Joe, I'll give you what I know.
The GM differentials are physically interchangable into the axle
housings from '47 to '72.
Splineage varies from year to year, the AD trucks and some later trucks
had the 17 spline, some later trucks had the 30 spline. My 1 ton AD got
a donor differential from a '71 3/4 ton pickup and it had 17 splines,
the ujoint was the same and I used my old axles with no modification,
it was a bolt in job.
The only way to be sure is to pull an axle on the project truck and see
how many splines it has. being a full-floating axle this is about a 15
minuet job, but have a new gasket (or silicone) ready before hand.
Then look at the splines on your potential donor truck, same splines,
you're go to go.
I wouldn't advise anyone to pay $400 for a used differential, keep
looking, I got mine for $50.00.
As to ratios, the big block automatic trucks had the best ratios, mine
was from a small block auto and it was 4.11 to 1. Big improvement over
the 5.34 to 1 it originally had.
John
On Friday, Jun 29, 2007, at 09:23 US/Eastern, oletrucks-digest wrote:
> Date: Thu, 28 Jun 2007 09:20:12 -0700
> From: Heather & Joe Way <joe@brakecylinder.com>
> Subject: [oletrucks] 3/4 ton Third Member Swaps
>
> I've been all over Stovebolt.com, and over to chevytrucks.org, and
> elsewhere, trying to verify information about swapping 1967-1972 3/4
> ton third members into earlier 3/4 ton rear ends.
>
> My understanding is that the '67-'72 pumpkin can be swapped into 3/4
> ton rears from the late '30s up through Task Force trucks. I can find
> a number of posts that seem to confirm this for AD trucks, but none
> for TF. I have access to a '52 3/4 ton rear for inspection purposes,
> and the pumpkin appears identical in every respect I can see to the
> 1959 rear my friend wants to change.
>
> I found one post that seems to say the AD and earlier axles are 17
> spline while the TF and '67-'72 axles are 30 spline, so if you're
> swapping into an AD or earlier rear you need to get the later axles
> and shorten them about 1/2".
>
> But I can't find any posts where the poster flat out states that he
> did this swap into a TF 3/4 ton rear end, with a '67-'72 pumpkin, and
> it worked.
>
> My friend has talked to a couple of differential "experts" and a
> couple of truck wrecking yards, and they all say they've never heard
> of this swap for any of the older trucks. I know I've read about this
> many times here and on the Stovebolt site, so I think these "experts"
> are wrong, but I would like to get some confirmation before I advise
> my friend to spend $400-$500 for a later third member.
>
> Who's been there and done that?
>
> Thanks.
>
> Joe
> - --
> Heather & Joe Way
> Sierra Specialty Automotive
> Brake cylinders sleeved with brass
> Gus Wilson Stories
> http://www.brakecylinder.com
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