Early American "big nut" axle

updated 15 jul2021

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. Many photos below.

Here you can clearly see the rather severe taper on the collar. That's two 6-inch rulers parallel to the threads.

It's not quite obvious how this assembles. Here's a key.

Check for wear/U-joint size

I do not know what is going on here. The Spicer 514G U-joint has cups 1.00" diameter, and the yoke has a 1.00" diameter hole. Cups are a press-fit into the yoke, and retained by inner C-clips. I have a yoke here however with 1.009" bores. It isn't simply stretched or damaged, the holes are round, no deformation or damage. It's a puzzle. The cups are loose in the yoke, and rattle. This combination would certainly cause noise and vibration issues, and probably worse problems.

The bores in the driveshaft ends is likewise 1.00".

Driveshaft removal

To get the driveshaft out of an assembled car requires a bit of fiddling.

STEP ONE is to loosen the big nut's grip on the yoke. Refer to the photos below; the tapered splined sleeve portion is severely tapered. Driving the nut towards the front of the car (right hand threads) tightens the yoke's grip on the pinion shaft. This must be loosened. It is tightened to 300 foot/pounds of torque. This is five-times the recommended torque on a wheel lug nut, and requires big wrenches. Please do not use pipe wrenches, they ruin the metal, but it's your car.

Even with a professional shop's floor lift this is not easy. The nut is also frozen on with rust and congealed grease. I think this method below is easier than the car on the lift. It is tedious, but easy, and safe. It requires the big wrenches.

Jack the car up on one side, on jack stands. Good ones, you will be applying lots of torque directly to the entire car. One wheel needs to be able to turn so that you can rotate and position the yoke.

TO LOOSEN: Put one wrench on the yoke, brace it on the ground solidly. Put the other wrench onto the nut and the far end onto the pad of a floor jack. This takes a bit of finesse, to take up all the slack, have the yoke wrench not fall off, etc (use clamps, elastic, tape, whatever). Then simply jack the end of the wrench. With the car's sills 16" off the ground, this ought to get you about 15 degrees of rotation, enough to flop the wrench over (now you know why they are angled like that, if you did not know before). Repeat.

STEP TWO is to get the drive shaft out. I believe this procedure below is in the service manual, but I'm too lazy to check.

Even with the yoke's death-grip on the pinion loosened, the drive shaft generally does not have quite enough fore/aft slack to come out. The following will provide that additional space:

With the car sitting on its tires, un-bolt the front spring eye perches, both sides, four nuts on studs, and remove the screw holding the brake line support, where the steel line meets the rubber hose. Disconnect the parking brake, at the adjusting rod. I forget if yo uneed to remove or loosen the shocks. Now jack the car up just high enough for the fromt spring perches to exit their studs. The axle assembly sits on the tires. Now roll the axle assembly back, carefully, the two or three inches necessary for the driveshaft to be removed.

Assembly

If you took it apart, assembly is fairly obvious; insert driveshaft, push axle forward, wrestle the spring perches back onto their studs. It can be difficult to push the axle forward from under the car; I use a ratchet cargo strap as a low-grade comealong. Works great.

If you didn't take it apart, as is common these days buying old disassembled abandoned project cars, or finding someone disassembled the rear U-joint to get the shaft out (my most recent case), or other crazy solutions applied, improvise. In my case, the axle is in place, I will install the driveshaft as a complication of transmission installation.

TO TIGHTEN: The same two-wrench/floor jack trick works, from the other side of the car. Before tightening the big nut, set the distance from the centerline of the U-joint's cups, to the face of the axle housing casting, to 4-3/16" (1958 TSM).

Pinion seal

Replacement seals may be available from AMC parts sources, such as Galvin's AMC. 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:

AMC 312 8171
AMC 320 3520
National 6855N
Timken 6855N
NAPA 49679
Sealed Power N-10180
Chrysler 2453144

25 March 2021 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 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.




Ready to install.



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