Posi rear differential factory option?
 

A few questions here.they come from seeing a SPID for a 72 K-10, (4spd equipped) on a FB truck page.
The SPID listed a POSI Traction Rear differential.
How common are the and/or are they hard to find?
Whatever Gear would have been in this Rearend? ( I’m assuming it would be 4.10)
If I were to find one, would it be possible to change the 4.10 to a more Highway sensible ratio (3.55 or lower )?
Last question would be: what would the Factory Part numbers be?
(So that I know that I’m getting an Actual POSI unit)
My current rear end is a “One Wheeler” and I’d sure like to change it over to a POSI unit.:smoke:

Re: Posi rear differential factory option?
 

A pic of my spid on my 72 K10 from my build thread. https://67-72chevytrucks.com/vboard/...2&postcount=14 Originally factory posi and 3.73 gears. If the gear ratio is not listed on spid it would be standard. Not sure what the standard gear is on a 72 4x4, would have to do some research. Believe the cutoff on the carrier is 3.73. If you want higher gears than that a new carrier is required or a carrier with higher gears than 3.73. Probably easier to get a new posi unit of your choice to go with new gears. Unless you are looking to only have original OEM parts in the truck there is no reason to install a 50 year old used posi unit. The clutches most likely will have significant wear. Who knows? You may get lucky. Just in cast you are not aware the carrier is the differential or Posi.

Re: Posi rear differential factory option?
 

Mine was a non-posi. I purchased a new aftermarket posi and installed it using the factory 3.73 gear set. It was tight enough that I could torque the wheel lug nuts with both wheels off the ground and the transmission in park. I have been thrilled with the new one.
BTW I have had a couple of truck that were factory posi. I believe they were a fairly common option.
Cheers.

Re: Posi rear differential factory option?
 

Mine is currently a 3.73 one wheeler.
I was kinda wanting to find a complete Posi rear end, Rebuild it, and swap it with my current one.
BUT, if it’s more feasible to purchase a NEW Posi third member, put in a 3.23 Gear (for a little better Highway rpm), I’ll do that, since I’ll be tearing it apart anyway.
When you say “Higher Gears”, are you meaning going up in the number (as in 3.73 to 4.10), or the other way (as in going to 3.23)
There are other questions with every one answered.:smoke:

Re: Posi rear differential factory option?
 

Look for a posi in a hiway-friendly ratio. Posi was available in an ratio. Personally, I would look for a more recent 12-bolt factory posi in 3.42:1, since ratios jumper from 3.73 to 3.08 in these trucks. 3.73 may be too grunty and 3.08 may not have enough grunt. 3.42 is the perfect sweet spot for running w/o overdrive yet works well if you do go with overdrive in the future. Probably your best bet would be buy an aftermarket performance posi, like Auburn gear

GM also offered posi for the front of the K-models, although seldom seen

Re: Posi rear differential factory option?
 

Yea that front factory Posi is longtime dream of mine… I’ve only seen a few in the time I’ve been messing with the old GM stuff.

Re: Posi rear differential factory option?
 

I have a factory posi unit sitting on my shelf. I traded the old interior out of my truck for it. Of course this was right after I rebuilt the rear end. So when ever I do the 5 lug swap, Ill put the posi in the rear end.

Re: Posi rear differential factory option?
 

I put an Eaton Tru-Trac in mine when I had the rear rebuilt.

https://www.eaton.com/us/en-us/catal...ferential.html

Re: Posi rear differential factory option?
 

My '68 has factory posi wit 4:11's. Not sure if posi is an upgrade option then or not.

Re: Posi rear differential factory option?
 

according to the 1972 Truck Price and Facts book, the K10 with 350 engine came with standard 3:07, optional 3:73 and that's all. Perhaps "special order" one could get other gears

Re: Posi rear differential factory option?
 

3.73 carrier is cutoff for lower gears (numerically higher) A different carrier is required for higher gears (numerically lower) So.... if you have 3.73 and want a 3.23 and it is available a new carrier is required.
If you have the higher gears and wanted to go to 3.73 or lower there are some companies that manufacture gears with thicker rings allowing the use of the carrier.

Re: Posi rear differential factory option?
 

Thanks, have not looked into this. The 3.73 gear option on my spid helps confirm your finding.

Re: Posi rear differential factory option?
 

Both of my 72 C/10's had factory 3.08 gears, both were factory series 3 carriers, one with the factory Positraction. Factory Positraction I installed a "thick" set of 4.56 gears in the factory unit and race it. My daily driver, the other 72, I went ahead and installed an Eaton series 4 carrier and a set of 3.73 gears. That was/is my tow rig/work horse I beefed up by welding the axle tubes and using a cover girdle. I like the Eaton Posi's because they are rebuildable and period correct. The new Eaton carriers are stronger than the factory carriers GM installed until 81 when the 12-bolt was discontinued. The "Truck 12-bolt" in my opinion, is just as strong as the Ford 9"s as long as they're backed up with cover girdles. Back in the day before every soccer mom drove a 4x4 the two wheel drive Positraction was king! ;)

Re: Posi rear differential factory option?
 

When I add posi I go with Auburn Gear or Detroit Tru-Trac, meaning go into rear end work. The trucks with factory posi I have run either came in the truck or I swapped whole rear. I have a later 3.42 posi to install in my '67 hot truck project and am now considering 3.73s and Tru-Trac behind the M22

That can't be all the information. It would depend more on what transmission the truck is equipped with than the engine. 3.73:1 was the most common in manual trucks (standard) with 4.10 being option). The TH 3.08 trucks came through with 3.08s with 3.73 & 4.10 being options.

