I've got some questions regarding rebuilding the rear diff

[AustinBelair]

I'm about to tackle the complete rebuild of my fathers truck for my mom which is a cancer survivor (For now) and I am lost on a few things.

I bought the CPP rear 5 lug axle conversion kit (That uses drums etc) but wanted to really go Disk, oh well.

I need to blow apart the stock rear diff housing to have the housing powdercoated. Once I have the housing coated I need to build it back out with all new hardware.

We are looking at a Posi and probably 3.73 gears. How do I know what Posi unit and what fitment I need (Do all GM 12 bolts translate?) How do I know what gears to get and what else will I need?

I will need to take all of it to a chassis shop and have them setup everything on a bench so I can just install the whole rolling assembly onto the frame.

[Ironangel]

Originally Posted by AustinBelair

I'm about to tackle the complete rebuild of my fathers truck for my mom which is a cancer survivor (For now) and I am lost on a few things.

I bought the CPP rear 5 lug axle conversion kit (That uses drums etc) but wanted to really go Disk, oh well.

I need to blow apart the stock rear diff housing to have the housing powdercoated. Once I have the housing coated I need to build it back out with all new hardware.

We are looking at a Posi and probably 3.73 gears. How do I know what Posi unit and what fitment I need (Do all GM 12 bolts translate?) How do I know what gears to get and what else will I need?

I will need to take all of it to a chassis shop and have them setup everything on a bench so I can just install the whole rolling assembly onto the frame.

Need to know what year and model the truck is? I'm going to guess it's a pre 72 since you've opted for the CPP 5 lug kit. To know what gear ratio and what series carrier your 12-bolt is, you need to pop the cover and note the numbers stamped on the edge of the ring gear such as (43-13)...Divide the ring gear number (large number) by the pinion number (small number) to get the ratio. Once you determine the ratio, you also know what series carrier you have. When you go to pop the cover, go ahead and set the rear axle on jack stands and rotate one of the rear tires one revolution while you count the revolutions of the drive shaft. If the drive shaft turns just a little past three turns you have a 3:08. If it's close to 3-1/2 turns you have a 3:42 and 3-3/4 turns would be a 3:73. When you determine your ratio, you also know what carrier was used. A series three will be 2:76-3:42 and a series four carrier will be 3:73-4:10...The (preferred) Posi unit is the Eaton EAT19556 for 3:73 and up. This is the factory unit used in 12T units to this day and in my opinion, is still second to NONE for reliability. There are stronger units, but NONE with the streetabilty and manners of the Eaton LS Positraction... https://www.ringpinion.com/b2c/Produ...rce=DiffWizard Google that part number because you can find new units for less than Randy's. You will need the "Master Installation Kit" as well as the ring and pinion gear set in the ratio you choose. "Yukon" is top shelf, USA Standard is equally as good for gear sets in my opinion. Do your research, buy a good manual and the tools necessary and you can build the rear end yourself. Or, pay the labor, but choose a reputable shop to set up the Posi carrier and gear set...Randy Lyman of "Randy's Ring & Pinon" has a very good manual for rebuilding these rear axle assemblies. If you plan to rebuild the axle assembly yourself I suggest his manual. Differentials: Identification, Restoration &...