28 To 31 Spline


10 Year Member
Oct 30, 2008
I have the stock 28 spline diff. Can I buy a 31 spline rebuild kit and replace the 28 spline spring with the supplied 31 spline spring, and get some 31 spline moser axles? Or am I stuck buying a 31 spline diff in order to use 31 spline axles? I would think that if I swapped the clutches and disc's from 28 to 31, and use the bigger spring, I could save a few hundred bucks and reuse the stock differential, but I am not entirely sure if that would work or not. Thanks.
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Stangnet Facilities Maint Tech... Er... Janitor
Admin Dude
Jan 4, 1985
Box behind Walmart
You need the axles (of course) and complete 31 spline traction lock.


You can get them from the junkyard if you can't find one on-line at a decent price.

Someone chime in if I'm missing anything: @Mustang5L5

Bob Hughes

Dec 26, 2011
Southamptonl, NJ
No that's it minus install kit. I swapped mine last year to 31-splines pretty cut and dry. You can get the whole trac loc units from am pretty reasonably priced


10 Year Member
Apr 23, 2008
NC State University
Gotta have a 31 spline carrier.

You can get the whole trac loc units from am pretty reasonably priced
Get rear-end stuff from Tousley Ford if you want to really save some money.

31 spine Cobra/GT500 diff: $215 https://parts.autonationfordwhitebe...fo&PartID=957827&siteid=214771&catalogid=5684

Full install kit, including GT500 pinion bearings, carrier bearings, axle bearings, all seals, etc. i.e. EVERYTHING thing you need to completely rebuild the rear-end: $90 https://parts.autonationfordwhitebe...fo&PartID=498790&siteid=214771&catalogid=5684

(you can get one for $10 cheaper without axle seals and bearings)

I'm assuming you aren't replacing the gears. If you were, those are around $120. https://parts.autonationfordwhitebe...o&PartID=1071356&siteid=214771&catalogid=5684

So you can build the rear end for $305 + axles + gear oil. You can find axles often on online classifieds if you look around. I picked up a brand new set for $150 from the Corral.

Pro-tip. DO NOT use the gasket that comes in this kits for the differential. It WILL leak. Use gray RTV instead.


10 Year Member
Oct 30, 2008
Thanks sneaky. Yeah I'm keeping the gears, and I'll re-use the bearings since I've already swapped them twice and the have less than 5k on them (unless the races are scored).

I see AM has some 31 spline moser axles... How hard is it to swap the abs ring from the stock axles to the new ones? Do they press off? I didn't pay attention when I swapped to 3.73's.

That sucks that I can't just throw the 31 spline rebuild kit into a 28 spline trac-lok. I figured the only difference would have been the clutches and metal rings inside the diff. When I replaced the stock clutches with the carbon fiber kit, I don't recall the axles physically splined in to the diff it's self. That's been years ago tho.
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