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AMC Model 20 Reconditioning

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    Posted: Dec/04/2018 at 4:33pm
Hi Team,

Just a quick question about rebuilding/reconditioning a 1979 AMC Model 20 rear axle (twin-grip with 2:87).  Its not work I have the experience/skills/tools to do myself.

What would you expect to pay for this work?  Assume replacing bearings/seals/gaskets at minimum, and possibly some other hard parts (yoke?), cleaning...etc.

Not changing gears or 'beefing' it up otherwise.

How long should that work take?  1-month or more...?


I'll accept ballpark and WAGs, any advice would be greatly appreciated, thanks!


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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Dec/04/2018 at 6:33pm
is there a reason to re-do it? rarely do axles in ordinary service need more than axle bearings and seals, and maybe associated brake work done. if they've been lubricated at all the ring, pinion, and inner bearings usually last "forever" (sic). U-joints are wear parts of course.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 83GT Quote  Post ReplyReply Direct Link To This Post Posted: Dec/04/2018 at 7:27pm
Originally posted by tomj tomj wrote:

is there a reason to re-do it? 

I pulled it from a Spirit I scrapped that had been sitting for many years.  I assumed it needed new bearings/seals/gaskets, for a mechanic to "give it the once-over" before I started using it.
I'm inexperienced when dealing with rear axles in general, so I didn't trust myself to decide if it was ready to use.Embarrassed
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 1970390amx Quote  Post ReplyReply Direct Link To This Post Posted: Dec/04/2018 at 11:55pm
I assume the diff is out. I would put it on jack stands and take the cover off. Dump the oil out and look at it. Is it clean, dirty full of metal? You get the idea. There should be a test in the service manual to check the twin grip clutches. Spin the yoke, does it spin smooth or is it rough and rough. Clean out the center section with solvent, brake cleaner works good. Do the gears look in good shape, no rust and the gear faces look shinny. If the gears look good and no roughness in spinning it over I would run it as is. A shop should charge around 6-7 hours to rebuild one maybe less if the axle bearings are not replaced. Parts 200 to 400$ Just a guess. Around here labor is around 100$ an hour. Most shops have someone that will do diff work call around and ask questions.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Dec/05/2018 at 6:09am
Yeah, you're looking at about $1000 ($800-1200, depending on location and local rates) for a shop to rebuild assuming the ring and pinion gears are good to go. If not add another $300-400 or so ($200 for the gears, maybe another $100-200 for labor).

I've never had one that the inner bearings were bad in, and have had many with 150K+ miles on them. The inner bearings are well oiled and have little load on them. The outer axle bearings carry the weight of the car. One the Twin-Grip is verified to work correctly and gears are verified to be good all I'd do is change the outer axle bearings and seals and the pinion seal.

Did you ever drive that Spirit? If the rear gears were whining they will likely need to be replaced. If not they should be good. It's hard to look at gears and tell if they are worn in properly without some experience. I bought a set of used gears that looked good, but they made a bit of noise -- just enough to be annoying, but not enough not to run for a while. After a few years I finally had enough though, as the gears only whined much on long trips (didn't make a lot) at cruising speed, not around town.

There are several sites that show what to look for on gears. Here's a good one from Dana, who made many of the AMC axles in the later years (at least 70s-80s):
You may need to put some axle grease or machinist blue on the teeth before you an clearly see the pattern. Then again it may be obvious as soon as you look at the teeth -- won't know until you pop the back cover. Look at the teeth BEFORE you clean it to see if you can determine the contact pattern. If not, clean and use grease or blue and rotate it a few times.

Other than outer bearings and seals all I'd be concerned about is the Twin-Grip. The clutches do wear, but if it's working now you should be okay. Just remember to fill with limited slip specified gear oil or use a limited slip additive.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 83GT Quote  Post ReplyReply Direct Link To This Post Posted: Dec/05/2018 at 6:28am
Thanks a bunch AMC Team!  This really helps!!  I've got a good idea where/what/how to get the work done now and can move forward on getting a twin-grip under my Spirit.

No, I never drove, or even rolled the donor Spirit, but once I had the axle out I could spin the diff and verify it moves freely (which I why I kept this axle to use in my '83).  I pulled the cover, drained the fluid and didn't see anything terrible (metal flakes, mice...etc), so I thought it would be good enough to use after a reconditioning.

...now to find the right shop....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tomerekson Quote  Post ReplyReply Direct Link To This Post Posted: Dec/05/2018 at 7:02am
I found a local, independent 4x4 and Jeep shop to do my model 20's.  He doesn't like doing the Dana's, but does a good job.  He is now semi retired, and if I provide all the parts he charges about $150 to rebuild/set up the ring and pinion.  I use one piece axles, so I do the axles and wheel bearings.  i generally buy parts at Summit, but have bought some parts through online Jeep stores.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Dec/05/2018 at 12:43pm
? Aren't some of the post 1970-ish AMC Model 20 Twin Grip rear ends the non-serviceable "Cone Type" Twin Grip rather than the serviceable "Clutch Type" Twin Grip ?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ollie Quote  Post ReplyReply Direct Link To This Post Posted: Dec/05/2018 at 6:26pm
Call Mr Weaver in Jasper, TX. I am sure he will do it for you. He was a AMC dealer.

Having AMC Fun,
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 83GT Quote  Post ReplyReply Direct Link To This Post Posted: Dec/06/2018 at 8:56am
Originally posted by Ollie Ollie wrote:

Call Mr Weaver in Jasper, TX. I am sure he will do it for you. He was a AMC dealer.

Having AMC Fun,
Ollie

Thanks Ollie!!


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