12 bolt in a 84 GMC?

[natedogg]

I bought a wrecked 84 GMC C1500 Sierra Classic short bed for parts and was surprised when there was a 12 bolt rear end under it. The guy I bought it from said it was a 3:08, but I don't know. I don't think a 12 was offered in a 1/2 ton short bed or was it? Thanks for any info on this?

[Cabnchassis]

If it's an 84 it's probably not stock, from what I've seen around the web they stopped using the 12 bolt in trucks in 82. Really here nor there anyway because there is the good 12 bolt and the truck 12 bolt which is not known as the good 12 bolt lol. No I honestly dont have any experience with the truck version of the 12 bolt so I can't say.

The only thing that could be a problem is if that 12 bolt was born on the back of a 4x4 it might have a limited locking eaton diff which is pretty much a time bomb for anything that might get too much wheel spin on asphalt. They used the truck 12 bolt in a lot of k5s and 4x4 trucks and the eaton has been a 4x4 option on GMs since the early 70s. I've seen pieces of that eaton gear shoot through the pumpkin cover of a truck burning out on the pavement, when it blew apart, so i'd pop the cover and find out exactly what you got for sure.

[J Knight]

1/2 ton 12 bolts were no longer offered after 81, so Im sure someone swapped it in your 84.

[craig113]

Then I have a mystery vehicle? '84 C20 w/ 454 Dana 70 10 bolt with 4.10

[Cabnchassis]

Since it's a big block I would guess someone was trying to make a 1/4 mile truck or just something to burn out around town. It may have the good 12 bolt in that case tho and if so it's likey to be posi. A highway rear end in a wheel spinner. What trans does it have?

[gchemist]

12 bolts did come in 3.08 gears. I swapped one into my '83. The 700R4 first gear launches my truck hard. Top speed is also crazy with the 3.08 gears. My truck does 80 MPH on the toll road with such ease, I feel it's almost floating up.

[Axle]

12 bolts also came with the 2.56 ratio.
1981 had 2.56 upto 3.73's in the 12bolt. Any truck 12 bolt is the "bad" 12 bolt. Cars recieved the "good" 12 bolt (larger yoke shaft diameter. But not all car 12 bolts are large diameter).

Alex.

[motornut]

my 79 came with a 3.4
does your axle tube have any numbers,around the rear end?
think mine has the G80......

[piecesparts]

I have a 12 bolt under my 84 GMC, but it was put in after there was a hit on the original rear axle. It came out of an earlier truck. The rear end was a 3.42 posi gear, but is now updated to a 3.73 posi.