72 12 bolt 4 series to 3 series swap?
 

Have a 12 bolt with 3:73 and would like to put in 3:08. Not finding 3:08 for a 4 series

Do I need to sap out entire rear end housing or?

Re: 72 12 bolt 4 series to 3 series swap?
 

Just the carrier

Re: 72 12 bolt 4 series to 3 series swap?
 

Good to know

Are the stock axles, 72 6 cyl, 30 spline?

If so this 3 series Eaton and a 3 series gear set would work
https://www.summitracing.com/parts/etn-19587-010

Re: 72 12 bolt 4 series to 3 series swap?
 

I believe so, but wait for someone else to chime in.

Re: 72 12 bolt 4 series to 3 series swap?
 

30 spline

GM 12 BOLT SPECS
Applications:

1964 - 1982 Chevrolet & GMC 1/2 ton to 3/4 ton trucks

Full/Semi Float:

Semi floating rear axle

Ring Gear:

8.875"

Axle Spline Count:

30 spline

Axle Tube Diameter:

3.00"

Pinion Diameter:

1.625"

Pinion Spline:

30 spline

U Joints:

1310 series

Ratios:

2.50 - 4.56 : 1

Carrier Break:

2.50 - 2.76 | 3.42 -4.56

Identification:

12 bolt rear cover, passenger car version has a different cover and is easily distinguishable

GM 12 Bolt axle identification

Re: 72 12 bolt 4 series to 3 series swap?
 

but which side of the carrier break is 3.08(3.07)?

Re: 72 12 bolt 4 series to 3 series swap?
 

My understanding is 3:08 gears only are for 3 series carrier. 4 series starts at 3:73 and goes numerically higher

Truck 12 Bolt Series 2 = 2.73 - down (smaller numerically) Series 3 = 2.76 - 3.42 Series 4 = 3.73 - up (larger numerically)

Re: 72 12 bolt 4 series to 3 series swap?
 

GM 12 Bolt Truck carrier breaks:
2.73 & Down
2.76–3.42
3.73 & Up