Steve,
I'm in the same situation with my "38", though with a 3600 series, I think
your gears would probably be 5.14:1, same as mine. No matter, they're still
bad ju-ju for any speed at all. I have standing "yellow stickies" posted by
the phones of about all the rear-end places in the USA to call me if a used
set of 4-something ring and pinions magically appears in their shop. New is
out of the question. Correspondence with every major manufacturer in the
U.S. says "Sorry, you'll have to upgrade your differential." I think that a
3-something gear set is wishing upon a star, myself. Given my lack of luck,
I've pursued alternative methods. See, I have a NAPCO 4x4 and I just CAN'T
upgrade my front axle housing. I'd sooner shoot myself. (Front and rear
diffs are identical, hence the importance of retaining the stock front and
rear axle housings).
Your question raises a "possible" solution that I've been presented with by
a pretty knowledgeable fellow. I haven't done it yet as the bucks haven't
appeared for this experimental portion of my project, but I intend to give
it a try at some point in the future. Maybe someone on the list can shed
some light on whether or not this has worked/will work for sure.
Here is a "Proposed Solution" to the dilemma. I need "CONFIRMATION" or
"MYTH DESTRUCTION" here listmembers. Little help, PLEASE?
The TF series "Corporate 3/4 & 1-ton differential units have a smooth
round 10-bolt inspection cover. I've been told that the differential itself
is called a 10-1/4" drop out unit, getting it's name from the diameter of
the ring gear. The "drop-out" unbolts and is removed from the front of the
rear axle housing (and\or the rear of the front axle housing on my 4x4).
The axles are 17 spline axles, and the model number for the GM Corporate
axle is HO-72.
A later model corporate 3/4 and 1-ton differential drop out unit,
manufactured from the early 60's through the early 70's will directly bolt
in to the early style TF axle housing, but is designed for 30 spline axles.
The matching 30 spline axles from the same series differentials can be cut
down 1/2 inch at the differential end, which then becomes a direct bolt up
into our older housings.
Possible benefits of this swap include a stronger axle (30 vs 17 spline),
and the instant on the shelf, or in the junkyard availability of myriad ring
and pinion combinations, including posi-traction units. In addition, the
drop out assembly should have the backlash correctly set already, or can be
set for a reasonable price by a rear end shop as it's already out of the
truck. It then becomes an "owner can do" project, with the only really
technical portion being proper torque specs on the install.
I did ask an axle manufacturer about the strength portion, and he says
that it's very true. 30 "is" stronger than 17.
If this scenario is in fact true, then junkyards had better stand by for
the crowds! What do you say "oletruckers"???
Kevin P.
'59 Apache 38/NAPCO
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
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