GM12 bolt

[neba]

trying to identify what gears i have in GM 12 bolt. can find casting numbers but no stamped numbers. NF 3896839. has a plate with springs in center that i havent seen before.

[oldblue1968chevy]

whats the specs on your truck?

[neba]

69 chevy C10 long bed 350v8 400 turbo

[kgt]

sounds like a posi, out of a car, the springs with a plate reminds me of that style as opposed to the centrifical style in the trucks. When you say identify, do you mean you want to know the gear ratio of the set, if so divide the ring gear teeth by the pinion and that number is the gear ratio..

[LMN8UR]

Just because you find codes, doesn't mean a previous owner didn't change them. Do you have a tach, is your speedo right, do you have a posi?

[neba]

never thought of something so simple as counting teeth. or the fact someone could have changed it. found it has a 3.73 does the plate with springs behind pinion indicate its a posi?

[Fitz]

Originally Posted by neba

does the plate with springs behind pinion indicate its a posi?

on the left open carrier on the right Eaton Posi

[neba]

bought this truck over the winter so my goal was to change fluid and inspect gears. gears appear to have nice even wear pattern.fluid didnt show anything unusual.next my curiosity asked what gear ratio which i now know is 3.73 now im wondering if its an Eaton. the ring gear has a GM stamp and part # 3817778 would an Eaton diff have GM gears.it looks like picture Fitz posted. my lack of computer skills is preventng me from sending picture

[Fitz]

Originally Posted by neba

bought this truck over the winter so my goal was to change fluid and inspect gears. gears appear to have nice even wear pattern.fluid didnt show anything unusual.next my curiosity asked what gear ratio which i now know is 3.73 now im wondering if its an Eaton. the ring gear has a GM stamp and part # 3817778 would an Eaton diff have GM gears.it looks like picture Fitz posted. my lack of computer skills is preventng me from sending picture

The center section can be Eaton, Detroit Locker, open or whatever the factory or previous owner might have put in the housing. The center I posted was an Eaton BUT that does not mean thet the entire differential is an Eaton! The Eaton Diffs were installed by the factory only on 3/4 ton trucks and they have a totally different outer case. The cover looks like a car 10 bolt on steroids but, as has been pointed out here before, the number of "bolts" refers to the number of fasteners attaching the ring gear to the center section and only coincidentally matches up with the number of bolts holding the rear cover in place. In the case of the GM 1/2 ton truck and some car housings, the numbers are the same but really arent a guide.

As Shaky pointed out in the thread "rear end curiousity" a true 3/4 ton Eaton differential somewhat resembles a GM 10 Bolt differential which is obvious from his photo below:

[neba]

so it seems the center parts are Eaton. doesnt look like it has clutches like some GM units had. does it require a lube with additives for posi, $18 a quart or regular lube $4 a quart? thanks for the help.

[cdowns]

get poyal purple gear oil it has the additives in it and the synthetic is far superior than the economy stuff