The Stovebolt Site Forums The Shop Area Engine Shop ID a 14 bolt rear and gear ratio

Hi all.
My second post here. My wife says I spend too much time reading here, but hey, there's so much to learn!
I bought a 1950 3/4 ton pickup recently that went through a number of previous owners, so there's plenty of surprises.
My latest surprise ... I thought the 14 bolt rear was original, but after a thorough inspection it appears it may have been changed. My first clue was the pass side shock mount is in the wrong position! I did a quick ratio calc (raise the rear, turn each wheel one revolution, count the driveshaft revolutions) and I come up with 3.2 driveshaft revolutions. I did it 6 times, and 6 times it came up 3.2.
Can anyone help me ID this diff and gear ratio? Does the 3.2 sound correct?
I got some casting numbers off the pumpkin (driver side front of pumpkin).

After wire wheeling it looks like:
CTD
120
3877323

I can't seem to find a differential casting reference on the internet, so any help would be appreciated.
I know I still have to pull the cover and check the teeth, etc, but I'm not planning to do that just yet. Too many other truck-related projects going on at the moment!
Thanks in advance!
50-3600-Guy

Hey, you are welcome here all the time.By 14 bolt, you mean the later rear axle from the 70's on up with the irregular shaped rear cover? Unless that later 14 bolt rear end is altered, it should be wide enough to have the tire sitting the fenders edges. They come in all sorts of ratios,later ones have 3.20 something gears available I believe.

You are correct. Before I took the bed off, the 16" tires had about 1/2" clearance on each side.
Thanks for the help! I guess that means I have a late model 14 bolt rear. Now I just have to figure out the gear ratio, and what to do about the fender clearance. Maybe 15" wheels would help.
At any rate, I should be good to go with the gear ratio I have.
Thanks again!

Here are a couple of websites to help you ID your axle. If you do have a newer 14 bolt axle you should be able to ID it here.
By gasket shape
By rear cover PIC

To find the gear ratio, try this:

"Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 & 3/4 turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading."
(quoted from West Coast Differentials)

Interesting links Kenny.
I was wondering why they did not have the GM rear end that was used from the early `40s until `72 in the 3/4, 1-ton,and 1&1/2 ton trucks.I will see what I can find for a link,,
This is quite interesting
http://la.znet.com/~t1937/4WD.htm
and
http://www.pirate4x4.com/tech/bible/#GMRE

cool find
http://www.rearman.com/truck.htm

Thanks guys!
It looks just like the picture in the link Kenny W provided, plus I found a site that said 3:21 gears were available for the late model 14 bolt rears. So, it appears I have a late model rear with 3:21 gears, which is good news. I won't have to swap out the pumpkin for a higher gear ratio.
I will still pull off the cover and make sure nothing looks worn. I'll count the teeth then to verify the ratio.
Thanks all for the great replies!
50-3600-Guy