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Rear end rebuild / leaf springs

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72SST401 View Drop Down
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    Posted: Jul/25/2016 at 12:02pm
Would appreciate opinions:
72 Javelin 401/4spd, going to completely rebuild the rear diff. Suggestions on where to get the best parts & one piece axle, what gear ratio for a street machine?
Best rear leaf springs to use? Factory springs are 4 leaf, I like a more old school aggressive stance just not too high.
Thanks, Gary
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FSJunkie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Jul/25/2016 at 2:05pm
Yukon Gear & Axle should have rebuild kits for the AMC-20. The kits come with bearings and seals, but not gears or Twin Grip clutches.

Gear choice really depends on your tire size and what you want your car to do. Most muscle cars are geared high to give them excellent acceleration, but the side effect is poor high speed cruising ability. Low gearing gives you the opposite: poor acceleration, but excellent cruising ability AND top speed.

You know the "Beep Beep" song about the little Rambler toasting the Cadillac? Well that's actually how it works if the Cadillac is geared high and the Rambler is geared low. Running at high speed with high gearing will bring any engine to its knees.....and the little Rambler will pass you practically in 2nd gear.

It's fun to be that Rambler, because the big V8 muscle car toasts you at the stoplight, but you eventually catch up to him and pass him. Gosh it makes the guy in the muscle car madder than heck.










Edited by FSJunkie - Jul/25/2016 at 2:20pm
1955 Packard
1966 Marlin
1972 Wagoneer
1973 Ambassador
1977 Hornet
1982 Concord D/L
1984 Eagle Limited
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ccowx View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Jul/25/2016 at 4:09pm
If you are looking for a general cruiser with decent acceleration off the line and still has some top end, go with 3.54's. 3.15's are nice and give a better kick off the line than the standard 2.87's or similar but still are essentially a highway gear. After that 3.73 or 3.91 are nice but you are starting to limit yourself on highway runs. I have 4.10's personally and  they are great BUT you lose any highway cruise capability. Period. My personal radius is about 20-30 miles and that is it. I like the steep gears because most of my driving is around town with only a few high way miles and it gives a great kick in the #ss when you hit the gas. BUT please understand that you are now a "one zip code car" as far as your ability to go anywhere.

Really, how far do you want to take your classic? I look at it this way, I spend 90%+ of my time driving around the local town to cruises etc. How many of us take our machines to shows that are more than 20-30 miles away? Most of our driving is to the local A+W or around our home town. These are antiques not road machines.

Bear in mind, tire diameter is a huge factor too. I personally am running F70-14 repro tires that are 26.5" around. If you have larger tires take that into consideration. I would suggest that having your engine turning around 2500-2700 on the highway at 60 is good. Once you are over 3000 rpms you are into the "over stressing your engine/AMC famous oiling issue" territory and you should not be cruising on the highway for long distances.

It is all about you personally like. I find that 3000 rpms is the max I am comfortable with. The noise gets unpleasant and above 3k I find that the oil drain back issues start to matter. Many like to err more towards 2500-2800(or less) for long cruises.

Personal taste matters here, but above 3000 you need to think about engine damage. Between 2000-3000 it what you personally like.

Be realistic and think about your driving. Gear your car for you reality. If you spend 90% of your time around town then why gear your machine for highway? If you take long road trips often then gear accordingly. And take tires into account!!!!

There are many calculators online to tell you what you are doing.

Chris
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Red Devil View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Red Devil Quote  Post ReplyReply Direct Link To This Post Posted: Jul/25/2016 at 4:21pm
... easier decision with an overdrive transmission as less compromise required.

If it's a 2.23 gear T10, 3.54 rear gear is a good compromise with a mild street cam. More cam wants a 4.10 gear. Economy cam is ok with 3.15s. A 2.43 or 2.64 geared T10 is more forgiving of less rear gear.

Ratech is good for correct pinion shims. Their 319K kit gets everything for pinion & carrier.
http://www.ratechmfg.com/amc.htm

For ring & pinion, Motive, Yukon, Richmond, Alloy USA, G2, etc. ... lots of options. Not sure if one is better than others.

If ordering 1-pce axle shafts, best to order c/w the outer bearings and seals from the axle manufacturer to ensure they are matched. DoctorDiff is likely most economical, Moser likely most popular, a few use Dutchman.

Or could go for an non-AMC rear. Ford 8.8 seems popular ... but not a direct drop-in.

For rear springs, Eaton-Detroit are likely best.

Hope this helps,RD.
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72SST401 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72SST401 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/25/2016 at 7:02pm
Thanks for your input, I appreciate it!
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72SST401 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72SST401 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/25/2016 at 7:04pm
Thank you!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote White70JavelinSST Quote  Post ReplyReply Direct Link To This Post Posted: Jul/26/2016 at 10:22am
X 2 on what Red Devil said.

I'll add that Eaton Detroit Spring has the bushings, shackles/bolts, and u-bolts too. Can't remember if they have that unique front spring eye bolt.
70 Javelin SST, second owner, purchased 1972
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72SST401 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72SST401 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/26/2016 at 11:16am
I'm considering a 5 speed swap now, seems a better option. I do appreciate everyone's comments, thanks!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote uncljohn Quote  Post ReplyReply Direct Link To This Post Posted: Jul/26/2016 at 12:36pm
My AMC cars for the most part are hobby cars driven on the streets and some times long distances to car shows. I use street gearing depending on the car and I have several that range from OEM 2.53 with a torque flight transmission and a 2200 rpms stall lock up torque converter to 3.15 with a Ford T5. I shoot for about 32 mph in high or top gear what ever you want to call it and depend on the torque converter if automatic and low gear ratio in the transmission with plenty of gearing for acceleration.
I got rid of the 3.54:1 Posi rear axle in my Donohue 20 years ago. Droning along listening to the engine at freeway speeds on road trips became old hat and incredibly boring after a while.
My latest build uses a 700r4 and a 2200 rpm stall lock up converter with final gearing some where close to 40mph/1000 rpm but I do not have the transmission shifting as I want it to and will probably find gears that give me about 32 mph /1000 rpm.
The bottom line? Times have changed and for me short gears for drag racing are yesterdays news and I no longer want anything to do with them.

As to stance? AMC for some reason made all of their vehicles sit nose high when they built them. I do not worry about rear springs other than if they are needed, OEM replacements are just fine. It is the front springs I pay attention to. First thing I do when I rebuild the front end is take about a half a coil off of the top which will drop about 1 1/2 inches. If I want a nose down aka "trans Am" look I will take a bit more. Rule of thumb, one coil = 3 inches. And a good shock makes the front end behave and look good.


Edited by uncljohn - Jul/26/2016 at 12:42pm
70 390 5spd Donohue
74 Hornet In restoration
76 Hornet, 5.7L Mercury Marine Power
80 Fuel Injected I6 Spirit
74 232 I-6, 4bbl, 270HL Isky Cam
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