Rear gear id?

[Tripp]

Hey guys,
I purchased a 87 K-5 with a blown ring and pinion on E-gay. I am looking at replaceing the rear or rebuild. All I know is the #'s on the ring gear are: 1GM3984828 13 40 1178. I believe the number in the glove box is a C6. Can anyone help me and tell me what ratio this rear has. What is the best option for the replacement rebuild or replacement. What is a fair price. This K-5 is not going to be a daily driver just something to beat around in.
Thank you

 

[bigblock72]

40 divided by 13 equals 3.076 or 3.08.

Hope this helps.

 

[cuervo]

If you change the back ratio you will need to change the front to match. I believe the carrier break is at 4:10 which means you could not go taller then 4:10 without changing the carrier.

 

[Tripp]

Thank you for the input. So to match the front I need a 3.08 rear. How you guys think that will hold up while driving on the beach in soft sand? I am running 35 BFGs

 

[lectric80]

With 35's it is going to be very weak, as it probably already was. You lose a lot of torque due to the high gear ratio with very tall tires. Big factor in my changing from 3.08's to 4.10's, plus it will help me with towing.

 

[dhcomp]

If you already have a broken rear, find a deal on someone's stock axles with 3.73's or 4.10's, and swap both. THat will be your most economical option. If you are willing to swap wheels, get a 3/4 ton set of 8 lug axles. Those will be stronger, but will be harder to get and more expensive + the rims.

Good luck!

 

[4X4HIGH]

1GM3984828 13 40 1178

The first set of numbers/letters (1GM3984828) is the part number, the next number (13) is the pinion gear tooth count, the next number (40) is the ring gear tooth count, the next numbers (1178) is the month 11 = November and 78 = 1978.

I'm guessing you probably got the 1178 mixed up and it's actually 1187 since the truck is an 87 either that or that diff was swapped in and you better check the front gear ratio to make sure of the gear ratio in it (good idea anyways since you just bought the truck and people do stupid things sometimes and they may have swapped in that rear diff that doesn't match the front).

 

[Tripp]

GREAT INFO FROM YOU GUYS!! Greatly appreciated