Dana 70 HD questions

[Nates73Crewdually]

To start off I really know nothing about differentials and I’m trying to learn about them as well as my truck. If I ask stupid questions or don’t understand an answer I apologize I’m just trying to figure this all out.

So I know I have a Dana 70HD in my 73 dually. It’s cast into it so I’m pretty sure I have that much right. Above that C34764 is cast into the housing as well. I don’t know if that means anything or not.

More the part I’m trying to figure out and decode is the stampings on the ring gear.
It says 7 21 72 DANA 34837 B3R28X 41-11
And then about an inch further it says 272

After counting the teeth on the ring and pinion I get 41 ring and 11 pinion so I believe that to be 3.72-1. What do all of the other number mean? If anything useful I guess. The tags are no longer on the cover bolts. I assume at some point someone has been inside of it because those are missing and the bolts are all stainless which I don’t think is factory. But I also don’t know that for sure. Maybe they are?

I don’t know anything really about LSD’s but what I have gathered is if you spin the pinion and both tires spin the same direction then you would be looking at some sort of locked diff. Either locker, Lincoln locked or an LSD etc. If I spin one tire only that tire spin but if I spin the pinion both tires spin the same direction. So I think LSD?? I really don’t know I’m just spit balling. Between my little knowledge and speaking with MR. Google I’ve come up with more questions then answers. So if anyone could toss an answer or two my way that would be pretty cool.

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[Big Chip]

There should be a number stamped on top of the axle tube on the passenger side that will help in your search.

 

[Craig 85]

The 41-11 stands for 3.73 gears. Since your truck is a '73, I would venture to guess the 7 21 72 is the production date of the ring gear. Per Dana, 34837 is the part number of the ring gear.

 

[CSFJ]

I thought the way to determine ratio, was to divide the ring gear tooth count by the pinion. In that case, this would be a 4.10.


I guess I should have actually done the math on this one. Never stopped to consider that the 41 tooth ring was shared by multiple ratios.

 

[idahovette]

41 divided by 11 =3.72727272

 

[HotRodPC]

And rounded up, it's 3.73's. 4.10's are 41 and 10, 41 and 9 are 4.56's.

 

[Nates73Crewdually]

Thanks everyone for the replies.

Big Chip as for number stamped on the tube itself I didn’t see any. Unfortunately I do have a year or two.... maybe 46.... of grime on them that the pressure washer seems to be friends with and doesn’t want to remove. So maybe after a good degreaser and another round of pressure washing, maybe a wire brush I’ll be able to uncover them. I did find a tag that seems to be rivited or maybe screwed onto the diff itself on the passenger side on the front. If that’s what your referring to I believe it says 286-72 I didn’t seem to pull anything up googling that.

But everyone I really appreciate the help