8-3/4" axle length / housing questions

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Valiant Showoff

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Dumb question for the group that has me stumped. I bought a '68 Cuda that had an 8-3/4" with a 741 case with 2.93 open gears and good big-bolt pattern Yukon axles with green bearings. I bought a used complete 3rd member (741 case with 3.91 gears and the good Powr-Lock clutches), installed everything and it immediately ground off approximately 3/16" off the spline tip on the driver side axle (see pic). I measured the axles from flange to tip and they're 27-11/16" long which matches the dimensions in the thread stickied above for an A-body An accurate 8 3/4" rear axle width list which had me very confused. I reinstalled just the axles into the differential housing and the gap between the axle tips is about 1-15/16" (see pic), which is about double the distance across the center of the 3rd member receiving splines. Which then makes me question the housing width itself. Did someone previously cut down a bigger housing for this car and just do it wrong.

I used the "axle calculator" on Dr Diff's website to get the all the standard dimensions of the rear end assembly and measured all my stuff. Differential housing (width "C") is 52-5/8" per specs. Pass side (width "A") and Driver side (width "B") housing width are each 21-9/16" per specs. Brake gap on the axles are 2-1/2" per specs. Axle flange (width "D") is 57-11/16" per specs. The only dimension I have that doesn't match Dr Diff's table is the axles. My existing axles are 27-11/16", but his data says they should be 28-1/16" which explains why I have almost 2" between installed axle tips and why it spun off the end of the splines since it was barely engaged into the 3rd member. But all the data I've found online, including the thread above on FABO, shows the axles should be 27-11/16".

So the question is, can someone confirm 28-1/8" is the right axle length as obviously my existing 27-11/16" axles are too short? And why is all the other online data saying 27-11/16" is correct?

THANKS for the help guys!

spline tip.jpeg


axle gap.jpeg
 
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Your forgetting about the space added to the large bolt pattern axle flange possibly.
 
my axles are big bolt pattern (BBP) with 10" diameter x 2-1/2" brake shoes, not the narrower 1-3/4" that I think is standard?

So the big bolt pattern axles are longer than the small bolt pattern? Or is it the BBP and larger brake backing plate that is causing the length change? If so, I wonder if the last guy had a set of Yukon's custom cut to 27-11/16" thinking that was the right for an A-body but didn't factor in the differences in brake gap for the BBP axles?

I sent an inquiry to Dr Diff ahead of ordering new axles, but he's apparently out of the shop for a couple weeks.
 
It’s hard to say what the last guy did. The A body never had a bbp oem axle. So it’s most likely that Yukon axles were ordered incorrectly if it is a A body housing or, it’s a housing that was cut and not exactly to oem specs.
Case from Dr. Diff will get you straightened out and going again.
He is a member here.

Maybe someone here knows his screen name and will alert him to this post.
 
My factory sbp axle is 25 5/8 from end of axle to bearing adjuster mounting plate to housing.
 
Dr Diff got back to me and apparently it's a common problem with the old Power-Locks with 2 side gears. Looks like I need to upgrade my unit to the wide 1-piece side gears and replace my messed up axle.
 
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