updated 25 mar 2021
new ~ 2010
Even amongst AMCers there's a lot of rumor and fear about the driveshaft system on these early Americans, known as the "big nut" axle. "Modern" American cars have the rear U-joint yoke fixed to the rear axle pinion shaft; the big-nut rears instead have a splined slip joint with a compression sleeve tightened with a ... big nut, tightened to 300 foot-pounds. With big wrenches.
I was warned of the horrors of working on this thing, but it simply wasn't that bad. I bought a pair of 1-3/4" combo wrenches, Fuller brand, total cost for two including shipping: $50 (in 2009).
It did require different procedures but none of them were particularly difficult.
Since apparantly few people have seen these things I took some photos of the procedure to debunk the mystery.
Replacement seals do not exist as far as anyone knows. It is made by National and has an AMC part number of 3128171. It is Group number 9.064. The factory parts catalog shows that number superceded by 320 3580.
Not that this will help make parts appear, but as far as I know these were once replacement part numbers:
25 March update:
... the pinion seal used
on this rear was mysteriously used on 68-69 Javelin Sixes and is
presently being reproduced by the vendors. It is available at american
parts depot for $39 and bears the part number 990 6402B. I believe the
other vendors have it as well. Also, Brent Havecost has an NOS 3128171
Seal at www.nashparts.com for $59 -- Sean Crane
This is the rear yoke, slip-joint coupler and the eponymous big nut, cleaned for inspection.
Ready to install.
This is the pinion end of the rear axle. I took these photos to later identify the seal, which are apparantly no longer available. It doesn't look like rocket science to adapt something to though.