12-bolt gear ratio ID. Help needed!
 

Just picked up a 12-bolt from 70s model squarebody to put in my '85. I'm trying to discern what gear ratio it actually has. I popped the diff cover and looked for a few numbers on the ring gear (the large one) because I've heard that both the ring and pinion gear tooth count is stamped on it. I found these numbers below in sequence right next to each other on the edge right by teeth:

6270998
10:34
GM
4
73

These were on the face of ring gear where bolt heads are:

326663N GM36

Do I have 3:40 gear ratio? Seems like an odd one but it all seems pretty fresh in there though most things are stamped with GM.

Also, this number was on the differential housing on the outside but I know all that matters is on the inside:

3896839

Re: 12-bolt gear ratio ID. Help needed!
 

Yup!!
A 3.40 gear.

I had a 74 with that ratio.

Re: 12-bolt gear ratio ID. Help needed!
 

Man. That's great! I'll take that all day long. With my current th350 or my 700r4. Is that pretty unusual gear ratio? I've heard of 3:42 but never really 3:40.

Re: 12-bolt gear ratio ID. Help needed!
 

Can't answer for the number decoding but a 3.40(aka 3.42) isn't an oddball. My 75 has that in a 12 bolt. Option code HE4 - 3.40 ratio, on my truck's option list.

Just spin the yoke and count the axle rotations.

Re: 12-bolt gear ratio ID. Help needed!
 

So a 3.40 is actually a 3.42? Didn't know that. I know a 3.42 isn't oddball.

Re: 12-bolt gear ratio ID. Help needed!
 

Not it isn't.
12 bolt is a 3.40, 10 bolt had a 3.42.

Re: 12-bolt gear ratio ID. Help needed!
 

That's what I needed to hear! Thanks.

Re: 12-bolt gear ratio ID. Help needed!
 

Wondered why some threads mention one ratio and others mention the other. Now I can check the box marked "Learn something today" :)

Re: 12-bolt gear ratio ID. Help needed!
 

Exactly 😎

Re: 12-bolt gear ratio ID. Help needed!
 

Since we're here, how can I tell with the diff cover off if it's a posi (LSD) rearend? This is my first rodeo with gears so I'm just trying to make sure of what I have - especially since posi traction is supposed to have an oil additive. Both wheels do spin the same direction when I turn the other and it had a metal clip in one of the diff bolts (I read that means it could be). But I can't tell just by looking at gears inside diff.

What am I looking for to know for sure?

Re: 12-bolt gear ratio ID. Help needed!
 

A open rear end will have nothing but side and spider gears with the crosspin..
A posi can/will have an assortment of springs and/or governor with clutches..
Here's a open diff..
Google (posi differential) images and it'll show you a bunch of different ones..

Re: 12-bolt gear ratio ID. Help needed!
 

Definitely an open rearend. Thanks much!