well i was working on ye olde project 85 today, and i was reading through the RPO codes and discovered i have a GU6 3.42:1 ratio differential, but its open! anyone know if i can easily swap my ring gear onto a posi unit and make it 3.42 posi? i do not mind having drums, and will it be difficult? i have rebuilt engines, and have done minor internal transmission work, so im confident i can do it.

If it's a 10-bolt posi you can swap it over as long as it's a 3-series posi. 2-series is 3.08 and taller (numerically lower), and 3-series is 3.23 and shorter (numerically higher).

I'm not positive that 3.08 is the cutoff, but I think it is. Either way, What you have is a 3-series gear, and you need a 3-series posi carrier. This will pretty much about to a gear swap and you'll need shims and bearings and crush sleeves and all taht other stuff so you can get the gear pattern right. It takes a lot of tedious trial and error to get the gears set in there right. Definitely not a bolt it in and throw it together kind of job.

I've never done it yet though... probably will pretty soon here...

Putting a posi in your rear end isn't a big deal. All you have to do is get the right one and then set the bearing preload and backlash when you install it. You will need a set of shims to set the bearing preload and backlash.

http://www.nastyz28.com/forum/showth...ht=change+posi
This is for the 8.5 10 bolt, but it's teh same for the 7.5.

I recommend a new Eaton Posi. They are strong, rebuildable and can be tuned to fit your needs. They are $435 plus shipping, and about $40 for the bearings and shims to install it.