This '72 I had not nearly long enough had a 292/4spd/4.10s/fr & rr posi, all listed as options
Attachment 2265748
This '72, also owned but a short while, had 350/4spd/3.73/posi rear, all but 3.73 were listed as options

Re: Posi rear differential factory option?
 

that is all, period. the guide shows this in two places. you get one or the other and that's it. it all appears to come down to what engine and what series - C/K10 v. C/K20, etc.

Re: Posi rear differential factory option?
 

On this 67 short bed the posi is listed on the spid with 3.73 gears , it's a 292 3 speed on the column . and has the posi red tag on the diff . :chevy:

Re: Posi rear differential factory option?
 

Grumpy, thanks for the picture, I've been looking high and low for the plug tag, any suggestions??

great thread guys, love all the info

Re: Posi rear differential factory option?
 

What I am saying is that is incorrect or incomplete. My actual life experience says different. Sorry to disappoint you. A manual truck did not come standard with 3.08s in these years. I showed you two trucks I actually owned. I never read your book. I can show you more trucks, though.

The yellow '72 had TH350 and 3.08s and factory fr & rr posi. The green '71 had 3spd (changed to TH350) with 3.73s and factory posi. I installed detroit locker rr & TruTrac fr. This was early 80s, both truck original when I bought them, same as the two sort beds

Re: Posi rear differential factory option?
 

The red tag is easy to get at classic industries / classic parts / etc

The hard part to find is the rear diff plug with the shoulder the tag fits on . Here is a thread with more info .:chevy:

https://67-72chevytrucks.com/vboard/....php?p=8798887

Re: Posi rear differential factory option?
 

Tim, My ‘72 Jimmy is a factory 4 speed 350 and came with 3.07 gears. Or am I just not understanding this? The 350 V8 came with 3.07 regardless of transmission choice standard with options of 3.73 and maybe 4.10 in the half ton.

OP, I haven’t seen anybody else say it, but if you change the rear gear ratio in a K truck (4x4) you have to change the front to match. Several people have mentioned the 3.40/3.42 gear, but I don’t think you can get that ratio in a Dana 44 gear set for the front.

Unless someone has removed it, the Dana 44 in the front should have a ratio tag specifying what gear is in it. And if it is anything other than what that engine combo came with stock, it will be an option on the SPID.

Re: Posi rear differential factory option?
 

A scan of the back cover of a 72 Chevy truck brochure I have in hand. Hope it is readable. Odd that it lists 3.73 as standard on the K10. Mine has 3.73 listed on spid as added, can be seen in post. More confusion for me. See it now- note at bottom. 3.07 ratio used with all 350 engines. So, once 350 is added as option 3.07 is standard and 3.73 is option.
Another add- Yukon gear has a 3.54 ratio and 3.08 for the D44 front as well as others. Expecting some to not agree with me on this, the 3.54 is acceptable to use with a 3.42 rear in off road use.
Realistically, probably less than 100 rpm difference between the 3.08 and 3.73 final drive. So why go with odd gears.

Re: Posi rear differential factory option?
 

I've got this information from the 1971 salesman's price list, for those who need to see printed what I've been saying

Re: Posi rear differential factory option?
 

The generic information that now resides in the GM Heritage Center archive comes from this material.

https://www.gm.com/content/dam/compa...olet-Truck.pdf

(Part of my job for many years was to check these for content and accuracy before they were published).

K

Re: Posi rear differential factory option?
 

So - having said that - we really need a bit more detail about your truck.

I'm going to assume it's a '72 K-10, based on your question (but if not the analysis will be the same):

They were certainly available originally; finding them now might be problematic.

No way to know.

Looking at the base content chart in the Info packet I linked (page 6) shows the standard axle as 3.73.

Checking the powertrain charts (page 10) shows an optional 3.07 ratio in addition to the standard 3.73, confirming what Richard and Casper posted earlier.

So that's not too bad (50/50 odds) but you can't know because you don't know what the original owner chose. Plus open vs posi, so that takes you to one in four odds. Better than I thought it would be, and better than usual.

Yes.

That's a big "ask".

Rear axle assemblies proliferate based on series, C vs K (sometimes), locking vs open, rear axle ratio, brake series or friction material (sometimes).

So somewhere there is, or was, a chart of 20 or 30 permuations that laid out the various part numbers - and the corresponding code stamp on the axle tube.

Any commodity that proliferates like that (rear axle, transmission, engine, shock absorbers, springs, radiators, etc) has a chart like that, or did at one time.

You are going to be hard pressed to find that list of part numbers, after 50 or 60 years, and even if you did there's no guarantee that ratio or carrier is still what's inside the axle.

K

Re: Posi rear differential factory option?
 

The fact I was focused on in my comment on Casper's post is that 4.10s were also available, not only 3.08s & 3.73s. I'm not sure what ratio came standard with 350 w/MT. I always thought back then the 3.08s were standard for AT. That's what I recall in what I've owned. After looking, I see MT came standard with 3.08s as well... yuck. That option pricing list shows the price for all three ratios for given engine/trans. I'm more a from personal experience guy than putting all my faith in texts... right or wrong. I've seen too many exceptions. The red K/10 I showed had 4.10s and that price guide shows "C10 only". That truck was the coolest optioned truck I ever owned or saw. It started a lot of arguments until the skeptics were shown the obviously original SPID. I wish I knew where that truck is now.

I spoke of 3.42s for the rear in my '67 C1500. I like 4.10s in a 4wd, or maybe 3.73s in one for hiway use

Keith, I'm getting a blank page from your link