light duty and 12 bolt is heavy duty and they both come in posi and open versions.

1. When one speaks of a carrier, and having to get a 3 series carrier to run 3.XX gears, what part is being talked about? Is that the actual housing that you see from the back of the car or or is that the barrel shaped thing that fits sideways in the housing (the thing that the ring gear fits to)?

2. Also I'm assuming that the gear that gives the rear end the particular ratio is actually a combination of the ring gear and the pinion gear? So when you change your ratio from say a 3.31 to a 3.73 you are changing the ring and pinion and nothing else?

3. If you switch from a 2 something to a 3 something or a 4 something, do you have to swap the housing or can you just take out all the guts and replace them with something else? Like say if I wanted to swap my 10 bolt open 2.73 for a 10-bolt posi 3.73 can that be done with the same housing or what?

4. What exactly is different in the rear end components that makes it a posi vs non posi?

I've gotten some info from reading the 69 chassis overhaul and service manuals, but I still lack some basic understanding I think.

Cheers,
Andy

1. Yes. The carrier has to have the ring gear in a different spot for the pinion. Yes the barrel deal.
2. Yes, that may include reshimming them to get correct pattern.
3. Same housing, it is just that the offset the ring gear so that the pinion gear will work (since the size changes so much for each gear)
4. Instead of having spider gears you have a spring/clutch system that allows the wheels to lock under power.

Andy:

When talking about rear-ends and such, remember the 10 changed in the early 70's (I think) from an 8.2 to an 8.5 rear set-up. Also there is a difference between truck and car 12 bolts (GM). Parts will not interchange. I don't know a whole lot about them but have acquired some knowledge thru the years.

The reason there is a 2,3 and 4 series (there is no 4 series for the 10 bolt) is because of the size of the pinion gear head. A 2.73 has a 15 tooth pinion gear, a 3.73 has a 11 tooth pinion gear and a 4.88 has a 8 tooth pinion gear. You can imagine the difference in the size of these gears. The pinion is always mounted in the same loction, so the ring gear must be moved closer or farther away from the pinion center line to make up for the difference in the pinion head size. The 2 series has the ring gear mounting flange moved farther away from the pinion center line because the pinion gear head is so large. The 3 series ring gear flange is closer to the pinion center line and the 4 series is even closer because the pinion gear head is so small. Ford and some other brands only use one differential and they make up the difference with the thickness of the ring gear itself.

A 2.73 gear has 41 teeth on the ring gear and 15 on the pinion. Divide the number on the ring gear by the number on the pinion and that is the gear ratio. The ring gear and pinion gear are matched at the factory and can not be mixed with other gears. They must always be changes as a set.

All housings are the same, no matter what the gear ratio or type of differential (posi or standard).

A posi is a limited slip differential, no matter what someone else might tell you. Some people think that limited slip is not the same as posi, but that is not true. A posi, or LSD (limited slip differential) uses clutch plates or cones to provide friction between the right and left axle gears in an attempt to deliver equal amounts of torque to each wheel. Most have preload springs to apply more pressure to the clutches so that it will lock up better. You can not add clutches to a standard differential to make it a posi, although there are some locker kits that fit into a standard differential case to make it a locker. A locker is a LSD, but not the same as a clutch type. Here are some pictures that might help a little. http://community.webshots.com/album/55410292AnMJrE