question on 12 bolt rear end

[bulldawg1081]

ive always thought that on the driver side on the diff housing where the axle tube connects to the housing theres a code like gm 75,i always thought that meant that the rear end was built in the year of the number stamped after "gm"  well i called in the 3 number code on the top of my axle tube from the passenger side which is KDU and they told me that the rear axle was produced from '74 to '77 and is a 340 ratio but the number on the housing is gm 72...whats up with that???

[SUX2BU99]

Good question. I thought the same when I saw GM 77 on one that I pulled out of a 1977 truck. I bought one from a bone yard that has GM 73 on it, which I figured meant it was made in 1973.

[bulldawg1081]

i dont know...anyone else have any ideas?

[bulldawg1081]

seriously...anyone know something about this that i dont??

[VileZambonie]

Access of accurate information in this section will require knowledge of ring gear size. The size of the ring gear can be estimated using the following:

RWD Cars, Mini-Trucks, and Half-Ton Pickups:

Typically vehicles falling into these categories will use ring gears ranging from 7 1/2 inch to 8 7/8 inch, however; some models may utilize a 9 1/2 inch ring gear. The differentials used in these vehicles and in these size ranges typically use semi-floating axles, however; full floating type axles may also be available with ring gears containing these dimensions.



 3/4 Ton Pickups and Heavy Duty Trucks: 

These vehicles use full floating type axles. The ring gear sizes range from 9 1/2 to 10 1/2 inches on 3/4 Ton Pickups, and 12 inch to 12 1/2 inches for trucks with capacity ratings of 11,000 pounds or higher. Typically trucks with the higher capacity ratings will utilize "Drop-Out" style carriers or Non-Integral carriers.



 4x4 Vehicles: 

Typically 4x4 models incorporate front and rear differentials. The front differential may be smaller in size than the rear indicating less axle capacity in the front than the rear. Use caution when replacing gears on the front and/or rear units, gear ratio's are critical to proper operation and must be maintained.

Example:

A 4x4 truck may use a Dana/Spicer model 60 rear differential with a gear ratio of 4.10:1. The front may use a Dana/Spicer model 44 with a gear ratio of 4.0909:1. Notice the difference in gear ratio from front to rear. To ensure proper 4WD operation the ratio must remain the same respectively from front to rear.

Gear Ratio:

Gear ratio may be obtained using several methods:

Method 1:
Obtain the axle code, typically located on the capacity sticker/plate, on the vehicle. Utilize the Gear Ratio Identification chart to determine ratio or the Regular Production Option codes (RPO codes).

Method 2:
Remove differential cover and visually inspect the ring gear for the ratio, typically stamped on the side.

Method 3:
With differential carrier assembly removed from differential housing, count and note the number of teeth on the ring gear. Count and note the number of teeth on the pinion gear. Divide the number of teeth on the ring gear by the number of teeth on the pinion gear. The quotient will be the gear ratio to :1.

Example:
The ring gear has 41 teeth
The pinion gear has 10 teeth

41
----- = 4.10:1 gear ratio
10

Model Identification:

General Motors utilizes differentials manufactured by several manufacturers. Proper identification will be necessary for the purpose of ordering parts. Typically the differential housing has a stamp somewhere on the axle tube which identifies the differential. The differential carrier also contains such a stamp. A metal tag may be bolted to the differential cover which identifies the differential.

The most effective way to identify the specific model of a differential is to consult your local parts distributor and supply the information requested. This not only identifies the differential but advises you up front of parts availability for the specific model.

[VileZambonie]

http://www.novaresource.org/axle.htm

[chevyman72]

If I am not mistaken, I think the 12 bolt went out of production in 73.

[Josh454]

If I am not mistaken, I think the 12 bolt went out of production in 73.

I believe that was the car 12 bolt.  The truck version lived on until something like 82.

[Captkaos]

12bolt trucks were discontinued in 1981.